Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Oxidative Degradation of Amino Acids. The Urea Cycle
Birds, snakes, and lizards excrete uric acid.

Uricotelic animals (birds, snakes, and lizards) excrete amino nitrogen primarily in the form of uric acid (Fig. 19-25). Uric acid is also the major end product of purine METABOLISM in primates, birds, and reptiles. The uric acid molecule has a rather complex Structure: it consists of two fused rings making up the so-called purine Nucleus. Purines also include adenine and guanine, which are Components of the corresponding NUCLEOTIDES. The synthesis of uric acid from amino groups is a multi-step process because the purine nucleus is built up gradually from A number of simple precursors. Fig. 19-25 shows the Water/144.html">Origin of the carbon and nitrogen atoms of uric acid, as determined from experiments using isotopically labeled precursors. We will examine the complex pathway of purine and uric acid synthesis in Ch. 22. Here, it is sufficient to note that this process involves many steps and requires a significant expenditure of energy. Thus, uricotelic animals must pay a substantial price for the advantage they gain by excreting amino nitrogen in a semisolid form. To some extent, however, this is compensated for by the fact that not only amino nitrogen is excreted as uric acid; it is also the end product of purine Catabolism (Ch. 22).

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Fig. 19-25. Excretion of amino nitrogen as uric acid in birds, snakes, and lizards.

The nitrogen atoms of uric acid (shown in red) originate from the α-amino groups of Amino Acids; they are incorporated into the uric acid molecule via a highly complex pathway.

Uric acid is called an acid because it exists in tautomeric forms capable of ionizing to form urates. Na⁺ and K⁺ urates are only slightly more soluble in water than uric acid itself.

Fig. 19-26. San Lorenzo Island off the coast of Peru is one of those islands with guano deposits. Hundreds of thousands of birds nest on these islands, so over many centuries, entire mountains of guano, consisting mainly of solid uric acid, have accumulated here. Guano is a valuable fertilizer; a ton of it costs more than 100 dollars. In the second half of the nineteenth century, entire fleets of sailing ships distributed guano all over the world, trading with many countries.

Off the coast of South America, many islands that serve as nesting sites for massive bird colonies contain vast reserves of uric acid (Fig. 19-26). These rich guano deposits are mined, and the guano is used as fertilizer. In this way, organic nitrogen is returned to the soil and can be reused for Amino acid synthesis by plants and soil microorganisms.