Dicarbonyl/l-xylulose reductase (DCXR): The multifunctional pentosuria enzyme

Abstract

Dicarbonyls are formed during oxidation of glucose and unsaturated fatty acids. They form stable inter-and intramolecular crosslinks in protein, including collagen, and lead to formation of advanced glycation end products (AGEs) in tissue proteins. These processes are thought to contribute to the pathogencesis of vascular disease in diabetes and atherosclerosis. In this study, protective effects of some major flavonoids on dicarbonyl formation from glucose and polyunsaturated fatty acids such as arachidonic acid and linolenic acid were examined.When 100 mM glucose in PBS was incubated at 37℃, dicarbonyls were formed very slowly (0.7 μM/day). The reaction rate was so low that the inhibitory effects of flavonoids could not be correctly evaluated using the reaction system. By addition of 1 mM ascorbic acid to the reaction system, the dicarbonyl formation was accelerated about 80 times. Flavonoids inhibited the ascorbate-induced dicarbonyl formation from glucose to different degrees. The concentrations at which flavonoids inhibit the dicarbonyl formation by 50%(IC_(50)) were as follows: epigallocatechin gallate (EGCG), 1.0 μM; epicatechin gallate (ECG), 1.3 μM; myricetin, 1.4 μM; kaempferol, 7.2 μM; quercetin, 12 μM; luteolin, 75 μM;hesperidin, naringenin, apigenin, silimarin, genistein, epicatechin (EC) and epigallocatechin (EGC) > 200 μM.When fatty acid micelles were incubated in PBS at 37℃, dicarbonyls were formed to different degress, depending on the kinds of fatty acid: the more the fatty acids were undsaturated, the more dicarbonyls they formed, and only little amounts of dicarbonyls were formed from palmitic acid and oleic acid. Ascorbic acid accelerated the dicarbonyls formation from polyunsaturated fatty acids less remarkably than it did the dicarbonyls formation from glucose. Flavonoids inhibited the dicarbonyl formation from polyunsatirated fatty acids to different degrees. The IC_(50) values of flavonoids for the reaction system consisting of 2 mM arachidonic acid and 1 mM ascorbic were as follows: kaempferol, 9.0 μM; luteolin, 9.7 μM; myricetin, 11 μM; quercetin, 18 μM; EGCG, 26 μM; silymarin, 42 μM; ECG, 55 μM; apigenin, naringenin, hesperidin, genistein, EC and ECG > 200 μM.The above results suggest that EGCG and other flavonoids could efficiently inhibit the dicarbonyl formation from glucose and polyunsautrated fatty acids and that they could provide protedtion from atherosclerotic changes associated with diabetes and aging.