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.
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