Allantoic acid

Allantoin is the excretory product in most mammals other than primates. Most fish hydrolyze allantoin to allantoic acid, and some excrete that compound as an end product. However, most continue the hydrolysis to form urea and glyoxylate using peroxisomal enzymes.336 In some invertebrates the urea may be hydrolyzed further to ammonia. In organisms that hydrolyze uric acid to urea or ammonia, this pathway is used only for degradation of purines from nucleotides. Excess nitrogen from catabolism of amino acids either is excreted directly as ammonia or is converted to urea by the urea cycle (Fig. 24-10). 

Plants also form the ureides allantoin and allantoic acid, and in some legumes, such as soy beans, these compounds account for 70-80% of the organic nitrogen in the xylem. They appear to function in nitrogen transport.337 As indicated in Fig. 25-18, the hydrolysis to glyoxylate, NH4+, and C02 follows a different pathway than in animals. See also Chapter 24, Section C. 

Mammals other than primates further oxidize urate by a liver enzyme, urate oxidase. The product, allantoin, is excreted. Humans and other primates, as well as birds, lack urate oxidase and hence excrete uric acid as the final product of purine catabolism. In many animals other than mammals, allantoin is metabolized further to other products that are excreted Allantoic acid (some teleost fish), urea (most fishes, amphibians, some mollusks), and ammonia (some marine invertebrates, crustaceans, etc.). This pathway of further purine breakdown is shown in figure 23.22. 

In man and other animals that produce uric acid, the product is not normally toxic but may be upon occasion. The animals that produce allantoin seem to have an advantage over man, since they are not in danger of producing a harmfully insoluble end product. Most fish cleave allantoin through the action of the enzyme allantoinase to allantoic acid. Some fish and amphibia split the allantoic acid further to urea and... 

Urea, methylurea, and dimethylureas react with glyoxylic acid and its methyl ester to give a-substituted hydantoic acid derivatives (16) and substituted allantoic acid derivatives (17), which can be cyclized to 5-substituted hydantoins (18). Although allantoin (18) formation from urea and... 

Many animals degrade uric acid further (Figure 15.13). Urate oxidase converts uric acid to allantoin, an excretory product in many mammals. Allantoinase catalyzes the hydration of allantoin to form allantoate, which is excreted by bony fish. Other fish, as well as amphibians, produce allantoicase, which splits allantoic acid into glyoxylate and urea. Finally, marine invertebrates degrade urea to NH4 and COz in a reaction catalyzed by urease. 

Uric acid, the end point of purine degradation in primates, is excreted. Most other animals, however, oxidize uric acid to allantoin, hydrolyze allantoin to allantoic acid and subsequently convert allantoic acid to urea or other possible excretion products, depending on the animal (Figure 22.8). 

Hydrolysis of allantoin in non-primate animals yields allantoic acid. 

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