Nitrogen excretion

The main nitrogenous end products of protein metabolism are amino acids, ammonia, uric acid, and urea. Much has been written concerning the excretion of these substances by various animals and the relationship this bears to their evolution, habitat, and mode of reproduction. The excretion of guanine by spiders and trimethylamine oxide by marine teleosts are important exceptions. Guanine deposited in fish scales and pterines in butterfly pigments might also be regarded as excretory products if one were also to consider as such the keratin of mammalian hair. 

von Przylecki and R. Truszkowski, in Bamann and Myrback, Die Metho-den der Fermentforschung, Vol. Ill, Thieme, Leipzig, 1941 Academic Press, New York, 1945, p. 2489. 

Needham, Biochemistry and Morphogenesis, Cambridge University Press, Cambridge, 1942. 

Baldwin, An Introduction to Comparative Biochemistry, 3rd ed.. Cam bridge University Press, Cambridge, 1948. 

The end products of purine metabolism are uric acid in primates and uricotelic animals, allantoin in mammals other than primates, urea in the rest of the ureotelic animals, and ammonia in many of the ammonotelic animals. 

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The increase in diet-tissue spacing has been proposed to be caused by the effects of water and heat stress on urinary nitrogen excretion. The model has been described in detail previously (Ambrose 1991) and will be briefly summarized here Nitrogen is excreted mainly as urinary urea. Its 6 N value is substantially (2-5%o) more negative than that of the diet (Steele Daniel 1978 Yoneyama et al. 1983). Under heat and water stress the concentration... 

Uezu, N., Yamamoto, S., Rikimaru, T, Kishi, K. and Inoue, G. 1983 Contributions of individual body tissues to nitrogen excretion in adult rats fed protein-deficient diets. Journal of... 

Bliss, D.Z. Stein, T.P. Schleifer, C.R. Settle, R.G. (1996). Supplementation with G A fiber increases fecal nitrogen excretion and lowers serum urea nitrogen concentration in chronic renal failure patients consuming a low-protein diet. The American Journal of Clinical Nutrition, Vol. 63, No.3, (March 1996), pp. 392-398, ISSN 0002-9165. 

D. J. Wright and D. R. Newall, Nitrogen excretion, osmotic and ionic regulation in nematodes. The Organisation of Nematodes (N. A. Croll, ed.). Academic Press, London, UK. 1976. p. 163. 

Reduction in nitrogen excretion is often set equal to a reduction in nitrogen emission. This is only true in those cases where the excess nitrogen in the commercial fertiliser can be caught and retained in the agricultural nutrient cycle. If the nutrients are, however, discharged from the cycle and emitted... 

Brusilow, S., Tinker, J. and Batshaw, M. L. Amino acid acylation A mechanism of nitrogen excretion in inborn errors of urea synthesis. Science 207 659,1980. 

Protease Vegetable proteins Increased protein digestibility Reduced nitrogen excretion... 

Progesterone decreases the plasma levels of many amino acids and leads to increased urinary nitrogen excretion. It induces changes in the structure and function of smooth endoplasmic reticulum in experimental animals. 

Lactate versus Alanine as Metabolic Fuel The Cost of Nitrogen Removal The three carbons in lactate and alanine have identical oxidation states, and animals can use either carbon source as a metabolic fuel. Compare the net ATP yield (moles of ATP per mole of substrate) for the complete oxidation (to C02 and H20) of lactate versus alanine when the cost of nitrogen excretion as urea is included. 

Nitrogen balance occurs when the amount of nitrogen consumed equals that of the nitrogen excreted in the urine, sweat, and feoes. Most healthy adults are normally in nitrogen balance. 

Ammonia is also the major nitrogenous end product in some of the simpler aquatic and marine animal forms, such as protozoa, nematodes, and even bony fishes, aquatic amphibia, and amphibian larvae. Such animals are called am-monotelic. But in many animals, NH3 is toxic, and its removal by simple diffusion is difficult. Thus, in terrestrial snails and amphibia, as well as in other animals living in environments in which water is limited, urea is the principal end product (fig. 22.6). Urea formation also helps to maintain osmotic balance with seawater in cartilagenous fishes. In such animals, most of the urea secreted by the kidney glomerulus is reabsorbed by the tubules. Indeed, the amount of nitrogen excreted by the kidneys of fishes is small com-... 

Another form of detoxified ammonia that is used in nitrogen excretion is uric acid. Uric acid is the predominant nitrogen excretory product in birds and terrestrial reptiles (turtles excrete urea, whereas alligators excrete ammonia unless they are dehydrated, in which case they, too, excrete uric acid). Uric acid formed as a product of amino acid catabolism involves the de novo pathway of purine biosynthesis therefore, its formation from NH3 liberated in amino acid catabolism is described elsewhere (see chapter 23). In mammals, uric acid is exclusively an intermediate in purine... 

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