MECHANISM OF PROTEIN HYDROLYSIS AND IMMUNIZATION 45 



ably not readily absorbed through the intestinal wall. But the 

 leucocyte itself is suspended in the blood stream, and it must on 

 no account permit the formation of a poisonous group, or if such 

 is formed it must be retained within the substance of the leuco- 

 cyte until it is further transformed and rendered innocuous, or 

 is otherwise guarded (for example by the red corpuscles) or 

 extruded from the blood stream. 



Something as to what this implies may be conceived from an 

 attempt to visualize the protein molecule, even in the vaguest 

 way. A typical protein, for example, is globin, the basis of 

 hemoglobin. Plimmer gives this formula for globin : C 72fl , H 1174 , 

 N 194 S 3 O 214 . In the process of digestion, this enormously large 

 and complex molecule undergoes hydrolytic cleavage again and 

 again. A single molecule of protein thus cleaved (always in such 

 a way that each new molecule contains a modicum of nitrogen 

 along with the other elements) makes up successively molecules 

 of proteoses and of peptones and polypeptids, and ultimately, if 

 the cleavage is carried far enough, the disintegrated fragments 

 constitute the relatively simple amino-acids, which form the 

 building stones of all proteins, and of which almost a score of 

 different types are now known, a few of which have become rea- 

 sonably familiar in recent medical literature under the names of 

 glycine, alanine, valine, leucine, tyrosine, etc. The simplest of 

 these, glycine, has the formula C 2 H 5 NO 2 ; but the others are not 

 much more complex ; leucine, for example, being fairly typical, 

 with the formula C 6 H 13 NO 2 . 



It will be seen that something like two hundred molecules of 

 the amino-acids each with its single atom of nitrogen would 

 result from the final cleavage of a single ordinary protein mole- 

 cule. So the digestion or proteolysis of even a small group of 

 protein molecules is like the 'tearing to pieces of a building com- 

 posed of many thousands of individual bricks, stones, and timbers. 

 It is obvious that the task thrust upon the leucocytes by the in- 

 truding protein molecule is by no means a simple one. Yet 

 there is abundant experimental evidence that such proteolysis 

 of an invading protein does take place parenterally ; and to the 

 present writer, at any rate, it seems highly probable that it is 

 the blood corpuscles, rather than any of the more specialized 

 tissues, that perform this function. 



The fact that only small quantities of foreign proteins, if ex- 

 perimentally introduced into the blood stream, are transformed 

 (the major part being excreted by the kidneys unchanged), is 

 obviously consonant with the relatively small bulk of the leu- 

 cocytes in their totality, as above referred to. 



To be sure, there do remain residual molecules, after final 



