In summary, both glutathione and blood neutrophils contribute to ANIT hepatotoxicity. Glutathione contributes by virtue of its ability to form a reversible S-conjugate with ANIT that is critical in shuttling ANIT into bile. Where it is released in large and probably toxic concentrations. The possibility remains that this conjugate may be bioactivated by secondary mechanisms, but no evidence for a toxic glutathionyl conjugate of ANIT currently exists. Neutrophils and platelets both appear to play important roles in ANIT hepatotoxicity. The role of platelets is currently unknown, but studies in vitro raise the possibility that neutrophils may be activated during ANIT exposure to release cytotoxic proteases that cause injury to target cells. Although ANIT activates neutrophils in vitro, the mechanisms by which neutrophils are recruited into the periportal region and activated in vivo remain unknown.

The control of luminal thiol-disulfide redox state may be important for several intestinal functions, including absorption of iron or selenium and maintenance of mucus fluidity. Disulfides are present in the diet, and although luminal thiols are supplied in bile, little is known about the ability of the small intestine to reduce disulfides to maintain the luminal thiol-disulfide redox state. The objective of the current study was to determine whether the isolated, vascularly perfused jejunum, free from biliary thiols, could reduce intraluminal glutathione disulfide (GSSG) to glutathione (GSH). GSSG was introduced in a deoxygenated solution to inhibit the reoxidation of any GSH formed, and preparations were pretreated with acivicin to inhibit the degradation of GSH by gamma-glutamyltransferase. GSSG (250 micromol/L) was reduced to GSH, with the luminal redox potential (E(h)) for GSSG/2GSH changing from >0 to -111, -132 and -143 mV at 10, 20 and 30 min, respectively. The E(h) for luminal cystine/2cysteine was approximately 20 mV more reducing than that for GSSG/2GSH at each time point, suggesting that cysteine could function in the reduction of GSSG in the lumen. Measurements in specific regions showed that GSSG reduction was more rapid in the duodenum and proximal jejunum than in the distal jejunum. Preparations without acivicin treatment showed that E(h) values were unaffected by inhibition of gamma-glutamyltransferase despite differences in GSH and cysteine pool sizes. Rat intestine has a mechanism to adjust the luminal thiol-disulfide redox. In principle, dysfunction of this mechanism could contribute to malabsorption or other nutritional disorders.

Using a vascularly perfused rat intestinal preparation, we found that large quantities (i.e., 100-200 microM) of acid-soluble thiols accumulated in the jejunal lumen in 10-30 min and that the accumulation was largely unaffected by dietary food restriction for 24 or 48 h. Depending on the length of perfusion, cysteine comprised 20-40% of total luminal thiols, whereas glutathione (GSH) made up only 0-3%. To determine whether luminal cysteine accumulation resulted from mucosal secretion of GSH and subsequent degradation by brush-border gamma-glutamyltransferase (gamma-GT) and dipeptidases, acivicin or serine-borate was used to inhibit gamma-GT. Both agents inhibited gamma-GT activity by > 95%, reduced luminal cysteine by approximately 40-50%, and caused a modest elevation of luminal GSH to approximately 10-13 microM, indicating that GSH secretion does occur but cannot account for all of the luminal cysteine accumulation. Luminal thiol trapping experiments with Ellman's reagent supported this conclusion. Given that cysteine made up 15-20% of the mucosal thiol pool in jejunum, secretion of cysteine from mucosa to lumen likely accounted for the majority of luminal cysteine. Given the mucolytic nature of thiols and the role of cysteine in iron absorption, intestinal thiol secretion may be important in intestinal function.

Glutathione S-transferases in the small intestine function in detoxification of electrophilic compounds ingested in foods, dietary supplements, and orally administered drug preparations. Although the required substrate glutathione (GSH) is synthesized in the intestinal enterocytes, the rate of synthesis is slow compared to both the maximal GST activity and the rate of uptake of luminal GSH. GSH is supplied to the intestinal lumen in the bile, and normal luminal concentrations in the rat are about 250 microM. The present study was designed to test the hypothesis that exogenous GSH is used for intestinal conjugation by glutathione S-transferase. The results show that 250 microM of extracellular GSH stimulated conjugation of 1-chloro-2,4-dinitrobenzene by approximately 300% in rat intestinal enterocyte preparations. However, an unexpected finding was that most of this stimulated activity did not depend upon uptake of GSH by the enterocytes but was due to glutathione S-transferase associated with mucus. Immunohistochemistry of glutathione S-transferase in the intact small intestine confirmed that a portion of the GST is present in the mucus layer. The presence of this detoxication enzyme in the extracellular mucus layer provides a novel mechanism for preventing direct contact of potentially toxic dietary electrophiles with the intestinal enterocytes.

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