Polyol-pathway enzymes of human brain. Partial purification and properties of aldose reductase and hexonate dehydrogenase

Abstract

Aldose reductase and hexonate dehydrogenase were isolated from human brain and partially purified. The 2 enzymes exhibited distinctive substrate-specificity profiles with a variety of aldoses, and aliphatic and aromatic aldehydes. Aldose reductase exhibited a high affinity for DL-glyceraldehyde (Km of 62 .mu.M) and a low affinity (Km of 90 mM) for glucose, the physiological substrate of the polyol pathway. Hexonate dehydrogenase exhibited a relatively low affinity for D-glucuronate (Km of 4.6 mM) and a very low affinity for glucose (Km of 390 mM). Both enzymes exhibited a high specificity for NADPH and were inhibited competitively by NADPH+. Hexonate dehydrogenase was inhibited by iodoacetate, iodoacetamide, N-ethylmaleimide and p-chloromercuribenzoate. Preincubation with 2-mercaptoethanol resulted in activation. Both enzymes were inhibited by a number of barbiturates (barbital, phenobarbital and pentobarbital) and by the CNS drugs diphenylhydantoin and ethosuccinimide. The substrate specificity and patterns of inhibition suggest that the 2 enzymes isolated correspond to 2 of 4 previously reported aldehyde reductases isolated from human brain.