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Cysteine

Cysteine

Description

Cysteine is a nonessential amino acid, which means it is manufactured from other amino acids in the liver; it does not have to be obtained directly through the diet. It is synthesized in the liver from methionine.

Cysteine is an important free radical antioxidant in cellular systems. It blocks oxidants of the free radical type that may be engaged in certain forms of cellular pathology, including aging, carcinogenesis, diabetes mellitus, and the development of heart disease. Cysteine is incorporated in the cellular glutathione, which works along with vitamin E to protect cells against free radical oxidant damage.

Deficiencies of a nonessential amino acid will not occur if a well-balanced diet is consumed because the intake of proper foods will allow the body to produce exactly the amount of amino acid required to function optimally. For cysteine, deficiencies of methionine, vitamin B-2, or selenium will result in poor metabolism of cysteine and reduced antioxidant protection. Cysteine is best administered therapeutically as glutathione or in combination with adequate vitamin B-6 and vitamin B-12 and folic acid. Doses of cysteine are between 100 and 500mg per day.

Recommended Dietary Allowances

Cysteine is a nonessential amino acid. An RDA has not been established for cysteine but the RDA for methionine of 55mg for women and 110mg for men is applicable.

Excessive intake of cysteine can result in liver damage, formation of kidney stones, or even some forms of schizophrenia.

Food Sources

Cysteine is a nonessential amino acid, which means it is manufactured from other amino acids in the liver; it does not have to be obtained directly through the diet. It is synthesized in the liver from methionine.

Animal protein is known to be higher in sulfur amino acids such as cysteine than vegetable proteins; therefore, strict, amino acid-imbalanced vegetarian diets may lead to poor sulfur amino acid intake and deficiency signs. Of the vegetable proteins, beans are generally higher in sulfur amino acids than grains. Therefore, a balanced vegetable protein of grains and beans would be useful in providing adequate cysteine intake in the diet.

Method of Action

Cysteine is not only an essential constituent of proteins, but it also lies in the major route of incorporation into all organic sulfur compounds in the body. Cysteine is formed from methionine and serine.

Methionine is converted to S-adenosyl methionine (SAM) and on to homocysteine. Homocysteine then reacts with serine under the influence of the enzyme cystathionine beta-synthetase. This series of steps requires adequate levels of vitamin B-12, vitamin B-6, and betaine. A variety of genetic defects have been found relating to these enzymes, which may then dictate the increased need in certain individuals for enhanced levels of vitamin B-12 and vitamin B-6, or betaine to stimulate proper synthesis of cysteine. Cysteine can also be converted to taurine, which is a component of bile salts as well as a neurotransmitter.

Cysteine is also incorporated within the molecule glutathione. In 1929, F.G. Hopkins discovered a tripeptide glutathione and recognized it as a constituent of most cells. Glutathione is a natural antioxidant made up of glutamic acid, cysteine, and glycine. The actual activity of glutathione as an antioxidant is dependent upon the cysteine residue, which serves as a free radical-protecting agent. Insufficiencies of cysteine can reduce the antioxidant potential of the individual and make he or she more susceptible to free radical pathology.

Abstracts

References

Blackburn, G.L., Grant, J.P., Young, V.R., ed. Amino Acids Metabolism and Medical Applications.

Munro, H.N. & Crim, M.C. The Proteins and Amino Acids. Modern Nutrition in Health and Disease. eds. R.S. Goodhart & M.E. Shils, 6 ed., Philadelphia: Lea and Febiger, 1980.

Rudman, D., Kutner, M., Ansley, J., Jansen, R., Chipponi, J. & Bain, R.P., Hypotyrosinemia, Hypocystinemia, and Failure to Retain Nitrogen during Total Parenteral Nutrition of Cirrhotic Patients. Gastroenterology, 81:1025-35, 1981.

Stegink, L.D., & Den Besten, L. Synthesis of Cysteine from Methionine in Normal Adult Subjects: Effect of Route of Alimentation. Science, 178:514-6, 1972.

White, A., Handler, P., Smith, E.L., Hill, R.L., & Lehman, I.R., Principles of Biochemistry, 6th ed., New York: McGraw-Hill, 1978.

Young, V.R., Meguid, M., Meredith, D.E. & Bier, D.M. Recent Developments in Knowledge of Human Amino Acid Requirements. Nitrogen Metabolism in Man, eds: J.C. Waterlow & J.M.L. Stephen. London: Applied Science Pubs, 1981.

 


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