Maillard reaction of ribose 5-phosphate generates superoxide and glycation products for bovine heart cytochrome c reduction

摘要

Ribose 5-phosphate (R5P) is a sugar known to undergo the Maillard reaction (glycation) at a rapid rate. In a reaction with the lysines of bovine heart cytochrome c, R5P generates superoxide (O2^-) that subsequently reduces ferri-cytochrome c to ferro-cytochrome c. The rate equation for the observed cytochrome c reduction is first order in respect to cytochrome c and half order in respect to R5P. The addition of amines to the cytochrome c-R5P system greatly increases the O2^- generation with rates of approximately 1.0 μM min^-1 being observed with millimolar levels of R5P and amine at 37 °C. Pre-incubation of R5P with the amine prior to cytochrome c addition further enhances the rate of cytochrome c reduction approximately 2-fold for every 30 minutes of incubation. While clearly accounting for a portion of the reduction of cytochrome c, O2^- is not the sole reductant of the system as the use of superoxide dismutase only partially limits cytochrome c reduction, and the contribution of O2^- proportionally decreases with longer amine-R5P incubation times. The remainder of the cytochrome c reduction is attributed to either the Amadori product or a cross-linked Schiff base created when a Maillard reaction-derived dicarbonyl compound(s) reacts with the amine. It is believed these compounds directly transfer electrons to ferri-cytochrome c and subsequently become stable free-radical cations. ATP, a putative regulator of cytochrome c activity, does not inhibit electron transport from O2^- or the cross-linked Schiff base but does prevent R5P from reacting with surface lysines to generate superoxide. The spontaneous reaction between R5P and amines could serve as an alternative system for generating O2^- in solution.