Metabolism of Extracellular Glutathione in Rat Small‐Intestinal Mucosa

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Abstract
Metabolism of exogenous glutathione was investigated in suspensions of freshly isolated rat small‐intestinal mucosal cells. The cells catalyzed the oxidation of reduced glutathione (GSH) to glutathione disulfide (GSSG). Neither serine · borate nor methionine significantly influenced this reaction. Formed GSSG was further metabolized as indicated by its disappearance from the medium. Degradation of GSSG was stimulated by methionine and inhibited by serine · borate. Separation and identification of GSSG metabolites were achieved by high‐performance liquid chromatography. The results indicate that the preferred route for GSSG metabolism to the constituent amino acids in small intestine, is by hydrolytic removal of the two γ‐glutamyl groups of GSSG to yield cystinyl‐bisglycine which is subsequently hydrolyzed to cystine. γ‐Glutamyltransferase activity was compared in isolated intestinal, kidney and liver cells using γ‐glutamyl‐p‐nitrocarboxyanilide as substrate. Kiedney cells were approximately 5‐fold and 150‐fold more active than intestinal and liver cells, respectively. Serine · borate markedly inhibited, and glycyl‐glycine stimulated, hydrolysis of γ‐glutamyl‐p‐nitrocarboxyanilide in all cell types confirming the involvement of γ‐glutamyltransferase in the reaction. The hydrolysis of γ‐glutamyl‐p‐nitrocarboxyanilide was inhibited to approximately the same extent by either GSH or GSSG suggesting that both compounds interact at the donor site of γ‐glutamyltransferase. Comparison of the rates of glutathione metabolism by isolated intestinal and kidney cells suggests that the intestinal contribution to the degradation of extracellular glutathione may be physiologically more important than has previously been assumed.