Biochemistry of Amino Acids - A. Meister 1961

General Biochemistry and Physiology of Amino Acid Metabolism
Metabolism of the Amino Group
General Remarks

Amino Acid Breakdown typically proceeds via the Cleavage of the amino group from the carbon Skeleton. The carbon chains of Amino Acids are converted into intermediates that also arise during lipid and Carbohydrate METABOLISM; these products subsequently undergo complete oxidation. Amine nitrogen is converted into urea, uric acid, and ammonia. These three compounds represent the principal End products of Nitrogen metabolism, while other nitrogen-containing compounds—such as creatinine, indican, allantoin, free and bound amino acids, and Purines—are excreted in significantly smaller quantities.

Species-related differences observed in the major end products of nitrogen metabolism are of considerable interest. In mammals, urea is the primary end product, with small amounts of uric acid, creatinine, allantoin, and ammonia also being excreted. Urea is synthesized and accumulated within the body of certain aquatic vertebrates, such as elasmobranch fish; in some amphibians, it serves as the principal end product of nitrogen metabolism. Tadpoles excrete predominantly ammonia, but during metamorphosis, this mode of excretion shifts to urea formation. Birds and reptiles produce primarily uric acid. During The Development of the chick embryo, Ammonia is produced at the earliest stages, followed by a stage of urea production; upon hatching, the definitive excretory pattern—uric acid formation—is established.

In aquatic invertebrates, The production of ammonia constitutes the primary pathway of nitrogen excretion. It has been noted that the formation and excretion of urea, and particularly uric acid, are characteristic of terrestrial forms, whereas Ammonia Production is typical of aquatic ones. This suggests that, from an evolutionary perspective, the detoxification of ammonia through its conversion into other end products of nitrogen metabolism represents an essential adaptation accompanying the transition from an aquatic to a terrestrial habitat [50–52].

In certain marine species, trimethylamine oxide [(CH₃)₃N = O] serves as one of the excretory nitrogenous products. This compound may also appear in small quantities in mammalian urine, potentially as a product of bacterial trimethylamine oxidation [53]. In some spider species, guanine is one of the main end products of nitrogen metabolism, apparently replacing uric acid [54].