two kynurenine aminotransferases in human brain using trisacetate buffer rather than conventional phosphate buffer it was possible to detect two distinct proteins capable of producing the neuroinhibitory brain metabolite kynurenic acid kyna from lkynurenine in human brain tissue the two kynurenine aminotransferases kats arbitrarily termed kat i and kat ii could be physically separated by isoelectric focussing on a ph 310 ampholine gradient and more completely by differential elution from a deaesepharose column kat i showed a pronounced preference for pyruvate as a cofactor and had a ph optimum of 96 in contrast kat ii was virtually equally active when either pyruvate or 2oxoglutarate were used as the aminoacceptor and its ph optimum was 74 moreover kat i and kat ii differed with regard to their sensitivity to amino acids and as the aminoacceptor and its ph optimum was 74 moreover kat i and kat ii differed with regard to their sensitivity to amino acids and kinetic characteristics the existence of two separate enzymes capable of producing kyna in the human brain raises the question if and to what extent each of the enzymes regulates the cerebral synthesis of kyna and its possible role as a modulator of excitatory amino acid receptor function