Summary. Branched-chain amino acid aminotransferase (l-leucine:2-oxoglutarate aminotransferase, EC 22.214.171.124) activity was determined in several tissues of the mouse. Testis homogenates presented a specific activity very close to that of heart extracts which were the most active. Enzyme activity was detectable in testes from 5-day-old mice and increased steadily during development to reach a maximum at the 20th day of life. The transaminase was present in the cytosol of testicular homogenates and also associated, probably in the matrix, with a special type of mitochondria present in spermatozoa and gametogenic cells. The enzyme from testis is active against the three branched-chain amino acids and catalyses the reaction in both directions. Highest activity and lowest Km were obtained with l-leucine. Activity with l-valine was the lowest. The enzyme from the mitochondrial fraction showed identical properties to that from the soluble phase. The possible participation of this aminotransferase in a shuttle system transferring reducing equivalents from cytoplasm to mitochondria is postulated.
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