302 DOUGLAS C. PRATT, ALBERT W. FRENKEL, AND DONALD D. HICKMAN 



reduced and the amount of ATP utilized in the reaction. Chromatophores 

 in the hght may form only ATP (reaction 2), or only reduced DPN 

 (reaction 4), or both ATP formation and DPN reduction may occur 

 simultaneously in the presence of the required cofactor for both reactions. 

 Vernon and Ash [13] have studied reactions 2 and 4 in some detail and 

 have found that the amount of inorganic phosphate esterified in the light 

 was the same regardless of whether their preparations carried out a 

 simultaneous reduction of DPN, and they concluded that the light- 

 induced phosphorylation reaction and the photoreduction of DPN occur 

 independently of each other. We have observed, on the other hand, that 

 the rate of photoreduction of DPN is inhibited under conditions where the 

 preparations carry out the light-induced formation of ATP at the same 

 time [15], indicating a possible relationship between these two processes, 

 but this interaction appears to be different from the one exhibited by mito- 

 chondria. Only a more detailed analysis of the kinetics of the chromato- 

 phore reactions can clarify the conflicting reports in the literature. 



As mentioned earlier there is disagreement about the effect of ADP on 

 the photoreduction of DPN by R. rubrimi chromatophores. Vernon and 

 Ash [9] initially reported that ADP inhibited the photoreduction of DPN ; 

 in a later paper [13] such an inhibition was not observed. We have noticed, 

 however, that ADP alone does not bring about an inhibition of DPN- 

 photoreduction. A marked inhibition is observed only when inorganic 

 phosphate and Mg + + are added and the preparation carries out active 

 photophosphorylation [15]. 



The observations available thus far would indicate that the light- 

 induced reduction of DPN by R. riihrinn chromatophores may be achieved 

 without the utilization of ATP. Except for the requirement of an exo- 

 genous reducing agent [16] and the absence of oxygen production, this 

 reduction appears to be more closely akin to the photoreduction of pyridine 

 nucleotides by chloroplasts than to the dark reduction carried out by 

 mitochondria. 



PHOTOREDUCTION OF PYRIDINE NUCLEOTIDES AND THEIR POSSIBLE 

 ROLE IN METABOLIC REGULATION 



On several occasions Dr. D. I. Arnon has raised the question as to the 

 curious specificity of the purified photosynthetic pyridine nucleotide 

 reductase (PPNR) of San Pietro for triphosphopyridine nucleotide [17]. 

 The specificity of R. rubrum chromatophores for diphosphopyridine 

 nucleotide is equally puzzHng [15, 18]. It may, therefore, be of interest to 

 consider whether these observations can be brought in line with recent 

 views on the role of these two pyridine nucleotides in metabolic regulation 

 which have been reviewed most recently by Klingenberg and Biicher [19]. 



