Deamination

DEAMINATION, the removal of amino groups (in the form of NH₃ or free N). The deamination of amino acids and proteins by chemical means is achieved through the action of nitrous acid. In this process, the aliphatic NH₂-groups of amino acids react according to the scheme: R.NH₂ + HNO₂ -> R.OH + N₂ + H₂O.

COOH

COOH. This reaction forms the basis of the Van Slyke method (see Van Slyke methods). Guanidine, indole, pyrrolidine, and imidazole do not react with HNO₂; therefore, proline and hydroxyproline do not yield N₂ by this method. Tryptophan yields 1/2 of the total amount of N, histidine 1/3, and arginine 1/4. The NH₂-group bound in asparagine in the form of an amide does not react with HNO₂. Glycocoll and cystine yield 103–107% of the theoretical amount of N. The ability to react and the speed of the reaction increase in the presence of mineral acids. Peptides and proteins react according to the content of free NH₂-groups in them (with the exception of the guanidine group of arginine). During the deamination of proteins by the Van Slyke method, 1–2% of the total amount of N is split off. In the protein molecule, the ε-NH₂-groups of lysine are apparently free.

In the animal organism, the majority of amino acids are deaminated in the intestinal wall and in the liver. Deamination proceeds according to two schemes, but always by the removal of NH₃. 1. Hydrolytic removal of the NH₂-group with the formation of an hydroxy acid: R.CH(NH₂).COOH + H₂O -> R.CH(OH).COOH + NH₃. 2. Oxidative deamination with the formation of keto acids (Neubauer), which is the main pathway for the breakdown of amino acids: R.CH(NH₂).COOH + O -> R.CO.COOH + NH₃. Both reactions are reversible. The released ammonia is used for the neutralization of acids formed in the organism and for the formation of urea. The nitrogen-free part of the amino acid is burned or used for the synthesis of carbohydrates and fats. The deamination of purine bases occurs in most cases under the influence of hydrolyzing enzymes, purine desamidases (adenase, guanase), which convert adenine and guanine into hypoxanthine and xanthine, which are then oxidized to uric acid; nucleoside desamidases deaminate purine bases that are in a bound state in the form of nucleosides. Desamidases are contained in most organs, and they are especially abundant in the liver. Micrococcus ureae and soy beans contain the enzyme urease, which hydrolytically splits urea into NH₃ and CO₂, upon which one of the methods for the quantitative determination of urea is based.