PURIFICATION AND PROPERTIES OF ALDEHYDE REDUCTASES FROM YEASTS THAT CONVERT ETHYL 2-ACETAMIDO-3-OXOBUTYRATE TO OPTICALLY-ACTIVE ETHYL 2-ACETAMIDO-3-HYDROXYBUTYRATE

Yeasts were screened for strains that converted ethyl 2-acetamido-3-oxobutyrate (AAOB) to optically active ethyl 2-acetamido-3-hydroxybutyrate (AAHB). Sporoholomyces sp. AKU4430 was found to accumulate the D-threo isomer of AAHB by a whole-cell reaction. Candida albicans AKU4596 accumulated mainly the D-threo and L-threo isomers. The enzymes that reduced AAOB were purified from these two yeasts, and characterized. To judge from their substrate specificity, inhibition pattern, molecular structure, and reaction mechanisms, the enzymes were of the NADPH-dependent aldo-keto reductase family (probably aldehyde reductase, EC 1.1.1.2). The enzyme of Sporoholomyces sp. reduced AAOB more strongly than that of C. albicans. The stereoselectivity of the enzymes was low; three isomers of AAHB (L-threo, L-allo, and D-threo) were produced by each purified enzyme. The selective accumulation of an isomer(s) of AAHB by reaction in yeast cells probably occurred because of differences in isomer degradation.