| The Maillard or browning reaction between reducing sugars and proteins contributes to the browning, increased fluorescence and crosslinking of protein with age and proceeds an accelerated rate in diabetes. Autoxidation reactions, oxidation reactions caused by molecular oxygen, produce reactive sugar fragments which lead to the formation of Advanced Glycation End-products (AGEs) in tissue protein. To determine the products of carbohydrate autoxidation, I have studied the effects of oxidative and antioxidative conditions on the reactions of glucose and fructose in phosphate buffer at physiological pH and temperature. The autoxidation of glucose yields arabinose and glyoxal, while the autoxidation of fructose produces ribulose, tetros(ulos)es, glyceraldehyde and glyoxal. Autoxidation reactions are inhibited under oxidative conditions.Ribulose, tetros(ulos)es and glyceraldehyde are more reactive with protein than the products formed during glucose autoxidation, which explains why more protein browning and crosslinking are observed for fructose than for glucose. Most surprisingly, pentoses and smaller sugars do not require oxygen for the browning and crosslinking of protein. Since aminoguanidine inhibits the chemical modification of proteins by pentoses, I postulate that browning by pentoses also involve dicarbonyl intermediates. The Hodge pathway for the formation of AGEs involves the degradation of the Amadori adduct to dicarbonyl products without the requirement of oxygen. Thus, the Hodge pathway provides the most reasonable mechanism for the modification of proteins by pentoses while oxidation of hexoses is rate limiting.To determine the role of carbohydrate autoxidation in the Maillard reaction, RNase was incubated with glucose, fructose and arabinose under oxidative and antioxidative conditions. The rate of development of browning, fluorescence, pentosidine and CML was 3-fold greater for fructose than for glucose. Fructose also crosslinked protein more rapidly than glucose. The modification of protein by glucose and fructose was inhibited under oxidative conditions. Arabinose browned and crosslinked RNase more efficiently than glucose and fructose, and, in contrast to glucose and fructose, these modifications proceeded at similar rates under oxidative and antioxidative conditions. Aminoguanidine, a dicarbonyl scavenger, inhibited the browning and crosslinking of RNase in a dose-dependent manner, but the antioxidant carvedilol was not effective at preventing arabinose from modifying RNase. |
