Isoniazid hepatotoxicity: The relationship between covalent binding and metabolism in vivo

The relationship between the hepatotoxicity and metabolism of isoniazid and its metabolites, acetylisoniazed and acetylhydrazine, has been investigated. Toxic doses of acetylisoniazid and acetylhydrazine, radiolabeled in the acetyl group, were found to bind covalently to liver protein in vivo. This binding was mediated by the microsomal enzyme system as indicated by the effects of pretreatments altering the activity of these enzymes. Metabolic studies revealed that the pretreatment increased the metabolism of the acetylhydrazine moiety of acetyl-labeled acetylisoniazed and of acetylhydrazine itself by the microsomal enzyme system. Pretreatment with the acyl amidase inhibitor, bis-p-nitrophenyl phosphate, inhibited the hydrolysis of acetylisoniazed to isonicotinic acid plus acetylhydrazine and concomitantly decreased the covalent binding of acetyl-labeled acetylisoniazid. the changes in the metabolism of isoniazid, acetylisoniazid changes in acetylhydrazine effected by various pretreatments paralleled changesin the severity of the hepatic necrosis caused by the compounds. These results strongly suggest that acetylhydrazine is the metabolite responsible for the hepatic necrosis caused by isoniazid and that microsomal metabolism of acetylhydrazine in vivo leads to the production of a reactive acylating species capable of reacting covalently with tissue macromolecules.