METABOLISM OF PROTEINS 577 



It is not any difference in the kind of protein offered them which 

 determines the difference in structure and action between one organ 

 and another. In this quarry alongside of the plasma proteins the tissue 

 cells find what is probably more important for their individual nutrition, 

 the building-stones of the shattered food-protein molecules. They are 

 only under exceptional circumstances confronted with intact molecules 

 of food -proteins. ' The body cells do not know what the kind of food 

 was ' (Abderhalden). In the case of the more highly developed tissues, 

 at least, no mere change of food will radically alter the structure of the 

 cells, nor even, as we have seen, the composition of the tissue proteins. 

 A cell may be fed with different kinds of food, it may be overfed, it may 

 be ill-fed, it may be starved; but its essential peculiarities remain as 

 long as it continues to live. What may be called its organization, per- 

 haps at bottom a more or less metaphorical expression for its essential 

 physico-chemical make-up, dominates its nutrition and function. 



' We must assume that many of the enigmatical properties of living 

 matter depend upon the activity of intact protein molecules. We can 

 obtain some idea of the possible variety in "the combinations of the 

 '* building-stones " of the proteins by recalling the fact that they are as 

 numerous as the letters of the alphabet, which are capable of expressing 

 an infinite number of thoughts. Every peculiarity of species and every 

 occurrence affecting the individual may be indicated by special com- 

 binations of the "building-stones " that is to say, by specific proteins. 

 Consequently we may readily understand how peculiarity of species 

 may find expression in the chemical nature of the proteins constituting 

 living matter, and how they may be transmitted through the material 

 contained in the generative cells ' (Kossel). Add to the great variety 

 of compounds rendered possible by the enormous number of permuta- 

 tions and combinations of the protein ' building-stones,'* the still greater 

 variety rendered possible by the fact that the quantitative relations of 

 given amino-acids may vary greatly in different proteins, and it will be 

 seen what a practically infinite power of functional adjustment and 

 reaction, correlated with a practically infinite variety of chemical 

 changes in the midst of which the cell still preserves its specificity 

 through and through, may be conferred upon the living substance by 

 its content of protein. 



Some have supposed that the protein of the living substance is es- 

 sentially different from dead protein, especially in possessing a character- 

 istic instability, a prodigious power of dissociation and reconstruction. 

 All the older theories which attempted to explain this alleged difference 

 require revision in accordance with the newer chemistry of proteins, 

 and speculations on the subject are probably in any case premature 

 till the constitution of the proteins is thoroughly understood. In the 

 meantime it is enough to say that the velocity of the reactions into 

 which the proteins of living protoplasm or their constituent amino- 

 acids may enter must depend upon intracellular conditions, which may 

 vary rapidly and within wide limits. For example, enzymes may be 

 present in greater or smaller concentration, or be activated and aided 

 more or less powerfully by other substances, or by a more or less favour- 

 able chemical reaction of the medium. The protein itself, too, or such 

 part of it as is ready for decomposition, may exist in a physical con- 

 dition now more and again less favourable to the attack of the enzymes. 



* Twenty different amino-acids, each used only once, but in a different order, 

 would be capable of forming about 2,000,000,000,000,000,000 (two thousand 

 million times a thousand millions) of different polypeptides, all containing the 

 various amino-acids in the same proportions (Abderhalden) . 



