The Mechanism of Action of Uridine Diphosphoglucose Dehydrogenase

Uridine diphospho-α-d-galacto-hexodialdose (UDP-Gal-6-CHO) and uridine diphospho-α-d-gluco-hexodialdose (UDP-Glc-6-CHO) were produced from UDP-galactose through the use of galactose oxidase and UDP-glucose 4-epimerase. UDP-Glc-6-CHO was shown to be an intermediate in the reaction catalyzed by the enzyme UDP-glucose dehydrogenase, indicating that the conversion of UDP-glucose to UDP-glucuronic acid (UDP-GlcUA) proceeds by two steps with UDP-Glc-6-CHO as an enzyme-bound intermediate. The first step, the formation of UDP-Glc-6-CHO from UDP-glucose, involves nicotinamide adenine dinucleotide as the oxidant, is reversible, and is highly specific for the substrate structure. UDP-Glc-6-CHO was reduced to UDP-glucose by the dehydrogenase and NADH, while UDP-Gal-6-CHO would not undergo an analogous reaction. The second step, formation of UDP-GlcUA from UDP-Glc-6-CHO, evidently accounts for the irreversibility of the over-all reaction and is less specific toward substrate structure. Both UDP-Glc-6-CHO and UDP-Gal-6-CHO will undergo this oxidation reaction with the enzyme and NAD+, although the latter reacts at about one-sixth of the rate of the former. The oxidation of UDP-Glc-6-CHO to UDP-GlcUA proceeds at exactly the same rate as does oxidation of UDP-glucose to UDP-GlcUA. The reduction of the UDP-hexodialdose with NaB3H4 constitutes a very economical synthesis of radioactive UDP-galactose.