Proline
Description
Proline 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.
Proline is the precursor to hydroxyproline, which is a major amino acid found in the connective tissue of the body-collagen. Proline is different from other amino acids in that it has a secondary amino group and contains a pyrrole ring, such as found in hemoglobin and the cytochromes. This structure causes proline in a protein to impart a rigid protein structure.
Proline has no known therapeutic use. Its effect in human nutrition, other than as a source of nitrogen, is not well established.
Supplementation with proline has not been found to increase collagen formation.
Recommended Dietary Allowances
Proline is a nonessential amino acid, therefore RDA has not been established.
Food Sources
Proline 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.
Method of Action
Proline is an unusual, cyclic alpha-amino acid. As a consequence of its structure, it imparts a unique set of physical characteristics to proteins in which it is incorporated. Collagen, which is the most abundant protein in the body, makes up much of the organic mass of skin, tendon, blood vessels, bone, cornea, vitreous humor of the eye, and basement membranes of all organs. Collagen is composed of the metabolic by-product of proline called hydroxyproline.
A closely related protein, elastin, is found in the elastic, yellow, connective tissue fibers, which are abundant in ligaments and walls of blood vessels. Both collagen and elastin are rich in hydroxyproline, which comes from proline by way of a hydroxylation reaction requiring adequate levels of vitamin C (ascorbic acid).
Proline is incorporated within a growing protein chain at the ribosome and is then hydroxylated into hydroxyproline as the growing peptide chains are still attached to the ribosome along with rough, endoplasmic reticulum. There are a number of genetic defects that have been identified as they relate to the synthesis of collagen which produce connective tissue difficulties. These include Ehlers-Danlos syndrome, which, in some instances, is accompanied with recurrent joint dislocations and curvature of the spine.
Osteoarthritis may also be related to abnormalities in collagen structure as well as the bone disease, osteogenesis imperfecta.
References
Bishop, M.C., Smith, R., Ledingham, J.G.G. & Oliver, D. Biochemical Markers in Renal Bone Disease. Proc. Eur. Dial. Transplant Assoc., 8:122, 1971.
Munro, H.N. & Crim, M.C. The Proteins and Amino Acids. Modern Nutrition in Health & Disease, eds. R.S. Goodhart & M.E. Shils, 6 ed. Philadelphia: Lea and Febiger, 1980.
Nixon, S.E. & Mawer, G.E. The Digestion and Absorption of Protein in Man 2. The Form in which Digested Protein is Absorbed. Br. J. Nutr. 24:2451-58, 1970.
Weber, F.L., Jr., Maddrey, W.C., & Walser, M. Amino Acid Metabolism of Dog Jejunum Before and During Absorption of Keto-analogues. Am. J. Physiol., 232:F210-F214, 1977.
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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