L-Phenylalanine is an allosteric inhibitor of M1-type pyruvate kinase. Accordingly, the effects were studied of 20 mM phenylalanine on the metabolism of 5 mM [U-14C]glucose and 3 mM L-[U-14C]glutamate by isolated hemidiaphragms from starved rats. Phenylalanine inhibited lactate and 14CO2 production from both substrates and stimulated alanine release. It is concluded that pyruvate kinase may have a dual role in intermediary metabolism in skeletal muscle: the enzyme is a component of the lower glycolytic pathway and is implicated in a pathway of amino acid oxidation and alanine synthesis.

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