Metabolism of cocaine and heroin is catalyzed by the same human liver carboxylesterases

• Published in: The Journal of pharmacology and experimental therapeutics Vol. 279; no. 2; p. 713
• Main Authors: Kamendulis, L M, Brzezinski, M R, Pindel, E V, Bosron, W F, Dean, R A
• Format: Journal Article
• Language: English
• Published: United States 01-11-1996
• Subjects: Butyrylcholinesterase - physiology; Carboxylic Ester Hydrolases - physiology; Cocaine - metabolism; Heroin - metabolism; Humans; Hydrolysis; Liver - enzymology; Morphine Derivatives - metabolism; Narcotics - metabolism
• ISSN: 0022-3565

Concomitant i.v. use of cocaine and heroin ("speedballing") is prevalent among drug-abusing populations. Heroin is rapidly metabolized by sequential deacetylation of two separate ester bonds to yield 6-monoacetylmorphine and morphine. Hydrolysis of heroin to 6-monoacetylmorphine is catalyzed by pseudocholinesterase. The pathway for hydrolysis of 6-monoacetylmorphine to morphine in vivo has yet to be established. Pseudocholinesterase and two human liver carboxylesterases [human liver carboxylesterase form 1 (hCE-1) and human liver carboxylesterase form 2 (hCE-2)] catalyze the rapid hydrolysis of ester linkages in cocaine. This investigation examined the relative catalytic efficiencies of hCE-1, hCE-2 and pseudocholinesterase for heroin metabolism and compared them with cocaine hydrolysis. Enzymatic formation of 6-monoacetylmorphine and morphine was determined by reverse-phase high-performance liquid chromatography. All three enzymes rapidly catalyzed hydrolysis of heroin to 6-monoacetylmorphine (hCE-1 kcat = 439 min-1, hCE-2 kcat = 2186 min-1 and pseudocholinesterase kcat = 13 min-1). The catalytic efficiency, under first-order conditions, for hCE-2-catalyzed formation of 6-monoacetylmorphine (314 min-1 mM-1) was much greater than that for either hCE-1 or pseudocholinesterase (69 and 4 min-1 mM-1, respectively). Similarly, the catalytic efficiency for hydrolysis of 6-monoacetylmorphine to morphine by hCE-2 (22 min-1 mM-1) was substantially greater than that for hCE-1 (0.024 min-1 mM-1). Cocaine competitively inhibited hCE-1-, hCE-2- and pseudocholinesterase-catalyzed hydrolysis of heroin to 6-monoacetylmorphine (Ki = 530, 460 and 130 microM, respectively) and 6-monoacetylmorphine hydrolysis to morphine (Ki = 710, 220 and 830 microM, respectively). These data demonstrate that metabolism of cocaine and heroin in humans is mediated by common metabolic pathways. The role of hepatic hCE-2 is particularly important for the hydrolysis of heroin to 6-monoacetylmorphine and of 6-monoacetylmorphine to morphine.