METABOLISM OF PROTEINS 569 



even without their complete detachment from the protoplasm, indi- 

 vidual amino-acids or mixtures of amino-acids, or polypeptide groups, 

 are cut out of the protoplasmic mass. 



It is now necessary to follow, as far as is possible, the steps 

 in the degradation of the' body-proteins. Since there is reason to 

 believe that these, like the food-proteins, are first split up into the 

 amino-acids from which they were originally synthesized before 

 undergoing further decomposition, a study of protein metabolism 

 is to a great extent a study of the metabolism of amino-acids. In 

 this study it is for most purposes impracticable, even if it were 

 desirable, to distinguish between amino-acids directly derived from 

 the food, and which have not yet been, and may never be, built up 

 into tissue-proteins, and those derived from the tissue proteins. 

 There is nothing to indicate that the fate of a given ammo-acid, 

 once it has reached the blood, depends in the least upon its source. 

 It may be said at once that the katabolism of the amino-acids is not 

 a single and uniform process, one step in which inevitably follows 

 another till the final end-products are reached. On the contrary, 

 certain of the stages may become the starting-points of syntheses, 

 which may lead back to the original or to another protein, or it 

 may be to sugar or to fat. The extent of such synthesis, and 

 even in some degree the stage from which it starts, may be assumed 

 to depend upon the needs of the tissues and the relative abundance 

 of protein and of other foods. . 



/Formation of Amino-Acids from Tissue-Proteins.^fThat amino- 

 acids are formed in the metabolism of the cells and by the action of 

 intracellular enzymes is indicated by the fact that proteolytic en- 

 zymes (proteases) are invariably present in the tissues! and can 

 be obtained from them by appropriate methods e.g., by subjecting 

 the organ in a finely divided state to a high pressure and collecting 

 the expressed juice. Not only do unicellular organisms, like leuco- 

 cytes, yeast cells, and bacteria, which must naturally depend upon 

 themselves alone for all enzymatic reactions, yield ferments which 

 have the power of splitting proteins, peptones, and polypeptides 

 into amino-acids, but their existence has been demonstrated in 

 practically all the organs of the higher animals and man. When a 

 piece of liver, e.g., is removed with aseptic precautions and kept at 

 body-temperature, extensive auto-digestion occurs, and ammonia 

 and other basic substances, glycin, and tryptophane, appear among 

 the products. Tyrosin appears so early that it is scarcely possible 

 to doubt that it must be a product of protein decomposition in the 

 liver-cells under normal conditions a decomposition which could 

 be observed also in the organ in situ were the circumstances as 

 favourable. The circumstances are less favourable in an organ 

 whose circulation is going on, because the amino-acids are removed 

 by the blood as they are formed. Further, it is to be assumed that 



