48 TOXINS ANALOGIES WITH ENZYMES 



use by Martin and Cherry in their demonstration that diphtheria 

 toxin and antitoxin combine chemically. 



Enzymes are also substances of small molecule, and pass 

 through similar niters. When injected into suitable animals 

 enzymes give rise to the production of anti-enzymes, which are 

 exactly equivalent to antitoxins. Thus we see that in many 

 points the process of intoxication with the bacterial exotoxins 

 presents close analogies with the destruction of proteids, etc., by 

 enzymes; and to these we might add the suggestion that it is 

 very probable that these exotoxins act, partly at least, by a 

 process of hydrolysis. This suggestion is based partially on the 

 fact that the process of haemolysis is almost certainly one of 

 hydrolysis, and partially on the appearance of poisoned cells, 

 which look as if they had absorbed water and became partly 

 dissolved. 



There is, however, one feature in which the exotoxins and 

 their allies, the bacterial haemolysins, are absolutely different from 

 the enzymes. In the case of the enzymes a molecule attaches 

 itself to the substance to be attacked, water is absorbed, and the 

 whole complex molecule breaks down ; and in this process the 

 molecule of enzyme is set free, and is again ready to attack 

 another molecule. Thus a very small amount of the active 

 substance can decompose a large amount of fermentable substance. 

 The toxins do not behave in this way, and, as far as we know, 

 a molecule of toxin which has united with one molecule of proto- 

 plasm is never set free to attack another. 1 The proof of this is 

 not very direct, and rests mainly on the fact that the amount of 

 toxin necessary to kill two animals of the same species varies roughly 

 with their weight. Thus the minimal lethal dose of diphtheria 

 toxin for a guinea-pig of 250 grammes will not kill one of 400. 

 If the molecule of toxin could attack one molecule of cell 

 substance after another in the same way as an enzyme, we should 

 expect it to do so, though after a longer interval. It must be 

 confessed, however, that this proof is not very striking ; excep- 

 tions frequently occur, since, as a rule, older animals are less 

 susceptible than younger ones in proportion to the body-weight. 

 But it is certainly true with regard to the bacterial haemolysins, 

 since we can test them on the same sample of blood, and when 



1 It may possibly undergo dissociation, and be set free to attack another 

 molecule, but this is a different process : the molecule first attacked is not 

 injured. 



