Amino Acids

AMINO ACIDS, organic acids (containing the COOH group), in which one or more hydrogen atoms in the alcohol radical are replaced by amino groups (NH_a), due to which amino acids belong to so-called amphoteric compounds. Depending on the number of amino groups, mono-amino acids and di-amino acids are distinguished. According to the number of carboxyl groups, amino acids are divided into monobasic and dibasic. The position of the amino group relative to the acid group is denoted by Greek letters: alpha-amino acids (amino group is connected to the carbon closest to the acid group), beta-amino acids (amino group is connected to the second carbon, counting from the acid group), etc. The most important are alpha-amino acids, as they are part of all natural proteins. Amino acids acquired important significance in biology and medicine after the work of Kossel, Fischer, Abderhalden and others. These authors definitely established that all proteins found in animal and plant organisms, when subjected to hydrolysis by strong acids or alkalis or the action of proteolytic enzymes, break down and give a mixture of amino acids. Thus, proteins consist of various amino acids connected to each other in one way or another. One protein differs from another either in the quality of the amino acids it contains or in the quantity of amino acids connected to each other. Proteins taken in food, passing through the digestive tract, are subjected to the sequential action of three groups of enzymes: pepsin (gastric juice), trypsin (pancreatic juice) and erepsin (intestinal juice). As a result, proteins in the intestine are already undifferentiated and represent a mixture of their breakdown products, i.e., amino acids. The latter are absorbed through the intestinal walls and enter the blood unchanged. Hence the great physiological significance of amino acids, which, strictly speaking, animals feed on instead of proteins. Abderhalden managed to prove that replacing food proteins with a mixture of amino acids makes it possible to maintain the nitrogen balance of the body without any violations. Entering the blood, amino acids are adsorbed by red blood cells (Zbarsky), then in the liver amino acids undergo partial deamination, i.e., the amino group is split off in the form of ammonia, another part of the amino acid is used for the synthesis of the most diverse proteins of the body (tissues, blood, hair, nails, etc.). The ammonia split off from amino acids is converted into urea or in some animals into uric acid; part of it is used to neutralize acids formed in the body. After the removal of amino groups, the remaining ordinary organic acids from amino acids undergo further oxidation. In the bodies of animals and plants, amino acids also serve as material for the formation of amines (see Alkaloids, Proteinogenic Amines). The most important amino acids: amino acids of the fatty series - amino-acetic acid (glycine), aminopropionic acid (alanine), aminocaproic acid (leucine), aminosuccinic acid (aspartic acid), etc.; aromatic amino acids - phenylalanine, tyrosine; heterocyclic amino acids - tryptophan, etc. The quantitative determination of amino acids is very important in the study of protein metabolism, for which a number of methods have been developed. The most important are: the Van Slyke method, based on the property of nitrous acid to decompose amino acids with the release of nitrogen in the form of gas. The released nitrogen is measured in a special device and serves as an indicator of the amount of amino acids. The Sorensen method (so-called formol titration) is also very widespread, based on the fact that formaldehyde, acting on amino acids, binds its amino group. After this, the hydrogen ion of the amino acid dissociates, and the amino acid can be titrated directly with alkali. In recent years, the colorimetric method of Folin has also become widespread, based on the fact that amino acids with beta-sulfo-naphtho-quinone give a yellow color. The intensity of the color can be used to judge the amount of amino acids. All the above-mentioned methods are used to determine amino acids in blood, urine and other secretions of the animal body, Zbarsky.