Familial hyperlysinemias are autosomal recessive disorders in the oxidative degradation of lysine. Hyperlysinemia type I is associated with a combined deficiency in lysine-ketoglutarate reductase and saccharopine dehydrogenase activities, the first two sequential steps in the lysine degradative pathway. In familial hyperlysinemia type II, only saccharopine dehydrogenase activity is deficient. We report here that these reductase and dehydrogenase activities occur on a single protein based on the following findings. (i) The activity ratio of reductase/dehydrogenase remained constant (close to unity) throughout a 500-fold purification of both enzyme activities from mitochondrial extracts of baboon and bovine livers. The activity profiles of the reductase and the dehydrogenase superimpose on each other as the enzyme was eluted from DEAE-cellulose and Sephacryl S-300 columns. (ii) Activity-staining of the native polyacrylamide gel showed that both activities migrated the same distance toward the anode. (iii) The highly purified enzyme with the reductase and dehydrogenase activities showed a single polypeptide band of Mr = 115,000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The native enzyme from baboon and bovine livers has an apparent Mr of 468,000 (Stokes radius = 69.5 A) as determined by gel filtration, which suggests a tetrameric structure of identical subunits. The presence in mammalian tissues of a single protein catalyzing both the reductase and dehydrogenase reactions explains the combined enzyme deficiency observed in hyperlysinemia type I. We propose that the bifunctional enzyme be called aminoadipic semialdehyde synthase.