METABOLISM OF PROTEINS 565 



at least a simpler one, is that no more extensive synthesis of proteins 

 occurs in the wall of the alimentary canal than is necessary for the 

 needs of the tissues composing it, and, perhaps in addition, for the 

 maintenance of the normal composition of the plasma, and that the 

 decomposition products of the proteins are mainly absorbed as such, 

 and pass in the blood to the tissues for which they are destined. If 

 this is the case, the blood-proteins can no ionger be looked upon as 

 representing the main current of protein supply for the organs, but 

 rather the store of protein material proper to the circulating tissue 

 blood itself, and which confers on it certain chemical and physico- 

 chemical properties (e.g., the due degree of viscosity) necessary for 

 its function. Slowly accumulated, under ordinary conditions, and 

 slowly consumed, this protein store may, of course, be at the dis- 

 posal of the organs in an emergency for instance, in starvation 

 or may be rapidly recruited from the organ-proteins, as after 

 haemorrhage, just as in prolonged hunger the proteins of skeletal 

 muscle may be utilized to feed the heart. That the blood-proteins 

 can serve as nutritive material for the cells without undergoing 

 digestion in the alimentary canal is well shown by the observations 

 of Carrel and Burrows on the growth of isolated tissues in a medium 

 composed of clotted blood-plasma. But, as previously pointed out 

 in another connection (p. 444), a portion, and probably a large 

 portion, of the digested protein is absorbed from the intestine by 

 the blood in the form of amino-acids. Considerable quantities of 

 these compounds can be separated by dialysis from blood drawn 

 off during the absorption of proteins or by the process of vivi- 

 diffusion (p. 48) (Abel). Among these amino-acids, glycocoll, 

 alanin, glutaminic acid, and leucin, have been identified. While 

 the quantity of amino-acids in the blood, which is very small in the 

 fasting animal, is decidedly increased during protein digestion, it 

 is probable that even in starvation amino-acids derived from the 

 decomposition of the body-proteins are not entirely lacking. It 

 has been surmised that they constitute the form in which proteins 

 are transported from tissue to tissue, as well as the form in which 

 proteins are normally utilized by the cells. Although this cannot 

 be regarded as yet established, there is reason to believe that the 

 amino-acids play a great part in protein metabolism, perhaps as 

 great a part as the dextrose does in the metabolism of the carbo- 

 hydrates. There is some evidence that serum-albumin is more 

 directly related to the proteins of the food than serum-globulin. 

 And it is said that during starvation the albumin is relatively 

 diminished, and the globulin relatively increased. It is, of course, 

 not at all improbable that the plasma-proteins have a double source 

 organ-proteins on the one hand, food-proteins on the other. In 

 any case, it is certain that serum-albumin and serum-globulin 

 cannot be interchangeable without far-reaching decomposition, for 



