Human microsomal N-oxidative metabolism of cocaine

The N-oxidative metabolism of cocaine has been described previously in some detail in the mouse, and is associated with hepatotoxic effects of cocaine in this species. As part of an effort to determine whether this metabolism-dependent toxicity is relevant to clinical reports of liver injury from cocaine, the in vitro N-oxidative metabolism of cocaine was examined in microsomal suspensions obtained from human liver specimens. Human hepatic microsomal suspensions were capable of metabolizing cocaine to norcocaine, and norcocaine to N-hydroxynorcocaine, which is the sequential oxidative metabolism observed in mice to lead to toxic metabolite formation. Additionally, incubation of 4-[3H]cocaine with human hepatic microsomal suspensions resulted in the formation of a metabolite that bound irreversibly to microsomal protein. Each of these reactions was inhibited or abolished by gassing the incubation mixture with carbon monoxide, omitting NADPH, or adding the cytochrome P-450 inhibitors SKF 525-A (50 microM) or n-octylamine (3 mM). Competing microsomal esterase activity reduced the apparent rate of N-demethylation of cocaine by 50%, but had much less effect on the N-hydroxylation of norcocaine. In general, apparent KM values for both the N-demethylation and N-hydroxylation reactions in human liver microsomal suspensions were greater than those observed in comparable incubations using mouse hepatic microsomal suspensions, whereas Vmax rates were lower. The extent of irreversible (i.e. nonextractable) binding of cocaine following its metabolism in vitro was comparable between human and mouse microsomal suspensions, however. Similarities in the N-oxidative metabolism of cocaine suggest that humans and mice share the same bioactivation mechanism of cocaine-induced liver injury.