Nutritional summary

Function L-cysteine (Cys) is a component ofproteins and provides the sulfur in iron-sulfur proteins, pantothenate. tRNA, proteoglycans and many other compounds. It is a precursor of glutathione (antioxidation) and taurine (conjugation, membranes, vision ). Cys is also an energy fuel; its complete oxidation requires thiamin, riboflavin, niacin, vitamin B6, pantothenate, lipoate, ubiquinone, iron, zinc, and magnesium; disposal of the sulfur in Cys requires molybdenum.

Food sources: Plant-derived foods contain much less L-methionine than meals and other animal-derived foods. Nonetheless, all foods are likely to provide adequate amounts as long as total protein intakes meet recommendations. Requirements The combined requirement for L-methioninc (Met) and Cys is 845mg d (FAO WHO/UNU 19851. Very young infants and severely ill adults need some dietary Cys, because they may not be able to generate enough Cys from Met. Deficiency; Prolonged lack of both Cys and Met causes growth failure, loss of muscle mass, and organ damage.

Excessive intake: Very high intake of protein and mixed amino acids (more than three times the RPA or 2.4 g kg) is thought to increase the risk of renal glomerular sclerosis and accelerate osteoporosis. Consumption of Cys greatly in excess of recommendations generates large amounts ol'sullite which is not well tolerated by sensitive individuals, l ong-term health risks from the consumption of high-dose supplements have not been adequately evaluated

Endogenous sources

About half of all available Cys is metabolically derived from the Met metabolite L-homocysteine. The PLP-dcpendent enzyme cystathionine beta-synthase (ft 4.2. t .22) condenses homocysteine and L-serine to cystathionine which is then cleaved by another PLP-dependent enzyme, cystathiordne-gamma-lyasc I EC4.4.1.1). to release Cys. ammonia, and 2-oxobutanoate (transsttlfuration pathway),

Dietary sources

Foods contain Cys and cystine (CssC) as pan of their proteins, which makes total protein consumption the main determinant of Cys intake. The proteins in eggs, human milk, and grains (wheat, rye. rice) contain about 20 mgg; corn contains less than half as much, Meats and milk also contain only 9 13 mg/g. I leal treatment of foods slightly decreases the amount of Cys that can be absorbed and utilized (Dworschak, 1980).

The currently recommended dietary intake for sulfur-containing amino acids (Cys, CssC and methionine) is 13 mg kg d (Raguso el a!., 2000), but actual requirements may be considerably higher (Di Buono et at., 2001). Adequate Cys intake can reduce Met requirements by about 8mg kg tl)i Buono el at., 2001).

Digestion and absorption

Healthy individuals absorb amino acids and proteins m the proximal small intestine nearly completely. Food proteins are hydroly/ed by an array of gastric, pancreatic, and enteral enzymes, which generate Cys and cystine as part of oligopeptides and in free form. The former can be taken up through the hydrogen ion peptide cotransporier (PepTI. SLCI5A1).

di/tri peptides

Intestinal turnen di/tri peptides

Intestinal turnen

Capillary lumen

Brush border membrane

Basoiateral membrane

Capillary endothelium

Capillary lumen

Brush border membrane

Basoiateral membrane

Capillary endothelium

Figur* S.SO Intestinal absorption of Lcysteinc and L-cystinc

The main port of entry for free Cvs is the sodium-driven transporter B (Avissar ciul.. 2001). Smaller amounts enter with the sodium eotransporter ASC, One of the transporters comprising the ASC family, ASCT2, is expressed in colon, suggesting the capacity for Cys uptake beyond the small intestine (Utsunomiya-Tate ef ol, 1996).

Both Cys and CssC are shuttled across the brush border membrane in either direction in exchange for neutral amino acids by the amino acid transporter BAT l b" (SLC7A9): this transporter is anchored to the membrane and complemented by the glycoprotein subunit rBAT (SLC3AI) (Chairoungdua el a!.. 1909).

Significant amounts ofCys, both from the intestinal lumen and from blood circulation, are used for the cntcroeyic's own needs. The remainder is transported across the basoiateral membrane mainly by the sodium amino acid eotraitsporters A and ASC and the sodium-independent transporter asc.

The sodium-independent transporter LAT2 (SI ( 7AS) can exchange Cys and CssC for other neutral amino acids in either direction. Like the heteroexehangcr at the brush border membrane. L AT2 is anchored to the basoiateral membrane by a complementary glycoprotein, in this case 4F2 (SLC3A21 (Rajan et til.. 2000),

Weight Loss Enigma

Weight Loss Enigma

Finally Revealed The Revolutionary & Miraculous Weight Loss Secrets! Discover How to Command Those Unwanted Pounds To Take A Hike, So That You Can Get Into Shape & Lose Weight Easily Just Like You Dream Of In Just A Matter Of Weeks! You're About to Discover The Insider's Fat Burning Diet Tips to Easily Shed Off Those Extra Pounds And Obtain a Lovable Shape In No Time Flat!

Get My Free Ebook


Post a comment