Biomedical subjects

R C James

Publications and source records attributed to R C James.

57 records · Page 4Linked to original sources

PubMedDate not supplied

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.

Adolescent↗

PubMedDate not supplied

Comparison of human and mouse liver microsomal metabolism of bromobenzene and chlorobenzene to 2- and 4-halophenols.

Bromobenzene is metabolized by hepatic microsomes to two different epoxide intermediates, which then rearrange to form either ortho- or para-bromophenol. A rapid and sensitive technique utilizing HPLC with electrochemical detection is presented for the quantitation of these primary bromobenzene metabolites. This analytical procedure allows selective quantitation of phenolic products of microsomal metabolism without prior extraction. Application of this assay method to the microsomal metabolism of bromobenzene and chlorobenzene revealed that three important differences exist between mice and humans regarding the metabolism of these compounds. First, human liver microsomes have a greater affinity for halobenzene biotransformation to the hepatotoxic 3,4-epoxide, as indicated by the approximately 2-fold lower Km values for para-halophenol production compared with mouse liver microsomes. Second, human liver microsomes produce the hepatotoxic metabolite at a 2-fold greater rate than mouse liver microsomes, relative to the microsomal cytochrome P-450 content. And third, human liver microsomes produce less of the nonhepatotoxic ortho-halophenol metabolites at Vmax resulting in an average ratio of the hepatotoxic: nonhepatotoxic metabolite production that is 3.5 times higher than the ratio for B6C3F1 mice. These results indicate humans preferentially metabolize halobenzenes through the hepatotoxic 3,4-epoxide pathway, suggesting that humans may be more susceptible than mice to halobenzene-induced hepatotoxicity.

Animals↗

PubMedDate not supplied

Inhibition by ethanol of the metabolism of cocaine to benzoylecgonine and ecgonine methyl ester in mouse and human liver.

Previous studies have suggested that the esteratic metabolism of cocaine to benzoylecgonine may be inhibited by the presence of ethanol. In this study, the effects of ethanol on the esteratic metabolism of cocaine to benzoylecgonine and to ecgonine methyl ester were examined in vitro using 60,000g supernatant from mouse and human liver. The addition of ethanol (40 mM) to an incubation mixture containing cocaine (24 microM) resulted in substantial decreases in benzoylecgonine and ecgonine methyl ester formation in liver from both species. Sodium fluoride (40 mM), included in the experiment as a positive control, also produced marked inhibition of cocaine metabolism to benzoylecgonine and ecgonine methyl ester. Additional studies were conducted in vivo in which mice were administered cocaine (50 mg/kg, ip) with or without ethanol pretreatment (3 g/kg by gavage). Ethanol pretreatment resulted in 2- to 3-fold increases in peak hepatic concentrations of cocaine, ecgonine methyl ester, and the N-oxidative metabolite norcocaine, and the areas under the hepatic concentration vs. time curve (AUCs) for these compounds were doubled. In contrast, the hepatic concentrations and AUC for benzoylecgonine were halved. These observations are consistent with an inhibition in vivo of esteratic metabolism of cocaine to benzoylecgonine, resulting in higher cocaine levels and metabolism through alternative pathways. Such an interaction may be of importance in the reported effects of ethanol to enhance the activity and toxicity of cocaine.

Animals↗