Metabolic hydrolysis of isoniazid by subcellular fractions of rat liver

The characteristics of isoniazid amidase which hydrolyzes isoniazid to isonicotinic acid and hydrazine was examined in vitro using rat liver subcellular fractions. The activity of isoniazid amidase was estimated from the amount of hydrazine produced from a substrate, isoniazid, by means of GC-MS. High activity of the amidase was observed in the microsomal and lysosomal fractions, and at pH 7.4-7.8 in the microsomal fraction. The amidase was not inhibited by acetanilide, but by procaine and bis(p-nitrophenyl)phosphate. As expected, acetylisoniazid, a main metabolite of isoniazid, also inhibited the amidase. Not only the microsomal monooxygenase but also amidase was strongly induced by pretreatment with phenobarbital, 3-methylcholanthrene and rifampicin, respectively.

Is isoniazid-hepatotoxicity induced by the metabolite, hydrazine?

The effects of rifampicin (RMP) on isoniazid (INH) metabolism in rabbits were examined from the viewpoint of extensive hepatitis. After the RMP pretreatment, no remarkable changes were observed in the plasma levels of INH as well as its metabolites, acetylisoniazid (AcINH), acetylhydrazine (AcHz) and diacetylhydrazine (DAcHz) with the exception of hydrazine (Hz). After an oral administration of INH or Hz hydrate, the stochastic examination showed that the AUC0-8hr values of Hz plasma levels in RMP pretreated groups were significantly less than those in the control rabbits. RMP treatment was also shown to induce rabbit liver cytochrome P-450 activity. Histological studies demonstrated that Hz causes more remarkable hepatic necrosis in rabbits pretreated with RMP than in the control rabbits. These observations could suggest that Hz is a key intermediate of INH-hepatitis through the transformation of some hepatotoxic species by microsomal oxidation that is facilitated by RMP.

Studies of isoniazid metabolism in isolated rat hepatocytes by mass fragmentography

Isoniazid metabolism in isolated rat hepatocytes was studied by mass fragmentography using single ion monitoring. Isoniazid and its metabolites were determined as the trimethylsilylated derivatives of acetylisoniazid and diacetylhydrazine and of the benzaldehyde hydrazones of isoniazid and acetylhydrazine. Deuterated analogues served as internal standards. Hydrazine was quantitated as benzalazine using 15N-labeled hydrazine as an internal standard. The method is well suited for the microanalysis of isoniazid metabolism, from which it was clarified that the greater part of hydrazine, a hazardous metabolite of isoniazid, was formed through the direct hydrolysis of isoniazid itself as expected.

Metabolism of isoniazid and free hydrazine formation in isolated rat hepatocytes

Isoniazid (INH) was incubated with isolated rat hepatocytes, and the amounts of acetyl-INH, monoacetylhydrazine, diacetylhydrazine and hydrazine formed were determined by gas chromatography-mass spectrometry (GC-MS). Subsequent analyses of kinetic parameters of metabolic pathways and enzymological study of rat liver homogenate revealed that hydrazine was formed preferentially from INH, catalyzed by soluble hydrolytic enzymes.

Resection of aneurysm of the aortic arch without cardiopulmonary bypass

Four cases of aneurysm of the aortic arch were encountered within 1 year at the National Taiwan University Hospital, Taipei, Taiwan, Republic of China. All four patients underwent resection of the aneurysm and graft replacement of the aortic arch without cardiopulmonary bypass or hypothermia. Temporary external shunting with Tygon tube from the ascending aorta to the femoral artery, with and without permanent bypass grafts from the ascending aorta to the carotid arteries, was used in three patients. Permanent bypass grafts with multiple anastomoses was used in one patient. The over-all operative mortality rate was 25 percent. Some adjuncts are made to perfect the safe and simple technique of temporary external shunting, which has been reported previously from the same institution.