Any of a large group of organic compounds marked by the presence of both an amino (NH2) group and a carboxyl (COOH) group. Amino acids are the building blocks of proteins and the end products of protein digestion.
Approximately 80 amino acids are found in nature, but only 20 are necessary for human metabolism or growth. Of these, some can be produced by the liver; the rest, the “‘essential' amino acids,” must be supplied by food. Oral preparations of amino acids may be used as dietary supplements.
Arginine is nonessential for adults but cannot be formed quickly enough to supply the demand in infants and thus is classed as essential in early life.
Some proteins containing all the essential amino acids are called complete proteins. Examples are milk, cheese, eggs, and meat. Proteins that do not contain all the essential amino acids are called incomplete proteins. Examples are vegetables and grains. Amino acids pass unchanged through the intestinal wall into the blood, then through the portal vein to the liver and into the general circulation, from which they are absorbed by the tissues according to the specific amino acid needed by that tissue to make its own protein. Amino acids, if not otherwise metabolized, may be converted into urea.
SEE: deaminization; SEE: digestion; SEE: protein
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