Water and the Disposal of Nitrogen Wastes
Ammonia gas is toxic to most organisms and must usually be disposed of rapidly. In a certain sense, one can almost guess the mechanism of nitrogen-waste disposal if one knows the amount of water available to the organism in question. For example, bacteria and fish, which live in “infinite” water supplies, usually simply release ammonia into the medium, where organisms that are lower on the evolutionary scale can use it. Fish sometimes produce trimethylamine, another highly water-soluble compound, which is the characteristic “fish odor.” Most terrestrial animals do not have “infinite” water supplies, but mammals, which have bladders, usually live in conditions where adequate water is available. Their mechanism for disposal of most toxins is to prepare a water-soluble compound and then to excrete it through the urine. Thus, urea becomes a major by-product of nitrogen metabolism in mammals.
Reptiles and other desert animals do not usually have much water available, and birds cannot afford to carry the weight of a fluid-filled bladder.
These animals do not make urea; rather, they convert all their waste nitrogen to uric acid (Figure 23.17), the concentrated white solid so familiar in bird droppings. Some desert mammals, such as the kangaroo rat, which never drinks water but rather lives off metabolic water, also convert some of their waste nitrogen to uric acid to conserve the water used in urine.
Uric acid, the typical waste product from purines, can cause problems in primates due to its marginal water solubility. Deposits of uric acid in the joints and extremities cause gout. Other mammals do not have a problem with uric acid because they convert it to allantoin, which is very water-soluble.