In Vitro Competitive Inhibition of Plasma Cholinesterase by Cocaine: Normal and Variant Genotypes

Abstract

Objective: To determine the inhibitory constant, K_i, of cocaine for a number of the different genetic variants of human plasma cholinesterase. Design: In vitro analysis of plasma cholinesterase activity in the presence of cocaine as a competitive inhibitor. Methods: Six normal (UU) control sera and seven sera with the following plasma cholinesterase genotypes were assayed: AA, UA, AS, UF, US, AF and SS. Plasma cholinesterase activity was determined in the samples by colorimetric measurement of propionylthiocholine metabolism over a range of concentrations. Competitive activities were then determined in the presence of varying concentrations of cocaine. Double reciprocal plots (1/v vs 1/S) were used to calculate K_m and V_max for propionylthiocholine, and K_t for cocaine for each genotype. Results: The variant forms of the plasma cholinesterase had high cocaine K_t values all were approximately ten times greater than the K_t for normal plasma cholinesterase. Conclusions: Since the inhibitory constant is an indirect measure of an enzyme's affinity for a competing substrate, a high K_i for cocaine at recreational or therapeutic concentrations would translate into a longer in vivo half-life. Our results support the growing evidence that low plasma cholinesterase activity predisposes to cocaine toxicity.