136 Discussion 



Hollaender: In answer to your first question, I gave you the toxic 

 level for thiouronium. Routinely we use 6 mg. per standard mouse. 

 We get very good protection at 4 • 4 mg. If you go to 8-10 mg. the mouse 

 will be prostrate and a good percentage will die. The range in which it 

 is safe to use this compound is considerably greater than with cysteamine 

 or cysteine. 



Now in regard to your second question, on survival of these bacteria : 

 we emphasize in all our reports that we have determined the survival on 

 the basis of colony-forming organisms. We are very cautious on this 

 point because, as Dr. Stapleton has shown earlier, during discussion 

 after your paper, these bacteria are not dead when they are unable to 

 form visible colonies. They still can respire for a considerable time. As 

 to the question of whether we get more colonies after 72 hours, we 

 check this very carefully. We usually make two counts; for instance 

 when we study the effects of lower temperatures the plates stay for 24 

 hours at 18° and then for at least 24 hours at 37°. The delayed effect on 

 B/r, i.e. the appearance of late colonies, is very small if X-rays are used. 

 Now if you go to the long u.v. you may have to wait as long as five 

 days before all colonies appear, but that is another point. 



Alper: The same thing happens with Esch. coli B. I have checked this 

 in some, not all, experiments and found no more colonies after 48 than 

 after 24 hours. 



Hollaender : If you use u.v. it is a different story, you have to be more 

 careful there. 



Stapleton: With regard to Dr. Laser's question on why we call it 

 recovery, you can define recovery in many different ways and it is 

 apparently prevention of some lesion that would ordinarily lead to 

 death. It could be a lack of an inhibition. 



Laser: You could call it prevention rather than recovery. 



Stapleton: Yes. 



Van Bekkum: Dr. Hollaender, you have commented in your paper on 

 the protective effects of some jS-alkyl and iV-alkyl derivatives of cyste- 

 amine. Dr. de Groot and I have recently studied a few similar com- 

 pounds with regard to their protective activity in vivo (mice) and in 

 vitro (isolated rat thymocytes). The compounds referred to are the 

 following : 



(1) *S'-ethylcysteamine:— CgHs-S-CHa-CHa-NHa 



(2) iV-ethylcysteamine :— HS • CHg • CHg • NH • Cg Hg 



(3) iV-diethylcysteamine:— HS-CHa-CHa'N (C2H5)2 



In vivo the *S'-ethyl derivative (1) has some protective activity, which is 

 less than that of cysteamine. The iV-ethyl derivative (2) is more toxic 

 and less protective than cysteamine, the iV-diethyl compound (3) is still 

 more toxic to mice and has practically no protective activity in vivo. 

 On the other hand, in the thyrhocyte system both compounds (2) and 

 (3) afford considerable protection, while the tS-ethyl derivative (1) is 

 completely ineffective. It seems, therefore, that the amine group is 

 most important for in vivo protective activity, while the sulphydryl 

 group seems to be required for in vitro protection. 



