The Inhibition of Cholinesterase by Physostigmine and Prostigmine

It has been gencrally assumed that the inhibition of cholinesterase by physostigmine and prostigmine is non-competitive. This is implicit in the equation given by Matthes (1) relating the degree of inhibition to the concentration of physost)igmine, for it contains no term for substrate concentration, an omission justified only if the inhibition is non-competitive. However, it should be noted, as Matt,hes himself points out, that his equation fits only a certain range of the data, and that t,he discrepancies with low physostigminc concentration are probably not to be accounted for by experimental error. In any case, as Lineweaver and Burk (2) have made clear, the type of inhibition cannot be determined from this relationship. Some years later Easson and Stedman (3) reached the tentative conclusion that inhibition by prostigmine is non-competitive on the indirect evidence that, if enzyme and inhibitor are allowed to reach equilibrium before the subst’rate is added, hydrolysis of the latter, provided a sufficient excess is present, proceeds at a constant rate. Their theoretical relationship between prostigmine concentration and the degree of inhibition is essentially the same as that of Matthes: there is assumed to be a reversible but non-competitive combination between 1 molecule of inhibitor and one active center of enzyme to form an inactive compound. They state that it is necessary to leave prostigmine in contact with the enzyme for at least 9 hours before measuring the inhibition in order to insure equilibrium, and in their experiments they apparently left the mixture for about 24 hours. Similarly Roepke (4) states that approximately 8 hours are required for physostigmine and prostigmine to reach an equilibrium with the enzyme. Using an indicator method, he measured the rate of hydrolysis of varying concentrations of acetylcholine by a cholinesterase preparation which had been mixed wit)11 the drug and left overnight in the ice box. When the reciprocals of the velocities thus obtained were plotted against the reciprocals of the substrate concentrations (cJ Lineweaver and Burk (2)), it was found that the points lay upon two straight lines which did not cut the l/o axis at the same point, typical of non-competitive inhibition. On the other hand he failed to confirm Easson and Stedman, finding that the rate