Nutritional and functional importance of intestinal sulfur amino acid metabolism.

The metabolism of sulfur amino acids, methionine and cysteine, has been linked to several key aspects of human health and cellular function. In addition, the metabolism of dietary amino acids by the gastrointestinal tract is nutritionally important for normal function. In the case of sulfur amino acids (SAAs), in vivo, stable isotope studies in adults suggest that the splanchnic tissues utilize as much as 30-44% of the dietary methionine and cysteine. Similarly, the dietary methionine requirement is 30% lower in total parenteral nutrition (TPN)-fed piglets, a condition in which dietary nutrients largely bypass intestinal metabolism. These data suggest that intestinal metabolism of methionine is substantial, yet the intestinal metabolic fate of dietary methionine is largely unknown. Dietary cysteine likely plays a key role in intestinal epithelial antioxidant function as a precursor for glutathione. Moreover, cysteine and glutathione may also regulate epithelial cell proliferation via modulation of redox status. Recent evidence indicates that transformed colonic epithelial cells are capable of methionine transmethylation and transsulfuration. This review discusses the evidence of intestinal SAA metabolism and how this affects nutrient requirements and epithelial function.