Methionine

Description

Methionine is an essential amino acid. This means it must be obtained through the diet in adequate quantities to meet the body's needs.

Methionine is used in the manufacture of taurine, which is an important amino acid for cardiac function as well as serving as a brain neurotransmitter. Deficiencies of methionine which are found to be associated with a poor-quality dietary protein intake can result in taurine, cysteine, and one-carbon metabolite deficiencies.

Insufficiencies of methionine can result in poor synthesis of phosphatidylcholine and other phospholipids. These substances are essential for nervous system function as well as prevention of blood cell stickiness.

Supplementation with methionine is often seen in soy-based protein formulas to improve the protein quality. L-methionine supplementation of soy protein will raise its protein efficiency ratio by providing enhanced levels of this amino acid which is deficient in soy protein. Excessive intake of methionine can aggravate some forms of schizophrenia and encourage stuvite kidney stone formation in sensitive individuals. Therapeutic doses of methionine range between 500 and 1,000mg per day.

Recommended Dietary Allowances

The RDA for methionine has been established as 55mg per day for women and 110mg per day for men.

Food Sources

Foods high in methionine include:

Cottage cheese - dry	1,200 mg/cup
Cottage cheese - creamed	854 mg/cup
Fish and other seafoods	2,000-3,500 mg/lb.
Meat	750-2,500 mg/lb.
Poultry	1,500-2,000 mg/lb.
Peanut, roasted with skin	640 mg/cup
Sesame seed	1,400 mg/cup
Dry, whole lentils	350 mg/cup

Method of Action

Methionine is converted to S-adenosyl methionine, which then serves as a methyl group donor for the synthesis of substances such as ethanolamine. Ethanolamine is further methylated in the body and converted to phosphatidylcholine, which is found in lecithin.

Methionine is also converted into homocysteine, which is reconverted back to methionine through the trans-sulfuration pathway. Homocysteine should not build up in the body; if it does, it associated with an increased risk to heart disease and atherosclerosis. Poor conversion of homocysteine to methionine is caused by vitamin B-6 deficiency in genetically-susceptible individuals.

Methionine is incorporated into proteins. A major route of its metabolism involves conversion to S-adenosyl methionine (SAM).

SAM is a key intermediate in the transsulfuration pathway, which results in the manufacture of diverse substances such as taurine and carnitine. SAM is converted to homocysteine, which can be reconverted to methionine, but adequate levels of vitamin B-6 are required.

A genetic defect has been found which prevents proper conversion of homocysteine to methionine. This is associated with increased risk to atherosclerosis (coronary artery disease).

This block can be overcome by administering higher levels of vitamin B-6 and/or betaine, which promote these sluggish enzymes and facilitate better conversion of homocysteine to methionine. Plasma or urinary levels of homocysteine should be zero. Elevations indicate increased risk to coronary artery disease.

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 in Modern Nutrition in Health and Disease. eds. R.S. Goodhart & M.E. Shils, 6 edition, Philadelphia: Lea and Febiger, 1980.

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.

Stinett, J.D., Alexander, J.W., Watanabe, C., MacMillan, B.G., Fischer, J.E., Morris, M.J., Trocki, O., Miskell, P., Edwards, L., & James, H., Plasma and Skeletal Muscle Amino Acids Following Severe Burn Injury in Patients and Experimental Animals. Ann. Surg., 195:75-89, 1982.

Tallan, H.H., Rassin, D.K., Sturman, J.A. & Gaull, G.E., Methionine Metabolism in Brain. Handbook of Neuro-Chemistry. ed: A. Lajtha, 2nd ed, vol 2, New York: Plenum Pub Corp.,1982.

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.