Two major 2-aminoadipate aminotransferase (AadAT) activities of human liver extract were separated by DEAE-Sepharose column chromatography. The faster eluting enzyme was designated AadAT-I and the other one AadAT-II. AadAT-I had a hgih Km value for aminoadipate, 20 mmol/l, and a low Km value for glutamate, 1.4 mmol/l. In contrast, AadAT-II had a low Km value for aminoadipate, 0.25 mmol/l, and a high Km value for glutamate, 12.5 mmol/l. AadAT-I and AadAT-II were mainly localized in the supernatant and mitochondrial fraction, respectively. AadAT-I demonstrated only glutamate-2-oxoadipate or 2-aminoadipate-2-oxoglutarate aminotransferase activities. AadAT-II further showed the activity of tryptophan and kynurenine. On the basis of Km values and subcellular localization of the isoenzymes, a plausible role was suggested for them involving the metabolism of lysine and tryptophan.