[Regulation of enzyme activity in adsorptive enzyme systems. II. Influence of dextran sulfate on catalytic properties of lactate dehydrogenase (isozyme M4)].

It has been shown that the binding of pig skeletal muscle lactate dehydrogenase (isozyme M4) by dextran sulfate with weight-average molecular weight 500 000 is accompanied by a decrease of the rate of enzymatic reduction of pyruvate. The hyperbolic dependence of the enzymic reaction rate on NADH concentration observed for free lactate dehydrogenase is transformed in a sigmoidal curve in the case of adsorbed enzyme form (Hill's coefficient is equal to 2.1). The experimental data have been described quantitatively using the model of adsorptive enzyme system where the enzyme interacts reversibly with the support and co-operative interaction of substrate binding sites in the adsorbed enzyme molecule are realized. It is assumed that the value of microscopic dissociation constant for the complex of the substrate with adsorbed enzyme is being changed by a constant factor during saturation of the binding sites by the substrate in the enzyme molecule. The value of parameters of the model for the adsorptive enzyme system under study are determined.