4. Their action is specific in that each toxin acts on a particular kind of cell. The fact that a so-called toxin acts on several different kinds of cells, possibly indicates a mixture of several toxins, or action on the same substance in the cells.
5. Toxins are very sensitive to the action of injurious agencies such as heat, light, etc., and in about the same measure that enzymes are, though as a rule they are somewhat more sensitive or “labile.”
6. Toxins apparently have maxima, optima, and minima of temperature for their action, as shown by the destructive effect of heat and by the fact that a frog injected with tetanus toxin and kept at 20° shows no indication of poison, but if the temperature is raised to 37°, symptoms of poisoning are soon apparent. Cold, however, does not destroy a toxin.
7. When properly introduced into the tissues of animals they cause the body cells to form antitoxins ([Chapter XXVII]) which are capable of preventing the action of the toxin in question.
8. The determining test for a toxin is its action on a living cell.
It is true that enzymes are toxic, as are also various foreign proteins, when injected into an animal, but in much larger
doses than are toxins.
A marked difference between enzymes and toxins is that the former may bring about a very great chemical change and still may be recovered from the mixture of substances acted on and produced, while the toxin seems to be permanently used up in its toxic action and cannot be so recovered. Toxins seem very much like enzymes whose action is restricted to living cells.
Just as enzymes are probably produced by all kinds of cells and not by bacteria alone, so toxins are produced by other organisms. Among toxins which have been carefully studied are ricin, the poison of the castor oil plant (Ricinus communis); abrin of the jequirity bean (Abrus precatorius); robin of the common locust (Robinia pseudacacia); poisons of spiders, scorpions, bees, fish, snakes and salamanders.
It has been stated that some enzymes are thrown out from the cell and others are retained within the cell. The same is true of toxins, hence we speak of exo-toxins or toxins excreted from, and endo-toxins or toxins retained within the cell. Among the pathogenic bacteria there are very few which secrete toxins when growing outside the body. Clostridium tetani or lockjaw bacillus, Corynebacterium diphtheriæ or the diphtheria bacillus, Clostridium botulinum or a bacillus causing a type of food poisoning, Pseudomonas pyocyanea or the blue pus bacillus are the most important. Other pathogenic bacteria do not secrete their toxins under the above conditions, but only give them up when the cell is disintegrated either within or outside the body. For the reason that endotoxins are therefore difficult to obtain, their characteristics have not been much studied. The description of toxins as above given is intended to apply to the exo-toxins of bacteria, sometimes spoken of as true toxins, and to the vegetable toxins (phytotoxins) which resemble them.