Dynamic State of Body Protein

Proteins and other molecules derived from AAs in cells are catabolized sooner or later and must be replaced as needed to maintain cell function. Maintaining this balance requires a relatively constant supply of AAs, often referred to as the AA pool of free AAs available for synthesis of proteins or other nitrogenous compounds. The total AA pool in the human body includes AAs circulating in blood plasma and AAs found within cells. Under most conditions, the average person's AA pool contains about 150 g. Normally, the relative amounts of each AA in this pool stay fairly constant.4

The AA pool derives AAs both from the breakdown of body proteins and from the intestinal absorption of AAs derived from the digestion of dietary protein. The supply of AAs from the breakdown of body protein is referred to as endogenous AAs (derived from sources within the body), as opposed to exogenous AAs, derived directly from dietary protein or other ingested AAs.

The constant dynamic balance between protein degradation and PS is called protein turnover. Under normal homeostatic conditions in adult humans, the rates of PS and protein degradation are similar. During growth or the accumulation of muscle mass in adults, the rate of PS exceeds the rate of degradation, resulting in a net increase in the body protein content.

Normal PS requires an adequate amount of each AA to be available in cells. Since some AAs are needed in larger amounts than others, the pattern of AAs (relative proportions) in the AA pool is important. AAs present in excess of the amount needed for synthesis are typically utilized for energy by oxidation to carbon dioxide and water, with the nitrogen component disposed of primarily as urea in the urine.

The preferred AA pattern in cells is also partially maintained by converting one AA into another. This is possible only for the 11 dispensable AAs among the 20 AAs used in PS. The other nine AAs are considered to be indispensable or essential AAs that cannot be formed from another AA (or cannot be formed rapidly enough to meet needs) and must be supplied in adequate amounts from the diet. As long as there is enough total AA available, dispensable (nonessential) AAs can be formed from other AAs in adequate amounts to meet the needs of PS in cells.

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