37° DANIEL I. ARNON 



From the standpoint of photosynthesis, it was important to know if the 

 hydrogenases of photosynthetic bacteria could also reduce pyridine 

 nucleotide with molecular hydrogen in the dark, since this would provide 

 a mechanism, independent of light, for the formation of the reductant for 

 CO2 assimilation. In photosynthetic bacteria, the only cell-free hydro- 

 genase tested in this respect, that of R. riibriim, was reported to be unable 

 to reduce acceptors with potentials less than o volts [132] which would 

 thus exclude pyridine nucleotides (is^ = —0-32 V.). 



TABLE VI 



Pyridine Nucleotide Reduction with Molecular Hydrogen by Cell-Free 

 Preparations of Chroynatium 



(Ogata, Nozaki, and Arnon [91]) 



Each vessel included, in a final volume of 3 -o ml., a cell-free preparation (PS) 

 containing o • 3 mg. bacteriochlorophyll and the following in micromoles : tris 

 buffer, pH 7-8, 80; MgCl.2, 5 ; potassium phosphate, 5 ; KCl, 50; and when added, 

 DPN, 4; TPN, 4; and benzyl viologen, o-i (a gift of Dr. H. Gest). o-i ml. of 

 20*^)0 KOH was present in the centre well. The reaction was carried out in an 

 atmosphere of hydrogen at 25'^. Illumination, when given, was 35 000 Lux. At 

 the end of the reaction, an aliquot of the reaction mixture was precipitated with 

 saturated (NH4)2S04, pH 8, centrifuged and the optical density of the clear 

 supernatant fluid was measured at 340 m/x. 



The subject was reinvestigated by Ogata et al. [91], using the cell-free 

 hydrogenase of Chromatium. As in other photosynthetic bacteria (for 

 example, R. nibrum [132], an active hydrogenase was also found in 

 Chromatium. The Chromatium hydrogenase reduced DPN and TPN with 

 molecular hydrogen in the dark in the presence of benzyl viologen. The 

 enzyme was more active toward DPN than TPN (Table VI). 



These results indicated that in the presence of hydrogen gas, Chro- 

 matium cells do not require light for the reduction of pyridine nucleotides. 

 The role of light is then limited to ATP formation, without which CO2 

 assimilation cannot occur [121]. Photosynthesis by Chromatiu?n in the 

 presence of molecular hydrogen may, therefore, be summarized as 

 follows : 



