Carbonyl reductase occurs in multiple molecular forms. Sequence analysis has yielded a carboxyethyllysine residue in one of the enzyme forms, suggesting that pyruvate has been incorporated in a posttranslational enzymatic reaction [Krook, M., Ghosh, D., Strömberg R., Carlquist, M. and Jörnvall, H. (1993) Proc. Natl. Acad. Sci. USA 90, 502-506]. Using highly purified carbonyl reductase from human brain we show that pyruvate and other 2-oxocarboxylic acids are bound to the enzyme in an autocatalytic reaction. The resulting enzyme forms were indistinguishable from the native enzyme forms by electrophoresis and isoelectric focusing.