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Alanine Aminotransferase

II. THE BASIS FOR SUBSTRATE SPECIFICITY

From the ¹ From the Department of Biochemistry, University of California, Berkeley, California 94720

The substrate specificity of soluble alanine aminotransferase from pig heart was examined. On the basis of competitive inhibition studies it was concluded that, in many cases, the low reactivities of enzyme-substrate analogue complexes were due to the effect of the amino acid side chains on the maximum reaction velocity rather than to a change in the Michaelis constant.

Formate was found to stimulate alanine aminotransferase-catalyzed amino transfer from alanine to either pyruvate or -ketoglutarate, but to inhibit that from glutamate to either -ketoglutarate or pyruvate. Rates of removal of the -hydrogen atoms of glutamate and alanine in D₂O were affected by formate in the same manner as were the rates of transamination. Other monocarboxylate anions inhibited both transamination reactions. Formate acted as a noncompetitive inhibitor of -aminobutyrate transamination. Results were interpreted to mean that activation by formate was due to its binding to the site which normally binds the -carboxyl group of glutamate.

No effect of formate on the spectrum of the enzyme in the presence of an excess of alanine and pyruvate was observed, and its effect on the spectrum of the enzyme in the presence of excess glutamate and -ketoglutarate was slight.

Two other enzymes, d-alanine aminotransferase from Bacillus subtilis and aspartate aminotransferase from pig heart, catalyze amino transfer from alanine to -ketoglutarate, at rates which are increased by monocarboxylate anions.

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