Purification and catalytic properties of a novel acetohexamide-reducing enzyme from rabbit heart

An enzyme catalyzing the metabolic reduction of acetohexamide [4-acetyl-N-(cyclohexyl-carbamoyl)benzenesulfonamide], an oral antidiabetic drug, was purified to homogeneity from the cytosolic fraction of rabbit heart. The molecular mass of the purified enzyme was estimated to be 110 kDa by gel filtration and nondenaturing PAGE and 28 kDa by SDS-PAGE, suggesting that the enzyme is composed of four identical-size subunits. 4-Benzoyl-pyridine and p-nitroacetophenone, typical substrates of carbonyl reductase [EC 1.1.1.184], were not reduced by the enzyme. Of drugs with a ketone group tested, only acetohexamide was a good substrate of the enzyme. the enzyme effectively reduced analogs substituted with various alkyl groups instead of the cyclohexyl group in acetohexamide, although it had little or no ability to reduce analogs substituted with various alkyl groups instead of the methyl group in acetohexamide. The enzyme was inhibited not only by quercetin, a well-known inhibitor of carbonyl reductase, but also by phenobarbital, a potent inhibitor of aldehyde reductase [EC 1.1.1.2]. These results indicate that the enzyme purified from rabbit heart is a novel enzyme responsible for the reduction of acetohexamide and its analogs.