w. D. Mcelroy and j. w. Hastings 



167 



Properties of the Luminescent System 



Effect of pH, Temperature, Activators, and Inhibitors. The lumi- 

 nescent reaction has a rather sharp pH optimum at approximately 

 8.0 under conditions where none of the other diffusible cofactors are 

 limiting. The results in Fig. 2 indicate that this optimum can be 

 shifted by changing the ionic environment. In a sodium phosphate 

 buffer at pH 7.5 the temperature optimum is approximately 25° C 

 (see Fig. 3) with an experimental energy of activation of 18 kcal. 



20 25 



TEMP-C* 



Fig 3. Effect of temperature on firefly luminescence (McElroy and Strehler, 

 1949). 



Although several divalent ions will stimulate the luminescent reac- 

 tion when crude extracts are used, only Mg and Mn will function in 

 the purified system. In this its behavior seems similar to that of other 

 phosphate transfer systems in which ATP participates. Calcium is a 

 potent inhibitor of the light reaction, competing with the Mg++ ion. 

 The results in Fig. 4 indicate that the flash height as well as the decay 

 to the baseline level is affected by calcium. With a decreased rate of 

 light intensity decay a greater total light output is obtained with 



