Properties of Pyruvate Kinase from Soybean Nodule Cytosol

Abstract

The properties of pyruvate kinase from soybean (Glycine max L.) nodulecytosol were examined to determine what influence the N(2) fixationprocess might have on this supposed key control enzyme. A crude enzymepreparation was prepared by chromatography of cytosol extract ona diethylaminoethyl-cellulose column. ATP and citrate at 5 mm concentrationsinhibited pyruvate kinase 27 and 34%, respectively. Enzyme activationwas hyperbolic with respect to both K(+) and NH(4) (+) concentrations.In the presence of physiological concentrations of K(+) and highphosphoenolpyruvate (PEP) concentrations, NH(4) (+) inhibited enzymeactivity. Comparisons of kinetic parameters (V(max) and apparentK(a)) for NH(4) (+) and K(+) with inhibition curves indicated thatinhibition was very likely a result of competition of the ions foractivation site(s) on the pyruvate kinase. In addition, apparentK(a) (monovalent cation) and K(m) (PEP) were influenced by PEP andmonovalent cation concentrations, respectively. This effect may reflecta fundamental difference between plant and animal pyruvate kinases.It is concluded that control of cytosol pyruvate kinase may be closelyrelated to reactions involved in the assimilation of NH(4) (+).