Drug Biotransformation and Hepatotoxicity. Studies with Bromobenzene in Isolated Hepatocytes

Abstract

Cytochrome P-450-linked drug monooxygenation may occasionally give rise to the formation of electrophilic products which, unless further metabolized via various synthetic pathways, may interact with tissue nucleophiles to produce acute or chronic cytotoxicity. Epoxides and N-oxygenated derivatives are perhaps the best known examples of reactive metabolites which may cause cell death unless detoxified, primarily by glutathione (GSH) conjugation.

In recent experiments we have used isolated rat hepatocytes to study toxicity linked to metabolism of foreign chemicals, among them bromobenzene. Bromobenzene was rapidly metabolized in this system and gave rise to water-soluble as well as protein-bound metabolites. GSH conjugation contributed significantly to its metabolism as evidenced by a rapid decrease in cellular GSH level during incubation with bromobenzene. With hepatocytes isolated from phenobarbital-treated rats, bromobenzene caused a decrease in GSH to about 40% of the original level. However, there was evidence for GSH biosynthesis also during bromobenzene metabolism and the cells survived this treatment quite well.

When the rats were pretreated with diethylmaleate to lower hepatic GSH concentration prior to cell isolation, toxic effects of bromobenzene metabolism in the isolated hepatocytes became apparent. The GSH level, which had been lowered by approximately 70% by the diethylmaleate pretreatment, was further decreased upon incubation with bromobenzene and cell death was markedly accelerated. There was extensive covalent labeling of both protein and certain low-molecular cell constituents.

Cysteine and cysteamine inhibited the formation of protein-bound metabolites from bromobenzene in microsomes but the addition of extra cysteine (3 mM) to the medium (containing 0.2 mM cysteine) did not protect the hepatocytes from bromobenzene toxicity. Methionine, on the other hand, which did not cause a significant effect on bromobenzene metabolism in microsomes, prevented toxicity in hepatocytes, presumably by stimulating GSH synthesis and thereby decreasing the amount of reactive metabolite(s) available for interaction with other cellular nucleophiles.

It is concluded that GSH conjugation is of primary importance for the in-activation of reactive drug metabolites, such as those formed from bromobenzene, and that methionine is efficient in protecting against the resulting cell injury, probably by enhanced GSH synthesis via the formation of cystathionine.