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Enzymatic basis for the transesterification of cocaine in the presence of ethanol: evidence for the participation of microsomal carboxylesterases.

The enzyme-mediated transesterification of the 2-carboxymethyl ester of cocaine to a 2-carboxyethyl ester in the presence of ethanol has been characterized in mice by using both in vitro and in vivo systems. Hepatic subcellular fractionation of mouse livers demonstrated that cocaine transesterification activity was detectable only in the microsomal fraction. The in vitro microsomal transesterification of cocaine in the presence of ethanol was inhibited by fluoride and diisopropylfluorophosphate but was insensitive to physostigmine, thereby indicating that the reaction is specifically catalyzed by a carboxylesterase. The transesterification activity of hepatic microsomes showed saturable Michaelis-Menten kinetics in the presence of increasing concentrations of ethanol or cocaine. Cocaine transesterification specific activity in isolated mouse kidney microsomes was approximately 1.5-fold greater than that measured in the liver, suggesting that the kidney could play a significant role in ethyl ester formation. Collectively, these observations demonstrate that the transesterification of cocaine in the presence of ethanol is probably the result of hepatic and possibly renal carboxylesterases localized in the endoplasmic reticulum. The ethyl ester of cocaine was also detected in the livers of mice coadministered ethanol (3.0 g/kg intragastrically) and cocaine (50 mg/kg i.p.). Cocaine and cocaine ethyl ester displayed similar pharmacokinetic profiles, with hepatic half-lives of approximately 6.5 min. Pretreatment of the mice with 100 mg/kg tri-o-tolylphosphate completely blocked in vivo formation of the ethyl ester metabolite, further supporting the involvement of B-esterases in the transesterification of cocaine to cocaine ethyl ester.