Nitrogen Excretion 205 



those annelids which have been examined, purines are excreted and enzymes 

 for degrading them are lacking. 



Insects are predominantly uricotelic animals and differ greatly in their uri- 

 colytic enzymes. The distribution appears to be random and varies with time 

 in the life cycle (Table 31). Larvae of the blowfly, Lticilia, for example, con- 

 vert much of the uric acid from protein breakdown to allantoin; uricase is 

 absent from the pupae, however, and reappears in the adults. "^ 



Molluscs, like insects, differ in their uricolytic enzymes. In general, uricase 

 is found in lamellibranchs and in some gastropods. Allantoinase, allantoicase, 

 and urease are present in the marine lamellibranchs (Mytilus), but the urease 

 is lacking in Anodonta, and only uricase is found in gastropods {Helix, Plan- 

 orhis). Comparison of uricolysis in ammonotelic and uricotelic gastropods 

 would be of interest. Apparently, as with nitrogen breakdown, there is some 

 lability in purine degradation among molluscs. 



One cyclostome fish, Lmnpetra fluviatilis, like the annelids, has no enzyme 

 for degrading purines. The liver of the dipnoan Protopterus anesteus -** and 

 of the elasmobranch Raja contains uricase, allantoinase, and allantoicase. 

 Among teleost fish those of one group have uricase, allantoinase, and allan- 

 toicase, whereas those of another group lack the allantoicase; " their differ- 

 entiation (Table 31) seems unrelated to habitat. 



Amphibians degrade purines to urea but not to ammonia. Reptiles, whether 

 they are ureotelic or uricotelic, lack allantoinase and allantoicase; and some, 

 if not all, of them lack uricase; hence they excrete their purines as uric acid. 

 Likewise in birds uricolysis is absent; there may be neither uricase nor allan- 

 toinase in the liver. Mammals fall into two groups in purine metabolism; most 

 of them have uricase and excrete some allantoin. Man and other anthropoids, 

 howe\'er, lack this enzyme and excrete uric acid. Among dogs Dalmatian 

 coach hounds excrete approximately eleven times as much uric acid nitrogen 

 and half as much allantoin nitrogen as other breeds, apparently because the 

 Dalmatians fail to reabsorb uric acid in the kidney as other dogs do for con- 

 version to allantoin in the li\'er. The necessary enzymes are present in the 

 liver of all breeds. ^- 



Two evolutionary trends have occurred in purine metabolism: (1) Degrada- 

 tion completely to ammonia requires a chain of enzymes and is accomplished 

 only among a few groups of lower invertebrates. (2) Higher groups, both 

 invertebrate and vertebrate, break the chain at various points, and among the 

 most advanced groups (insects, birds, and most mammals) purines are excreted 

 as uric acid. The evolutionary trend here is elimination of enzymatic steps. 



Florkin ■'■' has suggested that the end-products of protein and of purine 

 catabolism tend to be the same. This is indicated in Figure 36. Animals, 

 whether ammonotelic, ureotelic, or uricotelic, tend to convert purines to the 

 same end-products as those of proteins. Here is an example of biochemical 

 con\'ergence. Exceptions are found among molluscs and mammals. 



REFERENCES 



1. Albrecht, p. G., ;. Biol. Cheni. 56:483-487 (1923). Urease and uricase in 

 marine molluscs. 



2. Bacon, R. F., Philippine ). Sci. 4: 165 (1909). Excretion by python. 



3. Bahl, K. N., Quart. J. Micr. Sci. 85, 343-390. (1945) Excretion in earthworms. 



4. Baldwin, E., Biocheni. ). 29:252-262 (1935). Arginase in invertebrates. 



