HUMAN LIVER CARBOXYLESTERASE HCE-1 - BINDING-SPECIFICITY FOR COCAINE,HEROIN, AND THEIR METABOLITES AND ANALOGS

Purified human liver carboxylesterase (hCE-1) catalyzes the hydrolysis of cocaine to form benzoylecgonine, the deacetylation of heroin to fo rm 6-acetylmorphine, and the ethanol-dependent transesterification of cocaine to form cocaethylene, In this study, the binding affinities of cocaine, cocaine metabolites and analogs, heroin, morphine, and 6-ace tylmorphine for hCE-1 were evaluated by measuring their kinetic inhibi tion constants with 4-methylumbelliferyl acetate in a rapid spectropho tometric assay, The naturally occurring (R)-(-)-cocaine isomer display ed the highest affinity of all cocaine and heroin analogs or metabolit es, The pseudo-or allopseudococaine isomers of cocaine exhibited lower affinity indicating that binding to the enzyme is stereoselective. th e methyl ester, benzoyl, and N-methyl groups of cocaine play important roles in binding because removal of these groups increased K-i values substantially, Compounds containing a variety of hydrophobic substitu tions at the benzoyl group of cocaine bound to the enzyme with high af finity, The high K-i value obtained for cocaethylene relative to cocai ne is consistent with weaker binding to the esterase and a longer elim ination half-life reported for the metabolite, The spectrophotometric competitive inhibition assay used here represents an effective method to identify drug or environmental esters metabolized by carboxylestera ses like hCE-1.