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)].