Enzyme inhibition by sodium alkyl sulphates.

velocities in the forward and reverse directions was 54.5: 1 , somewhat lower than that previously reported (McQuate & Utter, 1959). Product-inhibition experiments with a non-saturating concentration of the constant substrate showed that in every case competitive inhibition occurs. This is consistent with a rapidequilibrium random Bi Bi mechanism for the reverse reaction, a similar mechanism to that proposed for the forward reaction (Reynard et al., 1961 ; Ainsworth & MacFarlane, 1973). Analysis of the slope replots showed that there was linear competitive inhibition between ATP and both products. When pyruvate is the variable substrate, however, parabolic competitive inhibition occurred with both products. The experimental data for each of these plots were fitted to the relevant rate equation by the leastsquares method of Cleland (1967) and apparent constants were obtained. Fig. 2 shows the experimental data with pyruvate as the variable substrate and phosphoenolpyruvate as the product inhibitor. Parabolic slope effects are normally the result of multiple combinations of 'the inhibitor with the enzyme. It is curious that the parabolic effects were only seen with one substrate and not both, as would be the case if complexes of the form enzyme-productproduct occurred in a rapid equilibrium mechanism. One explanation is that the binding of pyruvate is not at complete thermodynamic equilibrium, and that the reaction possesses some non-rapid-equilibrium random Bi Bi character. Isotopicexchange experiments at equilibrium should confirm whether or not the enzyme really does catalyse a rapid equilibrium mechanism.