MECHANISM OF PROTEIN HYDROLYSIS AND IMMUNIZATION 71 



assumption is strongly supported by the observed similarity of 

 action of these toxins and sundry narcotic drugs of the familiar 

 alkaloids. And these alkaloids have known chemical formulae 

 that at once reveal their chemical relationship with the poly- 

 peptids. 



Morphine, for example, is C 17 H 19 NO 3 ; strychnine is 

 C 21 H 22 N 2 O 2 ; and glycyl-glycine, the simplest of the polypeptids 

 has the formula C 8 H 16 N 4 O 6 . If we knew how to combine the 

 amino-acid called valine (C^uNC^) with the amino-acid called 

 leucine (C 6 H 13 NO 2 ), we should have a molecule with the compo- 

 sition C 11 H 24 N 2 O 4 , in still closer simulation of the strychnine 

 molecule. The combination of two molecules of the amino-acid, 

 glycine with one of leucine, which has been effected, gives the 

 formula C 17 H 33 N 3 O 7 . 



Of course, we can by no means assume that because the com- 

 binations of atoms in a given pair of molecules are similar the 

 gross physiological effects of these molecules on the organism will 

 be identical or even comparable. To disprove any such hypoth- 

 esis, nothing more would be necessary than to consider the chem- 

 ical composition of certain other alkaloids, for example quinine, 

 which, with its formula C 20 H 24 N 2 O 2 seems to have close touch 

 with strychnine ; yet which, as every tyro in medicine knows, is 

 very fundamentally different in its physiological action. 



It should be understood, however, that in the modern view a 

 drug acts on only those tissues with which it can enter into 

 chemical union ; and that the markedly different physiological 

 actions of drugs depends upon the affinity for them shown by this 

 or that type of cell in the tissues of the body. Quinine and strych- 

 nine appear to us radically different drugs, because their effects 

 on the human system are so conspicuously diverse ; yet their 

 chemical composition proves their close similarity; and a rea- 

 sonable explanation of the difference in their effect is given if 

 we assume that the precise combination of "side-chains" in the 

 strychnine molecule chances to fit in with the scheme of the 

 molecules making up the substance of the central tissue of the 

 brain; whereas the side-chains of the quinine will link it with 

 other tissues that are in themselves no less profoundly affected 

 than are the brain tissues by strychnine ; but which are not so 

 vitally and intimately associated with the life-processes of the 

 organism as a whole. 



It is in the fact that most alkaloids, in common with toxins, 

 find their affinities in the cells of brain and spinal cord that the 

 seeming toxicity of these substances lies. Many a "harmless" 

 compound may affect muscle cells, let us say, far more pro- 

 foundly than the brain cells are affected by morphine or strych- 

 nine, yet have no "toxic" effect, because the muscles do not 



