492 ANIMAL BIOCHEMISTRY 



apparently contradictory finding can be reconciled by assuming major 

 biochemical disturbances when the tissue is transferred from the 

 animal. However, definite proof of such a hypothesis will be necessary 

 to remove the doubts thus cast on the concept of protein turnover in 

 intact animals. In this connection it may be mentioned that the 

 exchange rates for the proteins of animals fed on low-nitrogen diets 

 drop sharply. Apparently the animals have a mechanism for the con- 

 servation of proteins under these conditions. Protein breakdown does 

 occur, of course, and the amino acids are slowly metabolized and lost. 



Animals employ accelerated protein syntheses for replacement of 

 proteins lost during starvation, wasting diseases, and mechanical tissue 

 damage. Regeneration of liver is the classic example. When two-thirds 

 of the liver is removed surgically from young rats, regeneration of 

 the organ is complete in about 10 days in spite of the simultaneous 

 repair of the other tissues injured in reaching the liver. Work of this 

 sort raises the unsolved problem of termination of growth when the 

 organ is regenerated or the wound healed. Why should the remaining 

 liver tissue grow rapidly to its original size and shape and then stop 

 abruptly? Cancer tissue also grows very rapidly but continues increas- 

 ing. Perhaps it lacks the control system so mysteriously effective in 

 normal tissues. 



When animals are in nitrogen balance, protein synthesis and break- 

 down are equal. Isotopic studies have been made of the overall rates. 

 In man the rate is 0.6 to 1.0 g. of protein per day per kg. of body 

 weight. These values correspond to 40 to 70 g. daily for an adult 

 male, a quantity equaling the recommended daily allowance of dietary 

 protein. Dogs likewise synthesize about 0.6 g. of protein daily per kg. 

 of body weight. Rats with shorter protein life make 2 g. daily per 

 kg. of weight. 



Plasma and Milk Proteins 



Like all others, these extracellular proteins must be synthesized in- 

 side cells. In the case of the several plasma proteins, liver cells are 

 the major but not the sole synthetic source. The fibrinogen important 

 to blood clotting and serum albumin may be formed exclusively in the 

 livers of man and the other animals. About 80 per cent of the globulin 

 fraction also comes from liver with the remainder synthesized else- 

 where. The y-globulin group containing the antibodies arises from 

 the widely distributed cells of the reticuloendothelial system. Once 

 the proteins are formed inside cells, their method of secretion is still 

 puzzling. Dialysis can hardly be a factor with molecules of such size. 



The liver withdraws the essential amino acids from the blood and 



