474 ANIMAL BIOCHEMISTRY 



dergo further transformations and are excreted, primarily in urine. 

 Adenine and guanine, the purines of nucleic acids, are metabolized 

 as in Figure 20-4. Primates (including man), birds, and some reptiles 

 take the process only to uric acid and excrete that. Other mammals 

 and reptiles excrete allantoin, and the remaining animals break 

 purines down still further to mea in some cases and to ammonia in 

 others. 



By contrast, the free pyrimidines are degraded more extensively in 

 mammals than are the purines. Although the reactions are largely 

 luiknown, the nitrogen appears as urea and ammonia. The carbon 

 atoms are presumably luilized, but these reactions also are imknown. 



Animals synthesize purine and pyrimidine derivatives from amino 

 acids and various metabolic intermediates. These reactions have 

 been extensively studied and elaborate pathways postulated. Rather 

 than summarize these here, however, the reader is referred to recent 

 reviews on the subject. Once the nucleotides are formed, they are 

 converted to nucleic acids. The necessary condensation reactions have 

 been carried out, using extracts of cells and high-energy compounds 

 to drive the reaction. Although the process is complex, its isolation 

 from the living cell assures rapid progress in understanding many of 

 the individual steps. Great advances from work on the synthesis of 

 purines, pyrimidines, and all intermediates leading to nucleic acids 

 can be expected in the next decade. 



Protein Nutrition 



The class of foodstuffs called proteins has long been recognized as 

 an essential dietary component for animals. As commonly the case 

 for other nutrients, the need for protein varies with the species and 

 to some extent the individual. In part, the requirement for protein 

 is a requirement for nitrogen, since dietary protein contributes the 

 major portion of the ingested nitrogen available to the animal. 



Early workers knew that digestion hydrolyzed much of the protein 

 to amino acids. Yet they apparently felt that the protein also played 

 a role distinct from that of the amino acids themselves, since animals 

 failed to grow normally on diets containing amino acids in place of 

 proteins. Later discovery of threonine in proteins and its incorpora- 

 tion into experimental diets avoided part of the difficulty encountered 

 previously. Subsequent work on vitamin B^o, which often accompanies 

 animal proteins, further clarified the problem and supported the 

 concept dating to about 1900 that proteins served solely as sources of 

 certain important amino acids. More evidence has been accumulated 

 from the numerous studies showing that human beings, other mam- 



