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The distribution and some properties of branched chain acid aminotransferase(EC 2.6.1.42)in rabbit bone marrow have been studied. Rabbit bone marrow was fractionated by differential centrifugation into nuclear, mitochondrial and cytosolic fractions. The activity of branched chain amino acid aminotransferase was measured by the method of Ichihara and Koyama. Isozyme pattern of this enzyme was also examined by DEAE-cellulose column chromatography. The results abtained were as follows; 1. The activity in homogenate was found to be 259.47 unit/g of wet tissue. The activity of this enzyme was relatively low compared with that in rat tissues, i.e., liver, heart, intestine and pancreas. 2. The distribution of branched chain amino acid aminotransferase in the subcellular organelles showed that 89.6% of the activity was in cytosolic fraction, 9.0% in mitochondral fraction and 1.4% in nuclear fraction. 3. Cytosolic fraction of rabbit bone marrow contained EnzymeⅢ, but not EnzymeⅠand Ⅱ, of branched chain amino acid aminotransferase, EnzymeⅢ was elluted by 200mM phosphate buffer from DEAEcellulose column and catalyzed the transamination of all three branched chain amino acids. 4. EnzymeⅢ was purified about 61-folds increase in specific activity after chromatography on DEAEcellulose. 5. The best substrate among the activity of the EnzymeⅢ was about 45℃ and the optomal pH was 8.2. 7. The Km values for leucine, isoleucine and valine of EnzymeⅢ were 1.37mM, 1.43 mM and 2.11 mM. respectively. 8. The Km values for α-ketoglutarate and pyridoxal phosphate of EnzymeⅢ were 1.03 mM and 2.17×10^-3 mM, respectively. 9. The molecular weight of EnzymeⅢ was estimated to be about 50,000 by polyacrylamide disc gel electrophoresis.
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