30 A. KOHN AND M. LION 



did not have the same effect. Potassium iodide was more 

 effective than several other potassium saUs. The potassium 

 effect depended on the concentration of bacteria. 



Kohn: Dr. Avi-Dor mentioned the metabohc importance of 

 potassium and sodium, and that this effect required a certain 

 time to manifest itseff. In our laboratory Dr. Lion has per- 

 formed a number of experiments in which addition of salt was 

 made one second before freezing. Nevertheless the protective 

 effect was obtained. Regarding the ion effect reported by you, 

 is it a minimum effect or do you get an optimum curve? With 

 glucose the viability rises with concentration until a certain 

 maximum viability is reached. Further additions of glucose do 

 not alter the viability. With thiourea and sodium iodide, how- 

 ever, once you have reached the optimal concentration, further 

 addition decreases the viability rapidly. 



Avi-Dor: Regarding your first question: we measure respira- 

 tion which is a secondary effect of potassium. We do not know 

 whether the primary effect is instantaneous or not. As to your 

 second question, we did not reach an inhibitory concentration 

 with potassium. 



Mayer: Have you tried to correlate the fluorescence quench- 

 ing action of thiourea derivatives and their protective action? 

 What is your evidence that free radicals are involved? Does an 

 increase in thiourea concentration protect against a higher 

 oxygen concentration? Finally has dinitrophenol any effect? 



Kohn: Only three thiourea derivatives were found to be 

 active: thiourea, monomethyl thiourea and symmetric dimethyl 

 thiourea. Regarding the relation between oxygen concentration 

 and that of thiourea, no specific experiments were carried out. 



Shilo: In some bacteria, some unicellular algae and certain 

 higher plants carotenoid pigments protect against photo- 

 oxidative death. It occurs to me that a comparison between 

 bacteria containing carotenoid pigments and mutants lacking 



