Effects of pyrazole and 3-amino-1,2,4-triazole on the metabolism and toxicity of dimethylnitrosamine in the rat

Pretreatment of rats with pyrazole or 3-amino-1,2,4-triazole (3-AT) known inhibitors of alcohol metabolism, profoundly inhibited the metabolism of dimethylnitrosamine (DMN), both in terms of [14C]CO2 excretion and of the decline in the blood concentration. Additionally, 4-methylpyrazole, tetraethylthiuram disulfide (disulfiram), methanol, and ethanol inhibited the metabolism of DMN in the whole animal. In parallel experiments with [14C]aminopyrine, no substantial inhibitory effect was found with pyrazole, 3-AT, or disulfiram pretreatment. Investigations into the effects of pyrazole and 3-AT pretreatment on the acute toxicity and hepatotoxicity of DMN showed that pyrazole significantly increased the median lethal dose (LD50) of DMN and provided substantial protection against the hepatotoxicity of DMN, in that centriblobular necrosis was not seen at dose levels of DMN up to 25 mg/kg and early histochemical changes indicative of liver injury were not observed at a dose level of 15 mg DMN/kg. In contrast, 3-AT pretreatment did not affect the LD50 of DMN or provide any protection against the hepatotoxicity of DMN. Further, although both inhibitors delayed the incorporation of radioactivity from [14C]DMN into hepatic subcellular organelles, pyrazole was significantly more effective than was 3-AT.

Studies on the effects of orally administered Di-(2-ethylhexyl) phthalate in the ferret

A target-organ study of the effects of the phthalate ester di-(2-ethylhexyl) phthalate (DEHP) has been conducted in mature male albino ferrets. DEHP treatment caused a loss of body weight when administered as a 1% (w/w) diet for 14 months. Additionally marked liver enlargement with associated morphological and biochemical changes was observed. These changes consisted of liver cell enlargement, lysosomal changes, dilatation of the endoplasmic reticulum and the depression of a number of marker enzyme activities. The only other tissue observed to be affected by DEHP treatment was the testes where histological evidence of tissue damage was observed in some animals. Studies on the metabolism of [14C]DEHP in the ferret indicated that the diester was metabolised to derivatives of mono-(2-ethylhexyl) phthalate which were excreted in the urine both unconjugated and as glucuronides. The results obtained have been compared with previous studies in the rat and it is concluded that DEHP is hepatotoxic in both species.

Studies on the in vitro metabolism of dimethylnitrosamine by rat liver

The biotransformation of dimethylnitrosamine (DMN) to formaldehyde, generally attributed to the mediation of a demethylase enzyme associated with the microsomal mixed function oxidase system, has been investigated in rat liver preparations. All of the enzyme activity was found in the postmitochondrial fraction and the microsomes contained approximately 50% of this activity. The restoration of total activity resulting from the addition of the cytosol to the microsomal fraction was found to be due to presence of diffusible, heat-labile constituents in the cytosol. Enzyme kinetic studies revealed that DMN was metabolized to formaldehyde by either a multistep or a multicomponent process. DMN demethylase was found to be relatively stable to storage in contrast to cytochrome P-450 and a number of mixed function oxidase enzyme activities. In spectral interaction studies DMN was found to form an atypical interaction spertrum with either control, phenobarbitone-pretreated or phospholipid-depted microsomal preparations. DMN had little effect on Type II spectral interaction of aniline, but noncompetitvely inhibited the Type I spectral interaction of benzphetamine and biphenyl. Whilst the mixed function oxidase enzyme inhibitors SKF 525A and metyrapone markedly reduced the metabolism of ethylmorphine and anailine, DMN demethylase was little affected by the former compound and appreciably enhanced by metyrapone. Moreover, DMN demethylase was strongly inhibited a number of hepatic mixed function oxidases, but significantly reduced anaerobic nitroreductase activity. The results of these studies reveal important differences between the properties of the enzymatic systems which metabolize DMN and mixed function oxidase substrates, and are consistent with the conclusion that the degradation of DMN to formaldehyde by rat liver preparations is a multicomponent system not rate limiting with respect to cytochrome P-450.

The circadian variation of hepatic microsomal drug and steroid metabolism in the golden hamster

Rhythmic variations in a number of parameters of hepatic microsomal drug and steroid metabolism and binding were observed in the adult male golden hamster. With an illumination schedule of a light phase between 06.30 and 18.30 h, maximal activities were encountered between 04.00 and 08.00 h with corresponding minima between 16.00 and 18.00 h. In addition, kinetic changes were observed in both the metabolism and spectral interaction of aniline and biphenyl at times of maximal and minimal activities. Possible mechanisms responsible for the circadian variation in hamster hepatic microsomal xenobiotic metabolism are discussed.

Enhancement of hepatic microsomal drug metabolism in vitro following ethanol administration

1. Administration of ethanol intraperitoneally at low dosages (10-25 mg/kg) to rats stimulates hepatic microsomal mixed-function oxidase activity in vitro. 2. Pretreatment with ethanol administered orally has no effect on in vivo drug metabolism as measured by pentobarbitone plasma half-life and has no effect on the excretion of ascorbic acid. Ethanol administration does not enhance its own binding to cytochrome P-450. 3. These observations suggest that the administration of ethanol, at moderate dosage, does not give rise to induction of hepatic cytochrome P-450. 4. Unwashed hepatic microsomes are contaminated with alcohol dehydrogenase, but pretreatment with ethanol does not increase microsomal generation of NADH. 5. Pretreatment with ethanol has no stimulatory effect on NADH-NADP+ transhydrogenation. 6. The stimulation of hepatic drug metabolism in vitro following administration of ethanol is not due to increased cytochrome P-450 nor to increased NADPH, per se, but appears to result from an increase in the activity of NADPH-cytochrome c reductase.