Amino Acid Pools and Distribution

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The distribution of amino acids is complex. Not only are there 20 different amino acids incorporated into a variety of different proteins in a variety of different organs in the body, but amino acids are consumed in the diet from a variety of protein sources. In addition, each amino acid is maintained in part as a free amino acid in solution in blood and inside cells. Overall, a wide range of concentrations of amino acids exists across the various protein and free pools. Dietary protein is enzymatically hydrolyzed in the alimentary tract, releasing free individual amino acids that are then absorbed by the gut lumen and transported into the portal blood. Amino acids then pass into the systemic circulation and are extracted by different tissues. Although the concentrations of individual amino acids vary among different free pools such as plasma and intracellular muscle, the abundance of individual amino acids is relatively constant in a variety of proteins throughout the body and nature. Table.,2.3 shows the amino acid composition of egg protein and muscle and liver proteins (9). The data are expressed as moles of amino acid. The historical expression of amino acids is on a weight basis (e.g., grams of amino acid). Comparing amino acids by weight skews the comparison toward the heaviest amino acids, making them appear more abundant than they are. For example, tryptophan (molecular weight, 204) appears almost three times as abundant as glycine (molecular weight, 75) when quoted in terms of weight.

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Table 2.3 Amino Acid Concentrations in Muscle and Liver Protein and in High-Quality Egg Protein

An even distribution of all 20 amino acids would be 5% per amino acid, and the median distribution of individual amino acids centers around this figure for the proteins shown in Table. ...2.3. Tryptophan is the least common amino acid in many proteins, but considering the effect of its large size on protein configuration, this is not surprising. Amino acids of modest size and limited polarity such as alanine, leucine, serine, and valine are relatively abundant in protein (8-10% each). While the abundance of the essential amino acids is similar across the protein sources in TabJ.e..2..3, a variety of vegetable proteins are deficient or low in some essential amino acids. In the body, a variety of proteins are particularly rich in specific amino acids that confer specific attributes to the protein. For example, collagen is a fibrous protein abundant in connective tissues and tendons, bone, and muscle. Collagen fibrils are arranged differently depending on the functional type of collagen. Glycine makes up about one-third of collagen, and there is also considerable proline and hydroxyproline (proline converted after it has been incorporated into collagen). The glycine and proline residues allow the collagen protein chain to turn tightly and intertwine, and the hydroxyproline residues provide for hydrogen-bond cross-linking. Generally, the alterations in amino acid concentrations do not vary so dramatically among proteins as they do in collagen, but such examples demonstrate the diversity and functionality of the different amino acids in proteins.

The abundance of different amino acids varies over a far wider range in the free pools of extracellular and intracellular compartments. Typical values for free amino acid concentrations in plasma and intracellular muscle are given in T.§ble 2.4, which shows that amino acid concentrations vary widely in a given tissue and that free amino acids are generally inside cells. Although there is a significant correlation between free amino acid levels in plasma and muscle, the relationship is not linear (10). Amino acid concentrations range from a low of »20 pM for aspartic acid and methionine to a high of »500 pM for glutamine. The median level for plasma amino acids is 100 pM. There is no defined relationship between the nature of amino acids (essential vs. nonessential) and amino acid concentrations or type of amino acids (e.g., plasma concentrations of the three BCAAs range from 50 to 250 pM). Notably, the concentration of the acidic amino acids, aspartate and glutamate, is very low outside cells in plasma. In contrast, the concentration of glutamate is among the highest inside cells, such as muscle ( Table2..4).

Table 2.4 Typical Concentrations of Free Amino Acids in the Body

It is important to bear in mind the differences in the relative amounts of N contained in extracellular and intracellular amino acid pools and in protein itself. A normal person has about 55 mg amino acid N/L outside cells in extracellular space and about 800 mg amino acid N/L inside cells, which means that free amino acids are about 15 times more abundant inside cells than outside (10). Furthermore, the total pool of free amino acid N is small compared with protein-bound amino acids. Multiplying the free pools by estimates of extracellular water (0.2 L/g) and intracellular water (0.4 L/g) provides a measure of the total amount of N present in free amino acids: 0.33 g N/kg body weight. In contrast, body composition studies show that the N content of the body is 24 g N/kg body weight (11, 12). Thus, free amino acids make up only about 1% of the total amino N pool, with 99% of the amino N being protein bound.

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