Abstract
The properties of pyruvate kinase from soybean (Glycine max L.) nodule cytosol were examined to determine what influence the N(2) 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 NH(4) (+) concentrations. In the presence of physiological concentrations of K(+) and high phosphoenolpyruvate (PEP) concentrations, NH(4) (+) inhibited enzyme activity. Comparisons of kinetic parameters (V(max) and apparent K(a)) for NH(4) (+) 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 NH(4) (+).