Intestinal metabolism of sulfur-containing amino acids in the rat

Prathapasinghe, Gamika A. (2004) Intestinal metabolism of sulfur-containing amino acids in the rat. Masters thesis, Memorial University of Newfoundland.

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Abstract

The portal-drained viscera play a fundamental role m the supply of dietary nutrients to the rest of the body. On a tissue mass basis these organs consume a disproportionately higher quantity of dietary available nutrients. Of the portal-drained viscera, the intestine is unique, being able to utilize nutrients from both luminal and arterial sources. With regard to sulfur-containing amino acids, the small intestine consumes about 30% of dietary available methionine though the gastrointestinal system represents only about 7% of the whole body mass. This organ is known to contain all the necessary enzymes of transmethylation, remethylation and transsulfuration. -- Although the A-V balance of sulfur-containing amino acids across portal-drained viscera has been demonstrated by previous studies, the fluxes of sulfur-containing amino acids across the portal-drained viscera in the rat are not known. The fate of exogenous methionine within the enterocytes has not been elucidated. Therefore, in this thesis, the fluxes of major thiols across the portal-drained viscera and the metabolic fate of [1-¹⁴C] methionine in isolated rat enterocytes were studied. Total cysteine was found to be exported to the portal blood in rats fed a 60% casein diet, a 20% casein supplemented with 0.6% cystine diet and a 20% casein supplemented with 0.5% methionine diet. The exported cysteine was entirely in the non-protein bound form in the portal plasma. Homocysteine was taken up from the arterial blood in rats fed a 2% methionine-containing diet. Glutathione was taken up by portal venous-drained viscera in all diet groups except the 60% casein fed rats. -- Exogenous labelled methionine was metabolized by enterocytes at approximately the same rate as in hepatocytes. However, there was no appreciable flux through the transmethylation pathway. Homocysteine was metabolized through the transsulfuration pathway into cysteine. These studies reveal an active sulphur-containing amino acid metabolism in the intestine.

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