[Effect of ammonium chloride and urea on the activity of lactate dehydrogenase. A general model of catalysis].

The dependence of structural and functional properties of lactate dehydrogenase from the pig muscle (isoenzyme M4) on pH (6.9-8.5) on the effect of different forms of non-protein nitrogen (e.g. urea and ammonium chloride) has been investigated. It was shown that Vmax changes negligibly at pH 6.9-8.5 but K(m) increase markedly with pH elevation. At the alkali value of pH pyruvate inhibition disappeared. We have found that urea and ammonium chloride at the non-saturation concentrations inhibited the activity of lactate dehydrogenase. The inhibition constants (Ki and Ka) for urea and ammonium chloride were obtained from inhibition data. When analysing works of different authors and own results we propose the general model of catalysis of lactate dehydrogenase, which includes the formation of normal enzyme-substrate complex, abortive enzyme-NAD-pyruvate complex and the effect of different effectors, including nitrogen containing substances.