CYSTEAMINE 



which ionizing radiations degrade polymers. We set out hoping to find such a 

 mechanism and after having studied the effect of X-rays on aqueous solutions of 

 about eight chemically different polymers, we find that almost every one shows a 

 different behaviour. Some degrade only in the presence of oxygen while others 

 degrade equally readily in its absence ; still others crosslink to give gels. The whole 

 picture is very complex and it is not possible to deduce a general mechanism involving 

 oxygen from experiments with one material. 



In particular the suggestion of Magat that DNA degrades according to this general 

 pattern seems to be unfounded since the initial X-ray induced reaction is independent 

 of oxygen. Further, both polystyrene in chloroform and DNA in water continue 

 to degrade after the irradiation with X-rays is complete (after-effect) but the addition 

 of an — SH compound immediately after irradiation hastens the decomposition of 

 polystyrene (see paper by Fox, page 61) while Errera has shown that it reduces the 

 ' after-effect ' of DNA. The same simple peroxide mechanism can obviously not 

 apply to both systems. 



H. B. New^combe : In Magat's discussion, one of the points made by Patt 

 appears to have been overlooked, namely that an effect can sometimes be demon- 

 strated even when the chemical is added just after irradiation {e.g. in the case of 

 cysteine and thymus leucocyte suspensions). 



At this time, the free radicals would have disappeared and it must be supposed that 

 the protection must occur either through reaction with the induced peroxides or 

 through an effect on the post-irradiation metabolism of the cells. 



R. Latarjet : L'effet final de la cysteamine est la resultante de divers processus 

 dont les importances relatives dependent des conditions experimentales. En par- 

 ticulier, a Faction protectrice /^r^'coce peut succeder une action tardive en sens contraire. 

 En effet, la cysteamine peut favoriser Faction radiomimetique des peroxydes radio- 

 formes. L'experience suivante en fait foi, effectuee par Brinton dans mon labora- 

 toire, conformement a une prevision theorique du Fox. 



Des bacteries (E. coli, souche B) sont mises, pendant 10 min., en presence d'hydro- 

 peroxyde de cumene a la concentration de 1,5 IQ-^* a 37° C. On les soumet alors 

 a une breve et violente illumination qui augmente beaucoup Faction toxique du 

 peroxyde (preuve que celui-ci agit par ses produits de decomposition). On elimine 

 alors le peroxyde par dilution, et on etale les bacteries sur gelose en presence de 

 diverses substances. Apres incubation, on denomblre les colonies bacteriennes. 

 Voici les resultats d'une experience : 



Cette experience montre que, loin de proteger, la cysteamine peut agir. apres 

 Firradiation, par un processus qui augmente tres notablement Faction cytotoxique 

 de certains intermediaires chimiques produits dans les tissus par Firradiation. 



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