PROTEIN METABOLISM 493 



with a supply of the dietarily non-essential amino acids from the tliet 

 or metabolic reactions combines them to form the plasma proteins. 

 When the diet provides all the essential components, the synthesis is 

 rapid, and high levels of the plasma proteins are maintained. If the 

 diet becomes markedly deficient in total nitrogen or any essential 

 amino acid, protein synthesis drops quickly and the concentration of 

 serum albtimin falls. While somewhat affected, the globulin content 

 of blood is more stable and fluctuates less extensively. Abnormalities, 

 such as hepatitis and cirrhosis, affecting liver function produce similar 

 results because the number of functioning cells is decreased. 



With a large blood supply, the mammary glands of animals might 

 use either the free amino acids or the serum proteins in the synthesis 

 of milk proteins. The serum proteins could function rather directly 

 with limited degradation and recombination into the new molecules. 

 Or they could be split into the individual amino acids for a complete 

 condensation process. In this last event, amino acids from the proteins 

 would probably intermingle to some extent with those free in the 

 blood. 



At least a partial answer is available. Cows and goats use only free 

 amino acids of the blood, and serum proteins do not participate ex- 

 cept as they may be degraded to free amino acids outside the mam- 

 mary gland. The so-called essential amino acids are incorporated as 

 such. Glutamine, glutamic acid, and serine are also taken from the 

 blood rather than being synthesized from other materials. It has not 

 been established whether the remaining amino acids are supplied by 

 the blood or are synthesized as needed. 



Milk production represents protein synthesis on a large scale, espe- 

 cially among dairy cattle bred for this purpose. The scope and com- 

 plexity of this operation explain in part the high ntitritional require- 

 ments of lactating animals. Although milk differs with species, the 

 variations are in the relative amounts rather than in the kinds of 

 proteins. Hence there is no need to assimie differences in mechanism. 



Enzyme Synthesis 



Partly because enzymes are relatively easy to determine quantita- 

 tively by studies of their catalytic activities, much work has been done 

 on enzyme synthesis. In general the earlier remarks on the biosyn- 

 thesis of proteins apply equally well to enzymes. Nucleic acids are 

 definitely involved, amino acids must be supplied, and energy must 

 be coupled to the process. In addition, there are some special features 

 of interest. 



Microorganisms containing little or none of a particular enzyme can 



