The characterization and regulatory properties of pyruvate kinase from cotton seeds

Pyruvate kinase (ATP: pyruvate phosphotransferase, EC 2.7.1.40) was partially purified from cotton seeds. The enzyme shows normal kinetics toward phosphoenol-pyruvate, ADP, and magnesium or manganese. Of nearly 50 metabolites tested, the enzyme is inhibited only by ATP, UTP, citrate, and malate, and activated by AMP, GMP, and fumarate. The inhibition by citrate and ATP is not due to metal chelation; both compounds appear to directly affect the enzyme. The kinetics of the activations by AMP and by fumarate suggest the existence of separate activator sites for the two compounds.It is suggested that cotton seed pyruvate kinase is a regulatory enzyme, although it differs markedly from the regulatory pyruvate kinases which have been described in animals and in microorganisms. This is the first instance in which regulatory properties have been reported for a pyruvate kinase from a higher plant.