DANIEL I. ARNON 529 



phorylation, which consumes neither CO^. nor water, would be a use- 

 ful device for generating Al P to ch ive the many ATP-dependent reac- 

 tions, notably the synthesis of polysaccharides, proteins, and fats. 



These theoretical deductions for higher plants have recently received 

 experimental support from the work of iMachuhlan and Porter (100) . 

 They reported the first known instance of utilization of light energy 

 in leaf tissue for the synthesis of starch from labelled glucose, under 

 conditions when C]0._. assimilation was excluded but cyclic photo- 

 l)hosphorylation coidd proceed. 



18. Pyridine Nuclf.otide Reduction by Hm)rogenase in the Dark 



In the examples of photosynthesis in which the contribution of light 

 was limited to ATP formation, no reductant was needed in the con- 

 version of glucose to starch. In the assimilation of acetate, hydrogen 

 is released for metabolic purposes and no addition of hydrogen donor 

 is required (97) . But the assimilation of COo requires, in addition to 

 ATP, a supply of a reductant, viz., reduced pyridine nucleotide. It 

 was stated earlier that in photosynthesis of green plants both of these 

 components of assimilatory power are formed at the expense of light 

 energy. It is necessary, therefore, to trace the transition from a primi- 

 tive photosynthesis, in which light is used only for the formation of 

 ATP, to the "advanced" type of photosynthesis, observed in gieen 

 plants, in which light energy is used not only for ATP formation but 

 also for the reduction of pyridine nucleotide and the accompanying 

 evolution of oxygen. 



In the photoassimilation of CO2 by Chromatium the added re- 

 ductant was hydrogen gas (129) . This is the simplest reductant usable 

 by living cells. Cell-free hydrogenases from non-photosynthetic bac- 

 teria are known to reduce pyridine nucleotides with molecular hydro- 

 gen (84, 118; cf. 126). 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 CO^ assimilation. In photo- 

 synthetic bacteria, the only cell-free hydrogenase tested in this respect, 

 that of R. ruhrum, was reported to be unable to reduce acceptors with 

 potentials less than volts (82) , which would thus exclude pyridine 

 nucleotides {FJ„ = —0.32 v) . 



The subject was reinvestigated by Ogata, Nozaki, and Arnon (115) , 

 using the cell-free hydrogenase of Chromatium. As in R. ruhrum (82) . 

 the hydrogenase of Chromatium was found to l)e localized in the 



