Properties of Pyruvate Kinase from Soybean Nodule Cytosol

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Abstract
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 (Vmax and apparent Ka) 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 Ka (monovalent cation) and Km (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+.