The sulfur amino acids, methionine and cysteine, are implicated in numerous biological functions and diseases, aside from their role in protein synthesis
Methionine is an indispensable amino acid and is transmethylated intracellularly to homocysteine via S-adenosylmethionine, the principal biological methyl donor in mammalian cells and a precursor for polyamine synthesist. Reduced -adenosylmethionine concen¬trations, as a consequence of low methionine intake or folate deficiency, mainly lead to a deregulation in DNA methylation.

Homocysteine is a sulfur-containing amino acid present in the blood and tissues but not incorporated into protein. Homo¬cysteine can be converted into cy teine via cystathionine through the trans-sulfuration pathway, an irreversible process Homocysteine can also be methylated back to methionine via the remethylation pathway. The combination of transmethylation and remethylation pathways comprises the methionine cycle which occurs in most cells. However, the trans-sulfuration pathway has a limited tissue distribution and is restricted to the liver, kidney, intestine, pancreas and adrenals.

Cysteine is considered a semi-indispensable amino acid whose availability is dependent upon methionine intake. However, dietary cysteine can satisfy a proportion of the sulfur amino acid requirement, the so-called cysteine-sparing effect on dietary methionine requirernent. Cysteine is a constituent amino acid of the tripeptide glutathione (γ-Glu-Cys-Gly), the major cellular antioxidant in mammals, and serves also as a precursor for the synthesis of taurine, pyruvate, sulfate and hydrogen sulfide (H2S) .

The gastrointestinal tract is a metabolically significant site of sulfur amino acid metabolism in the body and metabolises about 20 % of the dietary methionine intake which is mainly transmethylated to homocysteine and trans-sulfurated to .cysteine. The gastrointestinal tract accounts for about 25 % of the whole-body transmethylation and trans-sulfurarion.
The gut also utilises 25 % of the dietary cysteine intake and the cysteine uptake by the gut represents about 65 % of the splanchnic first-pass uptake.

Sulfur amino acids deficiency significantly suppresses intestinal mucosal growth and reduces intestinal epithelial cell proliferation, and increases intestinal oxidant stress in piglets.

Suggesting that intestinal metabolism of dietary methionine and cysteine is nutritionally important for intestinal mucosal growth. Besides their role in protein synthesis, methionine and cysteine are precursors of important molecules. S-adenosylmethionine, a metabolite of methionine, is the principal biological methyl donor in mammalian cells and a precur or for polyamine synthesis. Cysteine is the rate-limiting amino acid for glutathione synthesis, the major cellular antioxidant in mammal .

Bauchart-Thevret et al 2009 Intestinal metabolism of sulphur amino acids Nutrition Research Review vol 22 pp 175-187

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