Birds, excretion

Birds excrete nitrogen as uric acid Uric acid is a purine having the molecular formula C5H4N4O3 it has no C—H bonds Write a structural formula for uric acid... 

Figure 25-18 Pathways of catabolism of purine nucleotides, nucleosides, and free bases. Spiders excrete xanthine while mammals and birds excrete uric acid. Spiders and birds convert all of their excess nitrogen via the de novo pathway of Fig. 25-15 into purines. Many animals excrete allantoin, urea, or NH4+. Some legumes utilize the pathway marked by green arrows in their nitrogen transport via ureides.

Animals, such as fish, that live in an aquatic environment excrete nitrogen as ammonia they are protected from the toxic effects of high concentrations of ammonia not only by the removal of ammonia from their bodies but also by rapid dilution of the excreted ammonia by the water in the environment. The principal waste product of nitrogen metabolism in terrestrial animals is urea (a water-soluble compound) its reactions provide some interesting comparisons with the citric acid cycle. Birds excrete nitrogen in the form of uric acid, which is insoluble in water. They do not have to carry the excess weight of water, which could hamper flight, to rid themselves of waste products. 

Fish excrete excess nitrogen as ammonia, and birds excrete it as uric acid. Mammals excrete it as urea. 

Birds and reptiles are oviparous, and the cleidoic eggs that they produce contain all the nutrient required until hatching. This nutrient, which is mainly protein and lipoprotein, is synthesised in the liver and oviduct prior to oviposition. Lipoproteins are discussed in Section 4.5, and the control of egg protein synthesis in Section 10.3. Birds excrete a semi-solid urine, and this requires a lower water intake than is possible in ureotelic animals. The metabolic adaption that allows this to occur is the excretion of nitrogen principally in the form of uric acid. Uric acid is sparingly soluble in water and is present in avian ureters largely as a colloidal suspension. This is discussed in Section 5.4. 

Following is a stnictural formula for guanidine, the compound by which migratory birds excrete excess metabolic nitrogen. The hydrochloride salt of this compound is a white crystalline powder, freely soluble in water and ethanol. 

In the case of birds, flight imposes restrictions upon the weight of the animal. This precludes internal water reservoirs necessary for urea excretion. Birds excrete amino nitrogen as uric acid which requires little water. Since uric acid is also the end product of purine catabolism (Section 16.7), nitrogenous waste is disposed of in a common degradation product. 

Excretory processes for xenobiotics are best understood for mammals, with far less work having been done on birds, reptiles, and amphibians. Highly lipophilic compounds show little tendency to be excreted unchanged. In the absence of effective metabolism, they tend to have very long biological half-lives in depot fat. Thus, half-lives of about 1 year have been reported for p,p -DDE in birds, whereas higher... 

Unchanged p,p -DDT tends to be lost only very slowly by land vertebrates. There can, however, be a certain amount of excretion by females into milk or across the placenta into the developing embryo (mammals) or into eggs (birds, reptiles, and insects). 

In terrestrial animals, cyclodienes such as dieldrin, like other refractive lipophilic pollutants, can be excreted in their unchanged forms, notably with lipoproteins, which are exported into milk (mammals), eggs (birds, reptiles, insects), or developing... 

Ammonia is highly toxic. Fish excrete NHj directly birds convert NHj to uric acid. Higher vertebrates convert NHj to urea. 

In birds and amphibia, the green biliverdin IX is excreted in mammals, a soluble enzyme called biliverdin reductase reduces the methenyl bridge between pyrrole III and pyrrole IV to a methylene group to produce bilirubin, a yellow pigment (Figure 32-12). 

Dioxins have effects similar to and potentially even more far-reaching than those of DDT, because they apparently affect a wide variety of species. Predatory birds are especially susceptible, and there is growing evidence that humans may be at risk. Tests have shown that when the concentration of dioxins in the blood of laboratory animals reaches a critical level, reproductive and immune-system defects result. Moreover, recent data indicate that the concentration of dioxins in the blood of the average U.S. resident has nearly reached that level. A major reason is that dioxins are hydrophobic, so they accumulate in fatty tissue rather than being readily processed and excreted from the body. 

Excreted by many primates, birds, reptiles and insects)... 

In biological samples, fenvalerate neither persists for lengthy periods nor is readily accumulated (Smith and Stratton 1986 Cooper 1991). In general, fenvalerate is rapidly (i.e., Tb 1/2 of 6 to 14 h) excreted by amphibians, birds, and mammals has low persistence in various reptiles, terrestrial... 

Metabolism of fenvalerate proceeds by way of oxidation and hydrolysis to produce metabolites considered pharmacologically inactive or inferior to the parent compound. Insects and fish are extremely susceptible to fenvalerate when compared to mammals and birds. Interspecies differences are associated with rates of metabolism, excretion, absorption, esterase activity, and neurosensitivity. 

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