The reduction stereochemistry of the Schiff's base formed between pyruvate and the ϵ-amino of the catalytic lysine of 2-keto-3-deoxygluconate-6-P-phosphate aldolase of Pseudomonas putida was investigated. Reduction was stereoselective yielding 55.73% N6-[(1R)-and 44.27% N6-[(1S)-1-carboxyethyl]-S-lysine. Thus the reducing agent predominated slightly at the si face of the ketimine carbon. For comparison, the reduction stereochemistry of the pyruvate-lysine ketimine formed on d-amino acid oxidase during d-alanine turnover at pH 8.5 was also investigated. In this case reduction was random, consistent with nonactive site participation in that transimination reaction generating the ketimine, as postulated by other investigators of this enzyme. © 1980, All rights reserved.