OBSERVATIONS ON THE FORMATION OF THE PHOTOS YNTHETIC APPARATUS 3OI 



Some comments on light-induced pyridine nucleotide 

 reduction by bacterial chromatophores 



REDUCTION OF PYRIDINE NUCLEOTIDES BY CHROMATOPHORES AND 

 BY MITOCHONDRIA 



In line with the discussion carried out by Dr. Chance and by other 

 participants at this meeting on the reduction of pyridine nucleotides by 

 mitochondria in the presence of ATP (reaction i) (10-12), 



Succinate + DPX - + nATP ^^ 



fumarate + DPNH + H ++ nADP + nP, (i) 



one may compare this reaction with the hght-induced reduction of di- 

 phosphopyridine nucleotide carried out by R. ruhrum chromatophores 

 which carry out the following reactions [8, 13-15]: 



light . . 



ADP^P ^ATP (2) 



DPX - + FMXH, ^ DPXH + H + FAIN (3) 



dark 

 light 



DPX ^ + succinate 7 — ^ DPXH + H * + fumarate (4) 



dark 



Consequently, it might be postulated that the photoreduction of DPX 

 (reaction 4) bv R. ruhrum chromatophores could be due to the participation 

 of ATP formed according to reaction 2, and that the ATP thus formed 

 would be utilized for the dark reduction of DPX according to reaction 4. 

 It is, therefore, of interest to examine the existing evidence for and against 

 such a reaction scheme involving a combination of reactions 2 and i to 

 account for the light-induced reduction of DPX by bacterial chromato- 

 phores. 



The photoreduction of DPX (reaction 4) by bacterial chromatophores 

 and the dark reduction of DPX (reaction i) by mitochondria have the 

 following characteristics in common: both require DPX, succinate and 

 possibly Alg*^ (or Aln*^), while ADP appears to be inhibitory to DPX 

 reduction in both svstems, although there is not complete agreement on 

 the behaviour of the chromatophore system, as will be described later. 

 The two reaction svstems appear to differ in the following respects: 

 (rt) the mitochondrial system requires ATP, but such a requirement has 

 not been established for the chromatophore system, although the partici- 

 pation of endogenous ATP cannot be excluded at the present time ; {b) it 

 appears that added ATP will not substitute for light in bringing about a 

 dark reduction of DPX by washed R. ruhrum chromatophores [15]; {c) in 

 mitochondria there is a direct relationship between the amount of DPX 



