EFFECTS OF TEMPERATURE: CELLULAR SYSTEMS 



791 



a constant retardation effect are shown in Fig. 15-12. It may be noticed 

 that although N, iV'-diethylurethane is much more potent than urethane, 

 the temperature of minimal effectiveness is the same, indicating a similar 

 mechanism of action. Also the same cytological changes occur as a response 

 to urethane at all the temperatures and this points to a single mechanism 



10- 



LOG (I) 

 (mM) 



N,N-DIETHYLURETHANE 



12° 16' 



TEMP. - 



20' 



24' 



28' 



32° 



Fig. 15-12. Effects of temperature on the 

 retardation of cleavage of Arhacia eggs by 

 urethanes. The concentrations required for a 

 constant chosen effect are plotted. (From 

 Cornman, 1957.) 



of inhibition. These results were provisionally explained as follows: as the 

 temperature rises from 12° to 20° the binding of the inhibitor becomes less, 

 and at higher temperatures there is progressive acceleration of the dena- 

 turation of some important protein or enzyme system. It is possibly sig- 

 nificant that the temperature for the minimal effect is the normal biolog- 

 ical temperature for Arhacia. In other words, deviation in either direction 

 from the normal temperature leads to a reduction in the resistance to in- 

 hibition. 



At this point we may summarize some of the principal reasons for the 

 variation of cellular inhibition with the temperature. Let us assume it is 

 found experimentally that the inhibition increases with rising temperature. 

 The following four mechanisms must be considered. 



