DIGESTION OF THE PROTEINS 245 



minates the process. It will be recollected that a similar equilibrium 

 between the tissues and the blood obtains in the case of glucose and its 

 anhydride, glycogen, a subnormal glucose concentration in the blood 

 leading to the splitting up of glycogen and liberation of glucose by the 

 liver to replenish the blood and thereby the muscular tissues to which 

 it carries the carbohydrate fuel which furnishes the energy-equivalent 

 of muscular work. In the same way we may suppose that the amino- 

 acids in the tissues stand in a relation of equilibrium to the amino-acids 

 in the blood, on the one hand, and to the proteins of the tissue on the 

 other. 



The proteins which are found in the various tissues of the body are 

 highly specialized and characteristic of the tissue-elements in which 

 they occur. The proteins in the various Connective Tissues are especi- 

 ally diverse in their composition and characteristics. Thus the pro- 

 teins of fibrous tissue are extraordinarily rich in glycocoll, and those of 

 elastic tissue are especially rich in glycocoll and also in glutamic acid. 

 Among other highly specialized proteins may be mentioned the keratin 

 of horny epidermal tissues which is exceptionally rich in cystine, the 

 protamines which are exceptionally rich in diamino-acids, and the 

 mucins which contain an amino-carbohydrate radical. In a less degree 

 the proteins of every type of tissue and cell betray, either in biological 

 or physical behavior or directly, in chemical composition, evidence 

 of distinctive architecture. 



The question of the locality of Protein Synthesis has evoked a very 

 great deal of discussion and prompted a variety of investigations. 

 Arguing that only the normal blood-proteins, the serum-albumins and 

 serum-globulins could be tolerated in the circulation, and assuming 

 "that the amino-acids were not, as we now know that they are, absorbed 

 as such, Abderhalden supposed that the amino-acids which result 

 from digestion are synthesized into protein in the intestinal epithelium, 

 just as the fatty acids and glycerol are synthesized into fats during 

 their passage through the intestinal wall, but with this difference, 

 namely, that whereas the fats which are thus synthesized bear a very 

 close relationship to the fats which were present in the diet, the protein 

 which was presumed to be synthesized must be limited to the blood- 

 proteins characteristic of the species. 



This hypothesis, however, made it necessary to view the process of 

 protein synthesis as a very roundabout and uneconomical one, for 

 since the proteins of the tissues differ so markedly from one another 

 and also from the blood-proteins, the blood-proteins evidently could 

 not be built up directly into tissue-proteins, but must first be broken 

 down in the tissues themselves, their amino-acids resorted and rear- 

 ranged, and resynthesized into the characteristic proteins of the tissue 

 in question. Thus the synthetic work of the intestine would have to 

 be undone again in each of the tissues. Moreover in many of the 

 tissues the process of redegradation and resynthesis would involve an 

 extraordinary amount of waste of ami no-acid material. For example, 



