AROMATIC AMINOTRANSFERASE ACTIVITY OF RAT BRAIN CYTOPLASMIC AND MITOCHONDRIAL ASPARTATE AMINOTRANSFERASES

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Abstract
Abstract—Mitochondrial and cytoplasmic forms of aspartate aminotransferase were purified from rat brain homogenates and tested for their ability to catalyze transamination of various aromatic amino acids. The mitochondrial enzyme exhibited activity toward tyrosine and phenylalanine with 2‐oxoglutar‐ate as acceptor, although the specific activities were less than 1% of the corresponding aspartate activity when all substrates were 10 mM. Even less activity was seen with DOPA, 5‐hydroxytryptophan and tryptophan. The cytoplasmic aspartate aminotransferase was active toward tryptophan, 5‐hydroxytryptophan and DOPA, but these transaminations were favored by pyruvate or oxaloacetate rather than 2‐oxoglutarate as keto acid. Based on co‐migration of aromatic activities with the respective aspartate aminotransferases during isoelectric focusing and based on equal sensitivities of aromatic transamination and aspartate transamination to inhibition by vinylglycine, it was concluded that all activities resided in the aspartate aminotransferase enzymes. Some doubt exists, however, as to the physiological significance of these alternate activities in view of the requirement that aromatic amino acids must compete with aspartate for transamination by these enzymes.