168 



BIOCHEMISTRY OF FIREFLY LUMINESCENCE 



increasing concentrations of calcium ion. It is also significant that the 

 response to inorganic triphosphate is similar to the initial response to 

 ATP with and without calcium present. The results are in keeping with 

 the proposed scheme for luminescence ( see below ) . 



The luminescent reaction is essentially insensitive to azide, cyanide, 

 and fluoride although it is strongly inhibited by p-chloromercuroben- 

 zoic acid. The latter inhibition can be reversed by glutathione. The 

 results indicate a definite involvement of a — SH group for the lu- 



TIME-MINUTES 



CoCLX 10 



Fig. 4. The eflFect of calcium on the hght intensity and the total Hght emitted by 

 the purified luminescent system. Curve A, control; B and C contain a final 

 concentration of CaCL of 4 X 10* M and 6 X 10"^ M respectively. Curve E 

 relates to total light emitted when ATP is added initially, while D refers to 

 the secondary response to inorganic triphosphate (McElroy and Coulombre, 

 1952). 



minescent reaction. Inhibitors of the light reaction, which are of 

 particular interest with respect to the structure of luciferin, are those 

 listed in Table II. All compete with firefly luciferin. Of the various 

 substituted compounds 2-phenyl benzothiazole is the most effective. 

 Although 5-methyl- and 5,6-dimethylbenzimidazole are effective com- 

 petitive inhibitors neither crystalline vitamin B12 nor 1-a-D-ribofurano- 

 sido-5,6-dimethylbenzimidazole influence light emission. Substitution 

 in the one position of the parent compounds completely eliminates 

 inhibitory effects, whereas substitution in the two position potentiates. 



