Formation of a Protein Dilysyl Crosslink Containing an Oxidized, Iminoquinonoid Derivative of Pyridoxal

These diagrams show a possible mechanism by which the protein lysyl adduct of pyridoxal might, following its oxidation, "initiated" by singlet delta dioxygen, into a quinonoid derivative [as shown in past posting: (http://hardcorephysiologyfun.blogspot.com/2010/05/potential-for-crosslinking-and.html)], participate in the formation of an iminoquinonoid dilysyl crosslink. As discussed in the captions, this overall reaction I'm showing is very similar to the so-called "1,4-Michael addition" reaction that quinones and other aromatic compounds can participate in. Here are the references [Penning et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/9894013)(http://www.med.upenn.edu/ceet/documents/CRTHighlyCited2007.pdf); Sridhar et al., 2001: (http://linkinghub.elsevier.com/retrieve/pii/S0040402000009546); Mure, 2004: (http://www.ncbi.nlm.nih.gov/pubmed/14967060)]. I should also mention that the initial reaction with singlet delta dioxygen may appear to be an exotic one [see here: (http://hardcorephysiologyfun.blogspot.com/2010/05/potential-for-crosslinking-and.html)], but I don't think that either the reaction or its product, the quinonoid pyridoxal derivative (shown as the initial compound in the diagrams below, alongside the coenzyme Q10 benzoquinone ring), is exotic. Quinonoid Schiff-base intermediates are known to exist in the formation of copper-dependent amine oxidase enzymes [Mure et al., 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12135347); Kano et al., 1997: ()(http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=a608275k&JournalCode=P2)].

C(subscript)S Point-Group Symmetry in Compound(s) I: Role of Overlap of Thiolate 3p(z) AO and Porphyrin a2u MO

These diagrams show the bent angle of the sulfur-iron "bond(s)" [the bond comprises the various molecular orbitals (MOs) that allow for pi-type bonding interactions between the sulfur 3p(x) and 3p(y) AOs with the 3d(xz) and 3d(yz) AOs of iron (or, rather, the MOs of non-protein-bound heme that those AOs of iron contribute to the formation of)] and sigma-type interactions (I'd define a sigma-type interaction, in this context, as being "overlap of quasi-cylindrical symmetry about the S-Fe axis," or something like that) of the 3p(z) AO of sulfur with the 3d(z2) AO of iron (or, rather, the MOs of heme that the 3d(z2) AO of iron has already participated in the formation of) and the a2u MO of heme [see Ogliaro et al., 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11948872); Harris, 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11738185)]. The bent angle of protein-cysteine-liganded heme and also in nitrosylhemes (Porphyrin-Fe-N=O) and in iron(II)-hemes bound to O2 (unless the Fe is bound to both oxygens of O2, in which case it's C2v symmetry, I think) causes those heme species to be assigned to the Cs point group, in which the only possible symmetry operations are the identity operation and the reflection about a sigma(h) plane of symmetry (I think I've drawn it correctly) [for a depiction of the bent angles of the Fe-N and N=O bonds, away from the traditionally-assigned z-axis that is perpendicular to the plane of the porphyrin ring, that characterize Cs point-group symmetry, see Novozhilova et al., 2006, p. 2100, Figs. 6 and 7: (http://www.ncbi.nlm.nih.gov/pubmed/16464112)(http://harker.chem.buffalo.edu/group/publication/370.pdf)]. The only other characteristic of Cs point group symmetry is that it's "abelian," and I forget what that is or maybe don't understand it. I'd need some more math to understand the symmetry stuff fully [for the point-group character tables, see: (http://www.webqc.org/symmetrypointgroup-cs.html)]. One begins with a flow chart and then goes from there. The angle is bent, in part, "because" the angle allows for the constructive overlap of the sulfur's 3p(z) AO with the a2u molecular orbital of heme.

The other main point is that there's potential for confusion in the nature of the interaction of sulfur's 3p(z) orbital with the a2u orbital, on the one hand, and, on the other hand, with the 3d(z2) AO of iron. I don't have time, at the moment, to get into this and refer to various articles, but one might say that one "reason" that the sulfur's "ungerade" 3p(z) AO's can exhibit constructive overlap with the "ungerade" a2u MO of heme [this would be symmetry-forbidden in the planar, isolated porphyrin ring that exhibits D4h point-group symmetry, by virtue of its nitrogen atoms being coplanar with the rest of the porphyrin ring (rather than "domed" out of the plane of the porphyrin ring, as in Cs and C4v point-group symmetries)], for example, is that the a2u MO of heme cease to really be "ungerade" in Cs point-group symmetry (and also in the C4v point group). The 3d(z2) AO and other AOs of the isolated ferryl moiety are "gerade," but those AOs and the a2u MO are neither gerade nor ungerade in Cs point-group symmetry, given that there's no inversion point in Cs point-group symmetry (there's a center of mass of heme, but no inversion point is assigned to it). They're all either a' or a'', in the Cs point group. Anyway, the change in the Fe-S bonding angle changes the symmetry point group that one assigns heme to and, more importantly, changes many of the interactions of the porphyrin-localized MOs of heme with the MOs of heme that are predominantly-localized to the FeO moiety, & it's interesting.

These are the conventions, evidently [conventions that few researchers actually follow, evidently, given that it seems like it would become problematic in computer models of MOs (http://vitalii.chemicalblogs.com/2_computational_chemistry/archive/40_conventions_for_symmetry_notations.html)] for the assignment of the principal axis (a C1 axis that makes the molecule nonaxial, given that one ends up back where one started after rotating the molecule 360 degrees, or 2pi radians, about the z-axis). The z-axis that I've drawn intersects 2 atoms (the maximum number), and the x-axis is perpendicular to the plane of the isolated, nonsubstituted porphyrin ring. The sigma(h) plane is a horizontal reflection plane, and the "lines" or axes of the Fe-S and Fe=O bonds are in the same plane (those lines are coplanar, and their plane is coplanar with the sigma(h) plane). The molecule is symmetric upon reflection through the sigma(h) plane but has no other axes of symmetry or planes of reflection. The assignment of a molecule to a point group is not determined by x, y, or z-axis assignments, but one has to assign x, y, and z axes (and, ultimately, a spherical coordinate system) in order to superimpose 3-dimensional plots of MO probability density functions (the orbital "shapes" are dictated by the probability that an electron will be within the region that is defined by the surface of the orbital at any given instant) onto the "coordinate-system-independent" C2, C3, etc., symmetry axes and planes of reflection. Evidently, that's part of the reason there can be confusion, as far as orbital symmetry assignments go. Researchers assign different coordinate systems, and that can, apparently, alter the MO symmetry assignments, across different articles, even when different groups of researchers are referring to precisely the same molecule, in precisely the same electronic state.



The reverse face of heme is shown in the second diagram:

Occupancies of "Frontier Orbitals" of Compound I; Nature of Nonbonding Electrons in the Fe=O Oxygen

These diagrams show the orbital occupations of a commonly-encountered electronic state of perferryl heme (the quartet A2u state of compound I), and many of the "orbital occupancies" are, in other iron(IV) or iron(III) hemes, the same as these or similar to these. The nature of the "lone pair" of oxygen has been a source of confusion in articles, and it's almost certainly a sigma-type bonding interaction of the 2p(z) atomic orbital of oxygen and the 3d(z2) atomic orbital of iron, with minimal 3d(z2) character and somewhat minimal bonding character (i.e., a "nonbonding MO"). [Also, I forgot to put it on the diagram, but the formal charge on oxygen, in this species, would be 6 - 2 (lone pair electrons) - 1 [nonbonding radical in pi*(xz or yz) MO] - 1/2(4 electrons in pi(xz) and pi(yz) bonding MO's) = +1. I think the formal charge can be +2 or something else, in some of the transition states, but that's not the actual charge. For example, in one of the major iron(II)-heme species, porphyrin-Fe(II)-O-O(-), the actual charge is about -0.2 and not the formal charge of -1, apparently [found experimentally, as reported in the abstract of Jensen et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/15598490)(http://www.cicum.cup.uni-muenchen.de/ac/kluefers/homepage/L/BAC/heme1.pdf)]. With regard to the oxygen's lone-pair electrons, though, there may be some 2s character, too, and part of the confusion stems from the fact that, in dioxygen, the "nonbonding MO's" have predominantly 2s character and mix to only a minimal extent with the 2p(z) atomic orbitals of oxygen (they're not "sp3" orbitals) [this article discusses the absence of the supposed "rabbit ears," produced by sp3-hybridized, "lone-pair" MO's in the water molecule: Laing, 1987: (http://pubs.acs.org/doi/abs/10.1021/ed064p124), and this article includes a discussion of nonbonding MO's in general, I think: Hurst et al., 1999: (http://pubs.acs.org/doi/abs/10.1021/jp984565h)]. But other articles refer to dioxygen's nonbonding MO's as being the pi*(yz) and pi*(xz) antibonding MO's, which are singly-occupied in ground-state triplet dioxygen (there's also at least one excited triplet state of dioxygen). There's no single way to determine, without looking at experimental data [data from the use of different types of photoelectron spectroscopy (PES): (http://scholar.google.com/scholar?hl=en&q=%22bonding+character%22+%22lone+pair%22+nonbonding+molecular+orbital+%22photoemission+spectroscopy%22+OR+%22photoelectron+spectroscopy%22&btnG=Search&as_sdt=100000001&as_ylo=&as_vis=0)], the extent to which each of the various MO's exhibits "bonding character", within a given molecule. In general, the highest-occupied molecular orbitals (HOMO's, meaning the HOMO, the HOMO-1, the HOMO-2, etc.), which may be either bonding or antibonding MO's, are the nonbonding ("lone-pair") MO's [the highest-energy MO's, in which there is more separation of electron density throughout different parts of a molecule (more "charge separation," for example, in resonance structures within a given MO--not as much delocalization of electrons)]. But in a lot of nitrogen-containing compounds, there may be essentially no MO's that exhibit nonbonding character, as revealed by PES. Anyway, these supplementary data show some of the strange pi- and sigma-type MO's that describe the bonding interactions in the Fe(II)-O-O moiety of iron(II)-heme bound to dioxygen [the two MO's on the right (the 2nd and 3rd) of p.7 (the 2nd one is probably most similar to the "lone-pair" sigma-type interaction in the FeO moiety of iron(IV)-heme, as sketched in the second-to-last diagram below, but those MO's on p.7 of that pdf are pi-type interactions, not sigma-type interactions, as I've shown): (http://www.rsc.org/suppdata/CC/b7/b704871h/b704871h.pdf)]. Anyway, transition metals will tend ("prefer") to "obtain," via overlap with and donation from ligands, 18 electrons in organometallic complexes [the "18-electron rule" (http://scholar.google.com/scholar?hl=en&q=%2218+electron+rule%22+transition+metal&btnG=Search&as_sdt=100000001&as_ylo=&as_vis=0)], and the rest of the electrons are in other MO's of the O-Fe-S moiety and MO's of the porphyrin ring that are shared with the heme iron(IV) (or iron in another oxidation state). In one article that describes orbital occupancies of an iron(II)-heme, the iron(II) shares a total of 20 electrons with the porphyrin ring and the distal (i.e., O2) and proximal (i.e., histidine or cysteine) ligands. I've only shown the electrons that are in the HOMO's, or "frontier orbitals," whose occupancies are most likely to shift during the courses of Cytochrome P450-enzyme-catalyzed reactions, etc. Note that the diagrams should say "3pi(xz)" or "3pi(yz)" constructive overlap of the sulfur's 3px or 3py atomic orbitals (with the porphyrin's a2u molecular orbital).



These diagrams show the higher-energy steric (i.e., repulsion by electron clouds), as the porphyrin molecular orbitals are interacting, via the nitrogens, with the 3d(xy) atomic orbital of iron, in Coordinate System 1 [shown below and, in terms of the relative energy levels of the d(xy) and d(x2-y2) MO's of transitional electronic states of hydrogen abstraction reactions catalyzed by compound I, depicted in Ogliaro et al., 2000, p. 8984, Scheme 4: (http://pubs.acs.org/doi/abs/10.1021/ja991878x)(http://theochem.chem.rug.nl/~filatov/Pubs/JACS_122_8977.pdf)], or, in Coordinate System 2 [not shown below but shown above, in the diagrams of the "frontier-orbital" occupations of commonly-encountered iron(IV)-heme species], with the 3d(x2-y2) atomic orbital of iron. In coordinate System 2, the relative energy levels of the d(x2-y2) and d(xy) atomic orbitals [and the corresponding delta(d(x2-y2)) and delta(d(xy)) MO's of heme] are reversed, in comparison to the situation in Coordinate System 1. Another potential source of confusion is that the delta(3d(x2-y2)) MO of heme is sometimes referred to, in diagrams, as d(x2-y2) (as if it's an atomic orbital of iron), and the pi*(xz) and pi*(yz) MO's of heme are often depicted as being "d(xz)" and "d(yz)" (parentheses mean subscripts in the blogger software that I'm using to type this) orbitals (as if they're atomic orbitals of iron). The pi(xz) and pi(yz) bonding MO's are not-infrequently referred to as being "pi orbitals" or the like [see Lehnert et al., 2001, p. 8289, Scheme 4: (http://www.ncbi.nlm.nih.gov/pubmed/11516278)]. It's always clear that these are MO's, but it's potentially a source of confusion, in my opinion. I have an article in which the authors specifically address this issue of the two different coordinate systems, but I can't find it on my computer. It's potentially a serious source of confusion, partly because there is sometimes no mention of the assignment, of either one or the other coordinate system, that the authors of various articles might be making, in their computer modeling of electron distributions in MO's ("probability density functions," as in "density functional theory," or DFT) or the like.





These are approximations for the "lone-pair" MO (sigma(d(z2)), etc.) of oxygen, a MO of heme that is likely to exhibit minimal 3d(z2) character, and the frequently-unoccupied sigma*(d(z2)) (antibonding) MO of heme that becomes occupied during charge-transfer states (ferryl-to-ring and thiolate ligand-to-ring and substrate-to-oxo transitions during high-energy, intermediate electronic states, such as in the electronic states of transition states of overall Cytochrome P450-dependent reactions, or during the multitude of "configuration interaction orbitals" that form during the absorption of visible or UV wavelengths by heme, etc.):

"Sub-states" of Singlet Delta Dioxygen; Changes in the Occupancies and Symmetries of the Orbitals of Singlet States of Dioxygen During Reactions

This article [Kearns, 1969: (http://pubs.acs.org/doi/abs/10.1021/ja01052a003)] helps me understand the difference between the two "sub-states" of singlet delta dioxygen [as discussed here: (http://hardcorephysiologyfun.blogspot.com/2010/05/three-major-electronic-states-of.html)]. This article (along with other articles by Yamaguchi and colleagues) [Yamaguchi et al., 2009: (http://linkinghub.elsevier.com/retrieve/pii/S0277538709000515)] shows the singlet delta state as comprising two "sub-states," also, except Yamaguchi et al. (2009) discuss the 2pi* diradical states of dioxygen (and Fe(IV)=O hemes) in terms of broken symmetry molecular orbitals. I guess that those are eigenfunctions, in which the 2pi* molecular orbitals are either not equivalent at all times or cease to be equivalent, in terms of their spin-density distributions (orbital "shapes"), as the charge-transfer complexes or transition states begin to form, during reactions of dioxygen with other molecules. Another point that Yamaguchi et al. (2009) make is that, as the O2 3sigma(g) molecular orbital (the "single, sigma" bond, formed by constructive overlap of two 2pz atomic orbitals) [Ochiai, 1996: (http://pubs.acs.org/doi/abs/10.1021/ed073p130)] begins to break, that 3sigma(g) molecular orbital begins to exhibit diradical character, albeit to a lesser extent than the true diradical state that the triplet ground state of dioxygen exhibits. Yamaguchi et al. (2009) (p. 2047, Fig. 5A) depicted the 3sigma(u) type (neither of the orbitals, shown as the bracketed pair at the bottom right of Fig. 5A, is symmetric with respect to the center of mass of dioxygen) of "broken-symmetry eigenfunction" molecular orbitals, formed by the configuration of the 3sigma(g) molecular orbitals (configuration interaction means like rehybridization of existing molecular orbitals to produce a new pair of molecular orbitals, with more complex patterns of "mixing" of the original molecular orbitals). I've shown 2pi*(u) "broken-symmetry orbitals" and not the 3sigma(u) type of orbitals shown in Fig. 5A [Yamaguchi et al. (2009) also depicted the pair of 4sigma(u) broken-symmetry molecular orbitals, which are unoccupied (LUMOs), at the top right of Fig. 5A, in brackets], but the concept is similar (for the types of broken-symmetry orbitals I've drawn, see Yamaguchi et al., 1983, p. 104, Fig. 1B: (http://linkinghub.elsevier.com/retrieve/pii/016612808385012X)].


This shows the way in which the lower-energy sub-state (just singlet delta) of singlet delta dioxygen can be distinguished from the higher-energy, singlet delta* sub-state of dioxygen (see Kearns, 1969, p. 6556, column 1).



In contrast to the "situations" shown above, the 2pi* orbitals in the higher-energy, singlet delta* sub-state of dioxygen, shown below, remain singly-occupied by electrons of opposite "spins" (see Kearns, 1969, p. 6556, column 1), and the net spin multiplicity for the overall dioxygen molecule, in each of the two singlet delta sub-states and in the singlet sigma state, is 1 (a singlet state, in terms of the spin multiplicity). This is because the net "spin" is zero [S = 0, and the spin multiplicity is abs.val.(2S) + 1, or 1] if the two electrons of opposite spin are in a single orbital (as in the "molecule-interacting" situations for singlet delta, shown above, or for the singlet sigma state, shown in the last diagram of this posting) or in two singly-occupied orbitals that exhibit singlet coupling (as in the "non-interacting" situation of singlet delta and singlet delta* and in the "molecule-interacting" situation for singlet delta*, as shown below):




This diagram shows the changes that occur in the relative energies of the two sub-states of singlet delta dioxygen (they're degenerate before dioxygen approaches the alkene), upon the approach of dioxygen, in either of the two sub-states, to an alkene. The diagram also shows the change that occurs, in response to the approach of dioxygen to the alkene, in the relative energies of the 2pi* orbitals, such that the 2pi* orbitals cease to be degenerate (at the same energy level):


This diagram shows the orbital occupancies for the singlet sigma state, in which the higher-energy 2pi* molecular orbital is doubly-occupied (this state, overall, is also higher in energy than the two singlet delta sub-states are and is the highest of the commonly-encountered electronic states of dioxygen):

Note on Lysine pKa's and Potential Crosslinking Mechanisms, With Reference to B6 Neurotoxicity

I was going to note that a "high pKa" lysine residue would be more likely to be deprotonated at physiological pH than a low-pKa one, and I wanted to correct that, in reference to the posting I made yesterday on the potential for pyridoxal-derived Schiff bases to participate in the formation of crosslinks within or between proteins. I did most of this stuff in April, and I was also thinking of these other articles that describe the "pH gating" of lysine residues near the active sites of proteins. In the pH gating phenomenon, the pKa's of some lysine residues can shift in response to changes in the conformations of proteins or the like. I should say that the issue of lysine residues as being mediators of electrostatic protein-protein interactions, such as in histone proteins, has been a persistent point of confusion and complexity in the literature (http://scholar.google.com/scholar?q=histone+lysine+cationic&hl=en&btnG=Search&as_sdt=100000001) and in some textbooks, also. It's very complex and becomes confusing when one attempts to make generalizations. I remember becoming confused in looking at the notes in one of my classes, in which there was an attempt to make a "bottom-line" generalization about the interaction of "negatively-charged phosphate backbones" of DNA with cationic lysine residues of histone proteins, and there was some serious problem with the generalization. I'm not sure that it's even the case that the nucleophilicity of lysine residues increases in some kind of linear manner, with increasing pKa values. Also, in the reversible inhibition of some RNA polymerase enzymes by pyridoxal 5'-phosphate (PLP), for example, low-pKa lysine residues are the ones that form Schiff bases with PLP [Bull et al., 1975: (http://www.ncbi.nlm.nih.gov/pubmed/237508)] and thereby mediate the inhibition of the enzymes. It's worth noting that the formation of pyridoxal-containing Schiff bases does not necessarily guarantee that the pyridoxal moiety will be irreversibly bound to the protein, but the formation of pyridoxal-containing Schiff bases is apparently kinetically-favorable. Anyway, here's an example of the type of crosslink, resulting from a nucleophilic attack, by a nucleophilic serine residue, on the carbon of a pyridoxal-containing Schiff base [see Weng and Leussing, 1983: (http://pubs.acs.org/doi/abs/10.1021/ja00350a056); Hedstrom, 2002: (http://pubs.acs.org/doi/abs/10.1021/cr000033x)]. I'm not sure if this could be called a transimination reaction. I think it's only a transimination if an amine nitrogen attacks the Schiff-base carbon.

Potential for Undesirable or Untoward Consequences of Significant Vitamin E Supplementation: Significance of Antiandrogenic Effects

This article [Hartman et al., 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11170129)] shows that supplementation with only 50 mg of d,l-alpha-tocopheryl acetate [equivalent to 34 mg of "d-alpha-tocopherol" or 34 "tocopherol equivalents" or 50 IU of "vitamin E" (http://www.ajcn.org/cgi/reprint/48/3/612.pdf)] (I don't understand the rationale for using the IU units, and the whole system of expressing things in IU should be abandoned immediately. I honestly still don't understand the conversions and don't intend to learn about them in any more depth.) The main point is that all-racemic (a 50-50 mixture of R and S chirality for each of the three chiral carbons on the side chain) alpha-tocopheryl acetate is roughly 50 percent as potent, in terms of changes in serum vitamin E over time (not bioavailability, given that I'm talking about steady-state serum tocopherol concentrations) and affinity for alpha-tocopherol transfer protein, as either d-alpha-tocopherol acetate or unesterified, d-alpha-tocopherol is. So 50 IU of "vitamin E" decreased serum testosterone and androstenedione levels in humans, and the authors cited all sorts of other research that had shown that vitamin E supplementation had produced either antiandrogenic effects or decreases in serum thyroid hormone levels [Tsai et al., 1978: (http://www.ncbi.nlm.nih.gov/pubmed/347918)(http://www.ajcn.org/cgi/reprint/31/5/831.pdf)], etc. Also, Hartman et al. (1999) [Hartman et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10624701)] found that serum tocopherol concentrations were inversely associated with serum testosterone and serum androstenedione concentrations in older men, and that suggests that higher doses of vitamin E [higher than the miniscule dose of 50 IU that actually decreased serum testosterone in a statistically significant way (Hartman et al., 2001)] could produce even greater decreases in serum androgens (mainly androstenedione, dihydrotestosterone, and testosterone). Vitamin E, especially in the form of vitamin E succinate, also can inhibit androgen receptor expression or inhibit the transcriptional response to androgen receptor activation by androgens, in epithelial cells of the prostate [Zhang et al., 2002: (http://128.151.10.65/george-whipple-lab/documents/yeh-papers/CVY63.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12032296); Huang et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/19420015)] and probably in other cell types also, and there's a lot of excitement about the potential usefulness of vitamin E, as an antiandrogenic compound, in the treatment of or prevention of prostate cancer, etc. (http://scholar.google.com/scholar?q=%22vitamin+E%22+androgen&hl=en&btnG=Search&as_sdt=100000001). This antiandrogenic effect of vitamin E supplementation, in concert with its capacity to competitively inhibit the binding of ubiquinone to succinate dehydrogenase and other respiratory chain enzymes and thereby produce either a pro-oxidant and pro-apoptotic effect [Dong et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/18372923)(http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2668987/pdf/nihms100471.pdf)] or, via its conversion into tocopheryl quinone, an "antioxidant" effect (that's not much of an antioxidant mechanism, to inhibit complex I and II activity), would potentially be a problematic effect in many cases, in my opinion, such as in the absence of legitimate evidence of prostate cancer or benign prostatic hypertrophy/hyperplasia. It may well be that some derivative of vitamin E succinate will turn out to be less problematic, in terms of antiandrogenic side effects, than other approaches to the prevention of prostate cancer or the recurrence of prostate cancer, and so I can't completely dismiss the usefulnesses of some of these things. I generally think that the research on vitamin E supplementation shows beneficial effects in the short term, in a lot of contexts, but doesn't really look at the long-term effects. In my experience, vitamin E supplementation has seemed to be beneficial in the short term, in some contexts [such as in some sort of viral illness or the like (although I wouldn't suggest that any very specific benefits occurred)], but not really very much in the long term. For example, there's research showing that vitamin E can decrease microglial activation and produce other anti-inflammatory effects, but how much research looks at all the complexity of the long-term effects of vitamin E compounds? I've decreased to about 10-20 IU or something of supplemental vitamin E, and I obviously can't make any sort of recommendation to anyone. But when an article shows that severe vitamin E deficiency or mutations in alpha-tocopherol transfer protein can produce skeletal muscle myopathy and cerebellar ataxia, as many articles have shown [(http://scholar.google.com/scholar?hl=en&q=tocopherol+ataxia&btnG=Search&as_sdt=100000001&as_ylo=&as_vis=0); (http://scholar.google.com/scholar?hl=en&q=tocopherol+myopathy&btnG=Search&as_sdt=100000001&as_ylo=&as_vis=0)], that doesn't mean that one needs to take 400 or 800 IU or even 200 IU to prevent myopathy or ataxia from occurring. The RDA of 200 IU or whatever it is sounds totally arbitrary to me, and I'm honestly not convinced that vitamin E is necessarily essential. There's more evidence that it's essential than there is evidence that vitamin K1 and compounds within the vitamin K2 series are essential, but the fact that loss-of-function mutations in alpha-tocopherol transfer protein (a-TTP) produce ataxia doesn't mean that vitamin E's antioxidant effects prevent ataxia. It might mean that a-TTP exerts all sorts of functions that don't have anything to do with vitamin E transport. Just because an enzyme exists that is called "vitamin K 2,3-epoxide reductase" doesn't mean that vitamin K is essential or that there aren't many other functions and substrates of the enzyme or multienzyme complex. (Incidentally, I have an article in which the authors say that the vitamin K requirements of either mice or rats or both are 100 times those of humans. I can't find it at the moment, but that should raise red flags that something is wrong with the notion of the essentiality of vitamin K, in my view. Why would that be the case? It lends credence to the notion that it's not essential, in my view, and I'm not sure I know how to say why that's the case.)

The mechanisms by which vitamin E may decrease serum androgens are potentially worrisome to me, and androgen deprivation per se has the potential, in my opinion, in view of the research, to worsen the progression of cardiovascular disease and liver disease [Mu et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/19453544)] and to increase the risk of type II diabetes [Saad et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/20126841)(http://www.abem-sbem.org.br/public/uploads/02_reviso_ABEM_538.pdf)]. I think the adverse effects of androgen deprivation would be just as detrimental to women as to men, but that's just my opinion. In addition to the fact that both women and men "need" testosterone, one might expect androgen deprivation to indirectly augment the potentially-adverse effects of estrogen replacement therapy on smooth-muscle-cell proliferation or autoimmune disease, for example, thereby lessening the therapeutic effects, etc. That said, some of the research on the sexually-dimorphic aspects of androgen metabolism in the liver sheds light on the ways in which the antiandrogenic effects of vitamin E could, in my opinion, potentially become problematic in some cases. And much of the research on the adverse effects of androgen deprivation has been done only in male humans or animals, for some reason. I should add that I think that the research that supposedly shows sexually-dimorphic aspects of androgen deprivation is not actually showing that, but the effects on men are more readily apparent, from a statistical standpoint, for example, because of the higher levels of serum and intracellular (hepatic) androgens in men. Traish et al. (2007) [Traish et al., 2007: (http://www.antonioaversa.net/public/PDF/9.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/17727347)] made this observation in relation to research that had supposedly shown no correlation between plasma testosterone levels and the effects that testosterone clearly elicits in both men and women [see discussion of references 62 and 63 on p. 1226 of Traish et al. (2007)]. Traish et al. (2007) noted that the lower plasma levels of testosterone in women may decrease the robustness of or even serve to abolish statistical associations between plasma testosterone levels and testosterone-mediated effects in women, but the authors argued that investigations into the correlations between the plasma concentrations of dehydroepiandrosterone sulfate (DHEA-S), which is converted into androgens much more readily in women than in men, and androgen-mediated effects, in women, were more likely to be fruitful than attempts to show correlations between plasma androgens and androgen-mediated effects in women (Traish et al., 2007). I should mention that I don't think the use of exogenous DHEA is likely to be advisable, except under a doctor's supervision, and DHEA can produce strange effects on lipid and energy metabolism, etc. Traish et al. (2007) noted the tendencies toward "therapeutic nihilism" in relation to androgen metabolism in women, and, in general, a lot of the research on androgens in women seems strange to me. In this article, for example, [Andersson et al., 1994: (http://www.ncbi.nlm.nih.gov/pubmed/8062607)], the authors reported that the degrees of adiposity and insulin resistance correlated positively with plasma testosterone in women but negatively with plasma testosterone in men. But increases in adiposity are known to increase aromatase (CYP19, a cytochrome P450 enzyme that converts androgens into estrogens) expression in adipocytes and, presumably, also in all sorts of different cells in the brain and throughout the body. It seems likely that the elevations in androgens are not causing obesity or insulin resistance in women but are part and parcel of pre-obesity-or-obesity-associated endocrinological derangements, and obesity has generally been associated, as one would expect, with increases in estrogen-mediated effects or even disease processes, such as the development of breast cancer in women [Lorincz and Sukumar, 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16728564)]. It seems that inter-individual differences in the transcriptional responsiveness to androgens and perhaps the local conversion of estrogens into androgens might be more important aspects or mechanisms of androgen metabolism, in women, at least, and probably also in men, than changes in plasma testosterone, or other androgens, alone would be.

Anyway, in relation to the antiandrogenic effects of vitamin E, vitamin E compounds are likely to produce decreases in serum androgens, in part, in my opinion, by serving as pregnane X receptor (PXR) agonists and thereby inducing CYP3A4 (the cytochrome P450 3A4 enzyme isoform) and CYP3A5 expression in the liver [Landes et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/12504802)], and, in relation to the whole issue of gender and androgen metabolism, the protein content of CYP3A4 in the liver is twice as high in women as in men [Wolbold et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/14512885)]. But that doesn't mean that drug-or-nutrient-induced increases in CYP3A4 are less likely to be problematic in women than in men, in my opinion, to the extent that those increases may be problematic. CYP3A4 inducing drugs have been implicated in drug-induced antiandrogenic effects, and, for example, the induction of fatty liver disease in response to leuprorelin, a luteinizing-hormone releasing hormone receptor antagonist, was attributed to the antiandrogenic effects of leuprorelin [Gabbi et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/18097299)]. Androgen deprivation has also been implicated as a cause of depression, and the authors of this article discuss evidence that the incidence of depression has been found to inversely associated with serum testosterone [Saini et al., 2008: (http://www.endo.gr/cgi/reprint/358/17/1868.pdf)]. Anyway, I'll put the information on the induction of skeletal muscle myopathy, in response to high doses of vitamin E (as a competitive inhibitor of ubiquinone binding to mitochondrial respiratory chain enzymes), in another posting. Vitamin E also can induce CYP2B1, I think, and that hydroxylates androgens at a different position on the steroid ring than CYP3A4 does.

Potential for the Crosslinking and Aggregation of Proteins through Lysyl Adducts of Pyridoxal: Relevance to B6 Neurotoxicity

That diagram above (it also applies to pyridoxal and pyridoxamine) will help with the references to the sites of the reactions, as discussed in the posting.

I'm not finished with this yet, but the thought I'm trying to convey is that free pyridoxal, pyridoxal 5'-phosphate, or pyridoxamine could, potentially, mediate the crosslinking of proteins by their extremely reactive properties, and these crosslinks, initiated by the nucleophilic attack of nucleophilic amino acid residues [lysine is the most nucleophilic, in general, and cysteine and histidine residues can also act as nucleophiles in the formation of protein adducts of small-molecule drugs or other compounds, such as pyridoxal (the aldehyde form of vitamin B6), etc.] on the carbonyl carbon of pyridoxal or by the nucleophilic attack of the primary amine moiety of pyridoxamine on electrophilic carbonyl-containing compounds. Pyridoxal can basically form Schiff base adducts and Amadori adducts with any lysine residue (especially one that has a sufficiently-low pKa, thereby increasing the likelihood that the epsilon (primary) amine of lysine will be deprotonated within the physiological pH range) (http://scholar.google.com/scholar?hl=en&q=%22low+pKa%22+nucleophilic+lysine+residue&btnG=Search&as_sdt=100000000&as_ylo=&as_vis=1), and some of the general concepts (and some specifics) about the reactivity of pyridoxine-derived compounds are discussed in this article [Wondrak, 2008: (http://www.ncbi.nlm.nih.gov/pubmed/18473939)]. In my view, however, the pyridine ring of pyridoxamine is also so reactive as to potentially complicate the role of pyridoxamine as a "carbonyl scavenger." Although there's lots of research on the potential therapeutic effects of pyridoxamine in the prevention of advanced glycation end (AGE) product formation [I've shown the formation of an N-glutathionyl conjugate of the acyclic, aldehyde form of beta-D-glucose, followed by the formation of a protein AGE (via the nucleophilic attack by the thiol group of the cysteine moiety of the glutathionyl conjugate of glucose on one of the sulfur atoms of a protein disulfide linkage; I didn't show the rest of it, but the idea comes through--that particular conjugate has been shown to form, and I'll cite the article later], the capacity of pyridoxamine to form substituted pyrrole derivatives [I've shown a mechanism by which the pyridoxamine conjugate of levuglandin E2 (a 1,4-dicarbonyl derivative of PGE2 that is formed in vivo and discussed in the article by Wondrak (2008))] could potentially be problematic, in my view.

A major reason I'm thinking that the formations of these types of conjugates of B6-derived compounds could contribute to B6-induced neuropathy/neurotoxicity is that a major underlying mechanism of small-molecule-mediated neurotoxicity, such as can be induced by exposure to compounds such as acrylamide, 1,4-dimethylketones (i.e. 2,5-hexanedione), and carbon disulfide, is the formation of protein lysyl adducts and, subsequently, protein crosslinks that disturb axonal transport [Zhu et al., 1994: (http://www.ncbi.nlm.nih.gov/pubmed/7981420); LoPachin and Decaprio, 2005: (http://www.ncbi.nlm.nih.gov/pubmed/15901921)]. There's at least one article showing that pyridoxal phosphate induced the aggregation of proteins in the lenses (of the eyes) by forming lysyl adducts on the proteins, and glucose partially inhibited the pyridoxal-mediated increases in protein aggregation [Ganea and Harding, 1996: (http://www.ncbi.nlm.nih.gov/pubmed/8727969)]. The authors cautioned against the use of pyridoxal as an inhibitor of glycation, etc. It's important to note that pyridoxamine acts as a nucleophile, and pyridoxal acts as an electrophile. But I think some pyridoxamine can be formed from pyridoxal in vivo, even though most of the conversion is of pyridoxamine to pyridoxal. Anyway, another major point is that a big mechanism underlying the neurotoxicity of 1,4-dicarbonyl compounds (i.e. 2,5-hexanedione) is the formation of substituted pyrrole compounds, after the initial, nucleophilic attack by lysine, by a transimination reaction (I've shown this, in the diagrams, in the formation of a cyclic pyridoxal adduct of spermidine, formed via a transimination reaction, and in the formation of the pyrrole derivative of pyridoxamine), followed by the oxidation of pyrrole by triplet dioxygen and, along with the presence of a protein-bound pyrrole moiety of heme, for example, the formation of a bipyrrole crosslink.

Anyway, I stopped taking supplemental vitamin B6, but one should talk to one's doctor about these things. I wonder if the reason glutamate has been shown to ameliorate B6 neuropathy might be that B6 forms adducts with glutamate and other amino acids (glutamate in particular), such as glycine. If B6 caused glutamate depletion or something, maybe that could account for the effect of exogenous glutamate. And maybe the inverse relationship between liver glycine levels and liver PLP levels is due to the formation of glycine adducts of pyridoxal. I don't have time to cite those articles, but another possibility is that the inactivation of proteins (I've shown a possible mechanism by which the PLP-induced inactivation of succinate dehydrogenase, via the formation of lysyl adducts (this has been shown to occur, as cited below), could occur) could account for the strange effects of B6 on alkaline phosphatase expression, and I've talked about the way those effects don't really have any metabolic explanation. Anyway, it's interesting to think about. Also, the Schiff base formation and Amadori rearrangement mechanisms commonly occur when the carbonyl carbons of carboxylate groups on glucuronide conjugates of drugs or drug metabolites undergo nucleophilic attack by lysine (or other) residues to form drug-protein conjugates ("acyl glucuronide" mediated idiosyncratic adverse drug reactions, etc., with immunological sequelae).

I wonder if deferoxamine (DFO) might exert some of its therapeutic effects in diabetic neuropathy (or in the prevention of transglutaminase-mediated crosslinking of proteins) by serving as a carbonyl scavenger and not just by indirectly, via its iron chelation, limiting reactive-oxygen-species-mediated carbonyl stress (http://scholar.google.com/scholar?hl=en&q=carbonyl+cross-link+deferoxamine+OR+desferrioxamine&btnG=Search&as_sdt=100000000&as_ylo=&as_vis=0). Lysine is used to form the primary, terminal amine group of desferrioxamine B (a.k.a. deferoxamine), and lysine is the most nucleophilic of the amino acids. If DFO serves as a carbonyl scavenger, the N-(carbonyl-carbon-of-reactive-carbonyl-compounds)-DFO adducts might be substantially less likely than something like pyridoxamine to undergo "post-Amadori" oxidation and to form protein adducts. Also, the lysosomal localization of DFO might mean that the carbonyl scavenging by deferoxamine would be compartment-specific or whatever--"site-restricted" (such as to mitochondria, during lysosomal fusion events). Anyway, it's interesting to think about.

This shows the formation of a protein lysyl adduct of acrylamide:


This shows the formation of a glutathionyl adduct of glucose, following the hydrolysis of the cyclic lactone form of glucose that predominates in vivo, the Schiff-base-formation mechanism that applies to all or most of the reactions involving nucleophilic attack on the carbonyl carbon of pyridoxal (the catalytic mechanism of many or all PLP-dependent enzymes involves a transimination reaction, and protein-bound PLP exists as a Schiff base conjugate), the Amadori rearrangement that produces a stable, Amadori product (I guess this is an AGE also), and the nucleophilic attack by the thiol group of the glucose-GSH adduct on a disulfide linkage (the first step in the formation of a protein (a protein AGE) adduct of the glucose-GSH adduct):


Nucleophilic attack by primary amine group of glutathione (GSH) on the carbonyl carbon of the aldehyde group of acyclic D-glucose:

Formation of unstable Schiff base:

Amadori rearrangement of Schiff base:

Formation of Amadori product:

Attack by thiol group of glutathionyl-glucose adduct on disulfide linkage, to form protein adduct of glutathionyl-glucose (N-deoxyglucosyl-glutathione, I think, is the type of nomenclature used in the article):

This shows the formation of a substituted pyrrole-containing compound, via the nucleophilic attack of the primary amine group of pyridoxamine on levuglandin E2, I think it's called:


I think that the dehydration of this "pyrroline" (?) compound, to form the pyrrole ring, may involve carbocation intermediates, but I didn't show that:


This is the slow step by which triplet dioxygen forms a charge-transfer complex, in which an electronic transition from a HOMO of pyrrole to the unoccupied 4sigma(g) molecular orbital of dioxygen (the LUMO of dioxygen) occurs, followed by the transfer of that electron into one of the pi*x or pi*y antibonding molecular orbitals (these are each singly-occupied in triplet dioxygen, as shown in the diradical species I've drawn in the diagram) to form the superoxide radical anion, in a doublet state. There is actually an ion pair in the charge-transfer complex, with the anion being delocalized across dioxygen and the pyrrole nitrogen existing as a formal or actual cation, and the existence of this type of ion pair, specifically in the oxidation of pyrrole compounds, has been shown experimentally:





Formation of cyclic, pyridoxal adduct of spermidine via a transimination reaction of the Schiff-base "intermediate" (a cyclization reaction that precludes the formation of a more stable, Amadori product from the Schiff base):




This shows a nucleophilic attack by glutathione on the carbonyl carbon of pyridoxal:

This shows a possible mechanistic explanation for the apparent noncompetitive, mechanism-based inactivation of succinate dehydrogenase (SDH) by pyridoxal (actually, pyridoxal 5'-phosphate (PLP)), followed by the dissociation of succinate dehydrogenase from complex II, in the mitochondrial matrix [Choudhry et al., 1985: (http://www.ncbi.nlm.nih.gov/pubmed/3972121)]. This PLP-induced inhibition of SDH has been shown to occur and to, either definitely or almost definitely, depend on the formation of one or more protein lysyl adducts of PLP within or near to the ubiquinone binding site (Choudhry et al., 1985, p. 172). CoQ2 (this is a ubiquinone analog that is just, I think, ubiquinone with 2 isoprene units instead of 10) could block the PLP-induced inactivation, and the authors found that the formation of protein adducts of PLP did not occur at the active site of the enzyme (the dicarboxylate binding site). Apparently lysine residues are required for the association of SDH with the rest of the complex II multienzyme complex, and the PLP-induced formation of Schiff-base or Amadori lysyl adducts apparently prevented the normal electrostatic interactions that are required to maintain the binding of SDH to complex II (shown in my sketches as a single bond, for the sake of brevity/simplicity). So it sounds like PLP could form adducts at the ubiquinone binding site and also at the lysine-rich "domains" or portions of SDH that mediate its association with complex II (or the ubiquinone binding site is near to or a part of those complex II-binding residues). PLP and pyridoxal and pyridoxine (and pyridoxamine, presumably, such as after it forms adducts) are known to react with singlet oxygen at C6 of the pyridine ring [Ohta and Foote, 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12371824)], and I've shown a slightly-abbreviated "mechanism" for the so-called "-ene" reaction of singlet oxygen with pyridoxal to form a C6 hydroperoxide derivative of pyridoxal. The mechanism of the -ene reaction is not the same as the singlet oxygen cycloaddition mechanism, and the -ene reaction may involve a perepoxide transition state or a so-called exciplex transition state. I think that the electron transfer is from a highest-occupied molecular orbital (HOMO) of pyridoxal to the lowest-unoccupied molecular orbital (LUMO) of singlet dioxygen (singlet delta dioxygen), and this LUMO is the 4sigma(g) orbital (the 3pz sigma antibonding molecular orbital) of singlet dioxygen. Then there's one or more transitions of electron(s) from the LUMO(s) of the transition-state molecular orbitals of the pyridoxal hydroperoxide adduct to a HOMO of the pyridoxal hydroperoxide adduct, etc., as far as I know. I meant to show a perepoxide transition state, but that particular chemistry drawing program didn't easily allow me to show what was going on by drawing a singly-bonded dioxygen species (that would potentially be confused with triplet dioxygen, which is often drawn that way) and by drawing the zwitterionic perepoxide in the transition state). So I show something closer to an "exciplex," I guess. The hydroperoxide then forms a cyclic endoperoxide that undergoes dehydration, yielding a quinone-like coenzyme Q10 analog. The mechanism shown by Ohta and Foote (2002), in contrast, showed the formation of the final quinone-like compound via the nucleophilic attack of methanol on the quaternary C2 of the pyridine ring of pyridoxal. Incidentally, I don't think coenzyme Q10 supplementation is likely to be a good idea, in general (and I say this emphatically), but one would obviously want to discuss this type of issue with one's doctor.












This shows 1) the formation of a monoalky dithiocarbamate protein lysyl adduct (Protein-Lys-NH-CS2, a lysyl dithiocarbamate adduct, as an example of an N-alkyl dithiocarbamate), following the nucleophilic attack by a lysine residue on carbon disulfide, 2) the oxidation of two lysyl dithiocarbamate residues to form a dialkyl dithiocarbamate disulfide linkage between two proteins or two lysyl residues of the same protein or polypeptide, and 3) a redox disulfide cleavage reaction in which the N-alkyl dithiocarbamate moieties of the oxidation of the dialkyl dithiocarbamate disulfide linkage are oxidized, indirectly, via the oxidation of the disulfide sulfur atoms by singlet delta dioxygen. So singlet delta dioxygen oxidizes the sulfur atoms of the disulfide moiety, and then the oxidized sulfur atoms serve as oxidants of the lysyl dithiocarbamate moieties to lysyl isothiocyanate residues and, in the process, undergo reduction in the "reductive cleavage" or "reduction" of the disulfide linkage, overall, I guess you could say. The cleavage of a disulfide linkage is usually discussed as being a "reduction" of the oxidized thiol (-SH) groups, and I'm trying to address that in terms of the reactions in question. Then the isothiocyanate residues can form more stable dialkyl di-isothiocyanate disulfide (I think that's the terminology) crosslinks, and I didn't get around to showing that part yet. The point is that these crosslinks begin with the nucleophilic attack by a lysine residue onto carbon disulfide and are mechanistically reminiscent of the attack by lysine on the carbonyl carbon of pyridoxal, thereby potentially forming pyridoxal-mediated crosslinks, either by transimination, prior to the formation of an Amadori product of the pyridoxal adduct of a lysine residue, or by the hydroxyl radical-mediated oxidation of the methylene carbon (C9) of the hydroxymethyl substituent of the pyridine ring of the protein-lysyl pyridoxyl adduct (pyridoxine has two hydroxymethyl substituents, but there's only one on an adduct of pyridoxal that's formed as a result of nucleophilic attack by lysine at C8) (followed by the formation of a protein crosslink at that allylic [quasi-benzylic) methylene carbon] or by the oxidation by singlet oxygen of the C6 (or C2, I think) carbon of the pyridine ring, followed by the formation of a quinone-like compound (shown above) or by the formation of a protein crosslink at the C9 site, etc. There are all sorts of other reactions that can occur with the lysyl dithiocarbamate, dilysyl dithiocarbamate, or lysyl isothiocyanate residues, such as the formation of hydrophobic thiophene derivatives, etc., that could become crosslinked to the pyrrole moieties of protein-bound heme species, etc.