The Effect of Cysteine on Tetanus Toxin *

In the course of some experiments with tetanus toxin it was noted that cysteine, when added to the toxin with the intention of preserving it from oxidation, had, on the contrary, a deleterious effect. Other experiments showed that on diphtheria toxin there was but little effect, and on botulinus toxin and on ricin none at all. Locke and Maint have mentioned that cysteine can play a role in toxin production by both the diphtheria and the tetanus organisms, and they believed that it did this by withdrawing copper from the culture medium. They showed that anything tending to alter a favorable Cu/Fe ratio affected toxin production adversely, and, although chiefly interested in this phase of the subject, they mentioned that the potency of toxic filtrates could be decreased by admixture of cysteine. In the experiments recorded below cysteine HCl hydrate, neutralized with NaOH and sterilized by filtration, and saline dilutions of tetanus toxin precipitated with ammonium sulphate were used. The minimal lethal dose of the toxins for white mice weighing 20 g. varied between 0.001 and 0.0004 mg. Incubations were at room temperature. Saline solutions containing 0.05 per cent cysteine and 10 m.l.d. of toxin per cc. are, after standing for half an hour, completely atoxic for mice when 1.0 cc. amounts are injected, and as low a cysteine concentration as 0.01 per cent shows some destructive action. When larger amounts of both substances are used, 0.5 per cent cysteine and from 2500 to 25,000 m.l.d.'s of toxin per cc., the results are more striking. One cubic centimeter quantities of mixtures containing the smaller amounts of toxin are, after 24 hours, completely harmless for mice, and after two days all mixtures are atoxic. If the mixtures are made in serum or in defibrinated blood, however, the reaction is very largely inhibited. The reaction of cysteine with toxin does not seem to be of the same nature as is that of cysteine with copper. In the latter, slight warming of the copper-cysteine mixture is all that is necessary to