Studies on cholinesterase. 8. Determination of reaction velocity constants with a reversible inhibitor of pseudo-cholinesterase

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Abstract
The theoretical formulations of Goldstein (Jour. Gen. Physiol. 27: 529. 1944 ) were based on the assumption that the reaction between an enzyme (E) and its substrate (S) could be adequately represented by an equation of the form E + S[image]ES. These formulations were modified to comply with the correct equation for the overall reaction E + S[image]ES[long dash][long dash]*E + reaction products. The velocity constants for the reactions between the dimethylcarbamate of (2-hydroxy-5-phenylbenzyl)_trimethylammonium bromide (Nu 683) and the pseudo-cholinesterase of human serum were determined by the use of these modified theoretical equations. The results obtained were as follows: Michaelis constant K1 = (k2 + k5)Al = 8.8 x 10-10 mole l.-1, k5 = 0.0119 min.-l; (rate of hydrolysis of inhibitor I); (k 2+ k5) = 0.0119 min.-1; k2 = negligible value (rate of dissociation of EI); k1 = 1.35 x 107 1. mole-1 min.-1 (rate of combination of E with I). The corresponding velocity constants between acetyl-choline and the pseudo-cholinesterase were: Michaelis constant K5 = (k4 + k6)/k3 = 1.34 x 10-3 mole l.-l; k6 = 5.8 x 104 min.-l (rate of hydrolysis of S); k3 > 4.3 x 107 1. mole-1 min.-l (rate of combination of E with S). The significance of these results was discussed. In the particular case of Nu 683 as inhibitor of the pseudo-cholinesterase activity of dialysed human serum, it would appear that the "reversible reaction" between the enzyme and inhibitor could for practical purposes be represented by an equation of the form. E + I[long dash]EI-E + reaction products.

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