480 



ANIMAL BIOCHEMISTRY 



glycine by the mechanisms of Figure 20-3. Neither dogs nor mice 

 require arginine, at least under some conditions. The influence of 

 physiological state is illustrated in Table 20-1 by comparison of the 

 values for human beings. Arginine and histidine are not essential in 

 the diets of normal adults. When a state of positive nitrogen balance 

 is needed temporarily, a dietary requirement for these two amino 

 acids might be expected. The values of the table cannot be taken 

 as absolute since there appear to be rather large differences between 

 apparently similar and normal individuals. Thus allowances have 

 been suggested that are greater by a factor of two than the minimum 

 values tabulated here. 



Since proteins differ considerably in the relative proportions of 

 the amino acids they contain, they differ also in the quantities needed 

 to supply the required levels of all the essential amino acids. Table 

 20-2 compares proteins from several sources with regard to main- 



TABLE 20-2. Requirements of Various Proteins for Nitrogen Balance 



in Normal Men 



tenance of nitrogen balance in normal men. These requirements are 

 subject to all the conditions discussed above as affecting the needs 

 for essential amino acids. Comparisons of the levels in the table 

 reveal that it is advantageous to supplement vegetable diets with 

 animal proteins. Good feeding practice in the livestock industries 

 recognizes this fact and depends upon waste animal materials from 

 meat-processing and food-handling operations as cheap sources of 

 animal proteins. 



Protein Storage 



When protein intake is increased, excretion lags somewhat behind 

 ingestion, indicating that nitrogen is being stored. After a time the 



