446 J. S. C. WESSELS 



cyclic photophosphorylation to DNP. As regards the effect of DNP there 

 seems to exist an obvious difference between the mechanisms of photo- 

 synthetic and respiratory generation of ATP. 



With the exception of w-nitrophenol, all nitrophenols which have been 

 found to be capable of catalyzing cyclic photophosphorylation could be 

 converted into reversible oxidation-reduction systems by reduction of the 

 nitro group to the amino group. It was shown, however, that w-nitrophenol 

 is not reduced to w-aminophenol, but to an intermediate reduction 

 product which may be identical with ;;/-hydroxylaminophenol or m- 

 nitrosophenol or, more probably, with a conversion product of these 

 compounds. In this connection it is of interest to note that/)-nitrosophenol 

 was also found to be active as a catalyst of cyclic photophosphorylation. 

 At the moment an effort is being made to elucidate the structure of the 

 photoreduction product of w-nitrophenol. 



Chloroplasts are also capable of reducing DNP in the dark under 

 anaerobic conditions, but then the presence of FMN and of an excess of 

 TPNH is required. TPNH cannot be replaced by DPNH or ascorbate, 

 nor FMN by vitamin K3. 



Reduction of DNP has not been observed under aerobic conditions, 

 either in the presence or in the absence of KCN. As FMN catalyzes the 

 oxidation of TPNH by chloroplast preparations under aerobic conditions, 

 it seems probable that the dark reduction of DNP is due to the presence 

 of some enzyme which can transfer electrons from TPNH to FMN. This 

 enzyme may be TPNH diaphorase or TPNH — cytochrome c reductase, 

 both of which have been shown to be present in chloroplasts by Avron 

 and Jagendorf [2], and Marre et al. [3], respectively. In accordance with 

 this view it was demonstrated that chemically reduced FMN is capable 

 of reducing DNP. When a solution of FMN is illuminated anaerobically 

 by white light in the presence of TPNH or EDTA, the flavin is reduced 

 reversibly, as has been shown recently by Vernon [4]. Subsequent addition 

 of DNP in the dark resulted in reduction of DNP to aminonitrophenol. 

 TPNH was found to be incapable of reducing DNP. 



As yet no indication has been found that FMN has some function in the 

 photoreduction of DNP by chloroplasts. The formation of aminonitro- 

 phenol in light is affected neither by the addition of FMN, nor by the 

 addition of the flavin antagonists atebrin and chlorpromazine. 



References 



1. Wessels, J. S. C, Biochim. biophys. Acta 29, 113 (1958). 



2. Avron, M., and Jagendorf, A. T., Arch. Biochem. Biophys. 65, 475 (1956). 



3. Marre, E., and Servettaz, O., Arch. Biocheiti. Biophys. 75, 309 (1958). 



4. Vernon, L. P., Biochim. biophys. Acta 36, 177 (1959). 



