L-cysteine

Endpoint summary

Description of key information

Since there is no long term storage for amino acids in mammals, excess amino acids are degraded, mainly in the liver. Metabolism of amino acids involves removal of the amino group which in mammals is converted to urea and excreted in the urine. After removal of the amino group the rest of the acid is utilised as energy or used to synthetise other endogenous substances.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Comprehensive information on metabolism of amino acids arising from dietary protein is available. In the normal diet, the amino acids are ingested as components of food proteins and not as free acids. An intake of 100 g protein per day is not an unusual intake for an adult European individual. Deducing the amount of individual amino acids from soy bean protein, an intake of 100 g protein would amount to an intake of 2.2 g cysteine.

Dietary protein is denatured in the stomach due to low pH. The disruption of polypeptide folding makes the chain susceptible to proteolysis. Up to 15% of dietary protein may be cleaved to peptides and amino acids by pepsins in the stomach. In the duodenum and small intestine digestion continues through proteases in pancreatic juice and peptidases which are membrane proteins facing the intestinal lumen. The products of digestion in the duodenum and small intestine are oligopeptides and single amino acids, the concentration of oligopeptides being about three to four times that of single amino acids. Di- and tri-peptides and single amino acids are transported across the brush border membrane into intestinal epithelial cells. Di- and tri-peptides are absorbed at a faster rate than amino acids and result in greater increases in plasma levels than comparable amounts as single amino acids (Castro, 1991). Inside the cells the di- and tri-peptides are hydrolysed by peptidases to single amino acids. There is little regulation of amino acid absorption in the intestine. Regardless of whether amino acids enter the intestinal cells as peptides or amino acids, they enter the hepatic portal circulation as single amino acids.

Absorbed amino acids leave the hepatic portal system and enter the peripheral blood circulation. L-Amino acids are taken up by tissues for synthesis of cellular proteins and other physiologically active compounds. Excess L-amino acids are degraded mainly in the liver. Degradation of L-amino acids involves removal of the amino group, which in mammals is converted to urea and excreted in the urine. After removal of the amino group the rest of the acid is utilised as energy or used to synthesise other endogenous substances.

Tissue pools of free amino acids are approximately 0.5% of the amount of bound amino acids. The pool of free amino acids turns over several times daily to meet requirements. The daily protein turnover has been estimated to be 250-300 g in a 70 kg man (Young et al., 1976). There is no storage form for amino acids in animals except in the biologically active protein of all cells.

The utilisation of amino acids is mainly regulated at the level of their metabolism (Peters, 1991). Of the 20 proteogenic amino acids all but three may be metabolised in the liver. The branched chain amino acids (leucine, isoleucine and valine) are degraded in peripheral tissue, mainly in muscle but also in kidney and brain. When dietary intake of amino acids exceeds the needs there is an induction of enzymes in the liver for breakdown of excess amino acids. This system is better controlled for the indispensable amino acids than for the dispensable, where an increase in intake leads to a gradual increase in catabolism. There may nevertheless be a net efflux of amino acids into the circulation following excess intakes. The impact of insulin via the ratio of carbohydrates to protein in the diet is another mechanisms by which amino acid concentrations in the blood may be influenced, as well as by the competition of different amino acids for mechanisms that transport amino acids through membranes and into certain tissues. The kidney regulates amino acids in plasma through tubular reabsorption, occurring after ultrafiltration by the glomeruli. The amino acids are actively co-transported with sodium ions. The daily excretion of protein in urine amounts to 20-150 mg/day in humans, of which albumin represents about 60% (Tietz, 1986c).

Since there is no long term storage for amino acids in mammals, excess amino acids are degraded, mainly in the liver. Metabolism of amino acids involves removal of the amino group which in mammals is converted to urea and excreted in the urine. After removal of the amino group the rest of the acid is utilised as energy or used to synthetise other endogenous substances.