Inhibition of methyl-n-amylnitrosamine hydroxylation by diallyl sulfide and phenethylisothiocyanate in the rat

Formation of the stable 2-, 3-, and 4-hydroxy derivatives of methyl-n-amyln itrosamine (MNAN) probably reflects cytochrome P-450-catalyzed activation o f MNAN by I-hydroxylation. Here we studied inhibition of the oxidation of M NAN to hydroxy-MNANs (HO-MNANs) by freshly excised tissues from MRC- Wistar rats treated with the vegetable-derived chemicals diallyl sulfide (DAS) an d phenethyl-isothiocyanate (PEITC). Rats were gavaged with DAS (200 mg/kg), PEITC (163 mg/kg), or vehicle (corn oil) alone. After various times, the v ats were killed, the esophagus, nasal mucosa, and liver were removed, and t he tissues/tissue slices were incubated for two hours with 23 muM MNAN. HO- MNAN formation was measured by gas chromatography-thermal energy analysis. Significant (p < 0.01) 72-75%, 40%, and 44% inhibitions of total HO-MNAN fo rmation were observed for nasal mucosa removed at 3-18 hours, for esophagus at 18 hours, and for liver at 3 hours, respectively, after gavage of DAS. Significant (p < 0.03) 46-75% inhibition of HO-MNAN formations was observed for the esophagus at 2-24 hours after gavage of PEITC. In disposition stud ies, rats M were treated with DAS (200 mg/kg) in corn oil and sacrificed af ter various intervals. DAS was determined by gas chromatography of tissue h omogenate extracts. After gavage of DAS, its total recovery from all tissue s studied was 27% of the dose after 45 minutes and 15-19% after 90 and 180 minutes, with > 80% of the recovered DAS in the stomach contents. Up to 2% per tissue of the recovered DAS was found in the stomach wall, liver, and b lood. After intraperitoneal injection of DAS, less than or equal to2% of th e dose was recovered in the blood and less than or equal to0.7% in the live r Hence, gavage of DAS and PEITC significantly inhibited HO-MNAN formation for up to 18 and 24 hours, respectively, whereas DAS was >80% metabolized 9 0 minutes after its gavage. These findings suggest that longlasting inhibit ors or their metabolites, or inactivation of P-450 enzymes, were responsibl e for the persistence of inhibition of MNAN metabolism.