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

Abstract

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 ( O 2 - ) 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 O 2 - 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 twofold for every 30 min of incubation. While clearly accounting for a portion of the reduction of cytochrome c, O 2 - is not the sole reductant of the system as the use of superoxide dismutase only partially limits cytochrome c reduction, and the contribution of O 2 - 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 that 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 O 2 - 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 O 2 - in solution.