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Properties of Pyruvate Kinase from Soybean Nodule Cytosol ¹

Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon 97331

The properties of pyruvate kinase from soybean (Glycine max L.) nodule cytosol were examined to determine what influence the N₂ fixation process might have on this supposed key control enzyme. A crude enzyme preparation was prepared by chromatography of cytosol extract on a diethylaminoethyl-cellulose column. ATP and citrate at 5 mM concentrations inhibited pyruvate kinase 27 and 34%, respectively. Enzyme activation was hyperbolic with respect to both K⁺ and NH₄⁺ concentrations. In the presence of physiological concentrations of K⁺ and high phosphoenolpyruvate (PEP) concentrations, NH₄⁺ inhibited enzyme activity. Comparisons of kinetic parameters (V_max and apparent K_a) for NH₄⁺ and K⁺ with inhibition curves indicated that inhibition was very likely a result of competition of the ions for activation site(s) on the pyruvate kinase. In addition, apparent K_a (monovalent cation) and K_m (PEP) were influenced by PEP and monovalent cation concentrations, respectively. This effect may reflect a fundamental difference between plant and animal pyruvate kinases. It is concluded that control of cytosol pyruvate kinase may be closely related to reactions involved in the assimilation of NH₄⁺.

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