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AN EXAMINATION OF THE CATALYTIC PROPERTIES OF SEVERAL AMINOTRANSFERASES IN BRAIN AND LIVER BY MEANS OF AN IMPROVED RADIOMETRIC ASSAY WITH DINITROPHENYLHYDRAZINE

Abstract— The assay of aminotransferases, performed by solvent extraction of keto acids formed from labelled amino acids, has been modified to enhance the recovery of both aliphatic and aromatic keto acid products. The keto acids are first converted to their respective dinitrophenylhydrazones which are more completely extracted into less polar organic solvents. By this manoeuvre, both keto acid extraction is increased and the extraction of the precursor amino acid is reduced. Employing this technique, the kinetics of brain‐stem γ‐aminobutyric acid (GABA), tryptophan, 3,4‐dihydroxyphenylalanine (DOPA) aminotransferases and brain‐stem and liver tyrosine aminotransferases were examined. Brain‐stem aminotransferases, particularly the aromatic amino acid transferases, have a higher affinity for both the amino acid and the keto acid when the aromatic keto acid, phenylpyruvate (0·8 mM), is employed as amino group acceptor, whereas maximal velocities for aminotransferase reactions are much greater when α‐ketoglutarate (0·8 mm) is the amino group acceptor. Brain‐stem tyrosine aminotransferase exhibits a much lower affinity for tyrosine in the presence of either 0·8mm‐α‐ketoglutarate or 0·8 mm‐phenylpyruvate than does liver tyrosine aminotransferase. p‐Chlorophenylpyruvate and phenylpyruvate exhibit similar properties as amino group acceptors for brain‐stem tryptophan aminotransferase. Cysteine inhibits tryptophan aminotransferase when phenylpyruvate is the amino group acceptor, in a manner which is competitive with the amino acid. Benzoylformate inhibits both tryptophan and DOPA aminotransferases when phenylpyruvate is the amino group acceptor, but this inhibition does not appear to be competitive with phenylpyruvate.