Biliary excretion of glutathione conjugates of 4,5-expoxy-4,5-dihydro-1-nitropyrene and 9,10-epoxy-9,10-dihydro-1-nitropyrene in rats administered 1-nitropyrene orally and their further metabolism in the intestinal tract

Abstract

1-Nitropyrene (1-NP), a mutagenic and carcinogenic substance that occurs in the environment, is metabolized by reductive and oxidative pathways. 4,5-Epoxy,4,5-dihydro-1-NP (1-NP 4,5-oxide) and 9,10-epoxy-9,10-dihydro-1-NP (1-NP 9,10-oxide) are oxidatively activated metabolites of 1-NP, and react with glutathione. HPLC analysis of biliary metabolites from rats administered [3H]1-NP orally showed the presence of glutathione conjugates of 1-NP 4,5-oxide and 1-NP 9,10-oxide and their metabolites, cysteinylglycine and cysteine conjugates. During the 48 h following [3H]1-NP administration, 21.4% of the biliary metabolites were excreted as glutathione, cysteinylglycine and cysteine conjugates of 1-NP oxides. The proportions of glutathione conjugates of 1-NP 4,5-oxide and 1-NP 9,10-oxide were 2.6 and 10.4% respectively. Bile and pancreatic juice collected from normal rats metabolized glutathione conjugates of 1-NP oxides and produced the corresponding cysteinylglycine and cysteine conjugates. In particular, the gamma-glutamyltransferase activity of pancreatic juice was markedly high. When glutathione conjugates of 1-NP oxides were incubated with a cell-free extract of small intestinal contents, they decreased rapidly and cysteine conjugates increased via cysteinylglycine conjugates, indicating that pancreatic juice plays an important role in the small intestine for the metabolism of glutathione conjugates to corresponding cysteine conjugates. Although degradation activity of small intestinal contents for cysteine conjugates was very low, degradation activity of the contents of the cecum and large intestine was high and was inhibited by an inhibitor of cysteine conjugate beta-lyase (beta-lyase) activity, aminooxyacetic acid. Furthermore, the intestinal anaerobic bacteria Peptostreptococcus magnus and Eubacterium limosum showed high beta-lyase activity, suggesting that the cysteine conjugates might be further metabolized by beta-lyase of the normal bacterial flora in the lower intestine to the reactivated metabolites and reabsorbed.

Reduction of nitroaromatic compounds by anaerobic bacteria isolated from the human gastrointestinal tract

“…Nitro-PAHs are metabolized by both reductive and oxidative pathways (nitroreduction, ring oxidation, or a combination of the two) (8,9,18,29,32,33,37). The reduction of nitro groups is catalyzed by several mammalian enzymes and also by intestinal bacteria (4,5,14,15,17,23,24,32).…”

“…Various anaerobic human, monkey, and rat intestinal microflora are capable of rapidly metabolizing 1-nitropyrene, dinitropyrenes, and 6-nitrochrysene to aromatic amines (4,5,14,17,23,24). In addition, anaerobic bacteria commonly associated with the gastrointestinal tract reduce nitro-PAHs to aromatic amines, indicating nitroreductase activity in these bacteria (4,15,17,18,24). Kinouchi and Ohnishi (19) have studied the enzymes that convert 1-nitropyrene to 1-aminopyrene in Bacteroidesfragilis and have purified one of these enzymes.…”

Effects of indole-3-carbinol and phenethyl isothiocyanate on bile and pancreatic juice excretion in rats

“…Glutathioneconjugates generate in the liver by glutathione Stransferase and excrete into the gastrointestinal tract through bile. Glutathione-conjugates undergoes to the stepwise cleavage in the small intestine by γglutamyl transferase (γ-GTP) that is derived mainly from the pancreas (8,9). To evaluate the drug metabolism in liver, bile and pancreatic juice should be collected separately because the latter contains high γ-GTP activity (10) that degrades the glutathione-conjugates in bile.…”

“…As a result of Phase II reaction, the glucuronate-, sulfate-and glutathione-conjugates are produced and excreted into bile or urine. It has been reported that high γ-GTP activity is present in the pancreas and in pancreatic juice (8)(9)(10), and glutathioneconjugates are converted into cysteinylglycineconjugates by this enzyme activity in pancreatic juice. Cysteinylglycine-conjugates are further metabolized to cysteine-conjugates by aminopeptidase in pancreatic juice (phase III reaction).…”