Species Distribution and Properties of Hepatic Phenylalanine (Histidine): Pyruvate Aminotransferase

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Abstract
Hepatic phenylalanine(histidine):pyruvate aminotransferase (EC 2.6.1.-) activity is much higher in the mouse and rat than in other animal species (human, guinea-pig, rabbit, pig, dog and chicken). The activity is elevated in the mouse and rat by the injection of glucagon but not in other species (guinea-pig, rabbit and chicken). The enzyme was purified from the mitochondrial fraction of mouse liver to homogeneity as judged by polyacrylamide disc gel electrophoresis in the presence of dodecylsulfate. With histidine as amino donor, the enzyme was active with pyruvate, oxaloacetate and hydroxypyruvate as amino acceptors but not with 2-oxoglutarate. Effective amino donors were histidine, phenylalanine and tyrosine with pyruvate, and methionine, serine and glutamine with phenylpyruvate. The apparent Km for histidine was about 6.9 mM with pyruvate and that for pyruvate was 21 mM with histidine. The enzyme is probably composed of 2 identical subunits with a MW of .apprx. 40,000. The pH optimum was near 9.0. Isoelectric focusing of the purified enzyme resulted in the detection of 4 forms with pI [isoelectric point] 6.0, 6.2, 6.5 and 6.7, respectively, all of which were responsive to glucagon. These 4 forms were nearly identical with the purified enzyme before the focusing with respect to physical and enzymic properties. A possible mechanism of this multiplicity is discussed.