Inhibition of acetaminophen oxidation by cimetidine and the effects on glutathione and activated sulphate synthesis rates
Dalhoff, K.; Poulsen, H.E.
Pharmacology and Toxicology 73(4): 215-218
1993
ISSN/ISBN: 0901-9928 PMID: 8295849 Document Number: 412631
The aim of the present study was to examine the effects of the hepatotoxic drug, acetaminophen, on the synthesis rates of glutathione, activated sulphate (PAPS, adenosine 3'-phosphate 5'-phosphosulphate) and the acetaminophen metabolites, acetaminophen-glutathione and acetaminophen-sulphate after inhibition of cytochrome P-450 drug oxidation by cimetidine in isolated rat hepatocytes. The synthesis rates of glutathione and PAPS were determined simultaneously by an established method based on trapping of radioactivity (35S) in the prelabelled glutathione and PAPS pools. Preincubation of the hepatocytes with 60 mu-g/ml cimetidine for 30 min. did not affect PAPS (1.71 versus 1.78 nmol/10-6 cells) nor glutathione concentration (16.0 versus 16.4 nmol/10-6 cells). The subsequent incubation with 5 mM acetaminophen resulted in decreased PAPS synthesis in the cimetidine treated cells (0.79 cntdot 10-3 versus 0.92 cntdot 10-3 nmol/(10-6 cells cntdot hr)) (P lt 0.05). There was no difference in PAPS concentration or acetaminophen-sulphate synthesis (1.73 versus 1.79 nmol/10-6 cells and 13.0 versus 12.9 nmol/(10-6 cells cntdot hr), respectively). Decreased PAPS synthesis may be related to decreased ATP supply or may be the result of a feed-back regulation due to diversion of sulphur from glutathione synthesis to sulfoxidation. The glutathione synthesis was not significantly affected by cimetidine treatment (57 cntdot 10-3 versus 27 cntdot 10-3 nmol/(10-6 cells cntdot hr)). As expected acetaminophen-glutathione synthesis decreased by 38% (1.66 versus 2.68 nmol/ (10-6 cells cntdot hr)) (P lt 0.01). Also the glutathione concentration was lower in cimetidine treated cells (15.2 versus 15.9 nmol/ 10-6 cells) (P lt 0.05). We have previously shown that glutathione synthesis was reduced if substrate availability decreased (acetaminophen concentration lowered). Thus, the unaltered glutathione synthesis observed in the present study in which N-acetyl-p-benzoquinoneimine formation was diminished suggests that cimetidine does not inhibit all acetaminophen metabolites which utilize reduced glutathione.