PHOTOOXIDATION AND PHO TOR EDUCTION REACTIONS 249 



The effect of flavins upon the photooxidation of DPIPH2 was ex- 

 amined. Fig, 12 shows the effect of adding FMN to the R. nibniDi sys- 

 tem. At a level of 3 xlO-^M, FMN exerted an appreciable stimulation 

 with the NAD slow reaction but had no effect upon the coupled fumarate 

 reaction. FAD gave about one-third the stimulation in the NAD system 

 and had no effect upon the fumarate system. Riboflavin was inactive in 

 both systems. One striking feature of the FMN stimulation was the low 

 concentration at which FMN was effective, with half maximal stimula- 

 tion at a concentration of about 10"^ M FMN. The response in the 

 present system to FMN should be compared with the stimulating 

 effect this nucleotide has upon the DPNH- cytochrome c^ reductase 

 system in the purified fractions obtained by Horio and Kamen (36). 



.6- 



.4 - 



I 2 3 



MINUTES 



Fig, 12. The effect of FMN on DPIPH2 photooxidation 

 by R. nihyiim chromatophores. The experimental con- 

 ditions for Fig. 1 were used with 0,21 mgof BChl present. 



This dark enzymatic reaction is stimulated half- maximally at about 

 10-'^ M FMN, FAD is less active than FMN, but riboflavin resembles 

 FMN in its activity. It is possible that the same flavoprotein is in- 

 volved in the present photooxidation reactions and in the dark enzymatic 

 reactions studied by Horio and Kamen, 



Addition of quinacrine to the various oxidation systems produced 

 the interesting results shown in Fig. 13. A clear separation of the 

 coupled photooxidations with fumarate and NAD was observed, the NAD- 

 coupled photooxidation being completely inhibited while the fumarate- 

 coupled oxidation was markedly stimulated. This shows clearly that 



