PROTEIN METABOLISM 475 



mals, ami birds can grow or maintain themselves when led amino 

 acids in place of proteins if the other known necessary factors are 

 supplied. 



On the other hand, some of those microorganisms requiring ex- 

 ternal supplies of amino acids grow best when certain peptides are 

 present. This rather widespread observation indicates that particular 

 peptide linkages are important to cells having limited capacities to 

 make them. Therefore, inclusion of particular peptides in the medium 

 permits more rapid growth. Although the best evidence of this type 

 exists only for microorganisms, there are similar indications for ani- 

 mals in reports that growth is better on certain proteins than it is on 

 mixtures of amino acids matching the proteins in composition. Such 

 work can be interpreted in several ways. 



First, particular peptides may stimulate growth when fed to supple- 

 ment the synthetic capacity of the animal. Second, the protein may 

 be contaminated with some known or tmknown factor needed by the 

 animal and not otherwise supplied adec|uately in the diet. Mixtures 

 of isolated amino acids would be deficient in such materials. Third, 

 the analysis of the amino acid content of the protein may be inaccurate 

 or incomplete, making the mixture an imperfect match. Most analyses 

 recjuire hydrolysis of the protein, and several of the amino acids are 

 partly destroyed. Evaluation of this effect is difficult and may lead to 

 underestimation of amounts present. Furthermore, there is reason 

 to suspect that ornithine and citrulline may occur in at least some 

 proteins, but direct evidence is lacking. Hence there may actually 

 be components of whose existence we are unaware. As a consequence, 

 it cannot be assumed that proteins serve solely as dietary supplies of 

 amino acids. 



In blood transfusions advantage is taken of the fact that intact 

 proteins can be used directly when injected into the circulatory system. 

 Thus the protein taken from another individual can replace that of 

 a person or animal that accidentally loses blood. The same principle 

 is involved in the use of insulin by diabetics. However, there are 

 limitations to the practical application of this idea because injection 

 of proteins from other species or even other individuals can cause 

 serious physiological reactions. Awareness of this possibility has led 

 to the tyjiing of blood for transfusion to assure use of blood that 

 Avill not harm the recipient. 



The problem of food allergies probably is related to this inter- 

 ference by foreign proteins. Proteins of certain kinds apparently pass 

 directly or as large fragments through the intestinal wall into the 

 blood stream. These foreign macromolecules then produce the un- 



