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The activity and isozyme patterns of branched chain amino acid transaminase (EC 2.6. 1.6) in Sarcoma 180 ascites tumor cells which harvested at 3, 7 and 14 days after transplantation have been studied. The activity of branched chain amino acid transaminase was measured by the method of Ichihara and Koyama. Isozyme patterns of this enzyme was also examined by using DEAEcellulose column chromatography. The results obtained were as follows; 1. In the Sarcoma 180 ascites tumor cells, the transaminase activites for branched chain amino acids are relatively low compared with those in rat tissues, i.e., liver, heart, intestine and pancreas. 2. This enzyme was localized in both the cytosolic and mitochondrial fraction. About 78-84 per cent of the total activity was found in the cytosolic fraction, 15-25 per cent in the mitochondrial fraction. 3. The activity of this enzyme in Sarcoma 180 ascites tumor cells harvested at 7 days after transplantation was highest. 4. Cytosolic fraction of Sarcoma 180 ascites tumor cells contained isozyme Ⅰ and Ⅲ, but not isozyme Ⅱ, of branched chain amino acid transaminase. Enzyme Ⅰ was eluted by 0.02 M phosphate buffer from DEAEcellulose column and catalyzed the transamination of all three branched chain amino acids. Enzyme Ⅲ was eluted by 0.2 M phosphate buffer. 5. The specific activity of Enzyme Ⅲ was found 2.1 times higher than Enzme Ⅰ in Sarcoma 180 ascites tumor cells harvested at 7 days after transplantation and Enzyme Ⅰ and Ⅲ were contained 35 per cent and 65 per cent, respectively. 6. In Sarcoma 180 ascites tumor cells harvested at 14 days after transplantation, the specific activity of Enzyme Ⅲ was found 8.3 times higher than Enzyme Ⅰ and the distribution of Enzyme Ⅰ and Ⅲ were 40 per cent and 60 per cent, respectively.
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