BENT LEY GLASS 877 



lull cxteiu iiiul implicaiioiKs ol this discovery arc known, ihc cjuanti- 

 tative, or even the qualitative, requirements for ATP and IPNH in 

 whole photosynthesis cannot i)c considered as settled. 



In 1951, Vishniac and Ochoa, Tolmach, and Anion independently 

 showed that an isolated chloroplast system can reduce jjyridine nu- 

 cleotides and evolve oxygen, il "pulled" by a coupled reaction that 

 j)romj)tly utilized the reduced pyridine nucleotide which was formed. 

 San Pietro and Lang later succeeded in isolating from chloroplasts 

 an enzyme, which is now called photosynthetic pyridine nucleotide 

 reductase, and which acts preferentially on TPN rather than DPN. 

 Use of the reductase enzyme permitted major amounts of reduced 

 jjyridine nucleotides to acctnnulate, and obviated the need for the 

 pulling system. In 1954, Arnon and his coworkers found that the 

 remaining component of "assimilatory power, " ATP, is also pro- 

 duced in chloroplasts. The process was shown to be distinct from 

 the "oxidative phosphorylation" of mitochondria in two respects: 

 no oxygen is consumed; and no chemical substrate, such as succinate, 

 is required to provide free energy from the formation of the pyro- 

 jihosphate bonds of ATP. Only light is needed to drive the process. 



Although photosynthetic phosphorylation involves no net con- 

 sumption of molecular oxygen, the presence of oxygen was found 

 to be necessary for the process to proceed. In other words, oxygen 

 behaves as a catalyst rather than a substrate in this form of phos- 

 phorylation. A prime difference is to be seen in the inability of 

 isolated chloroplasts to form ATP in the dark by oxidizing the well- 

 known hydrogen donors of oxidative phosphorylation with molecular 

 oxygen. 



In work with spinach chloroplast systems, it was soon found that 

 flavin mononucleotide (FMN) and vitamin K serve as catalysts of 

 photosynthetic phosphorylation, and that at optimal concentrations 

 of these cofactors the process becomes independent of external oxy- 

 gen, although at low concentration of the cofactors oxygen remains 

 necessary. This latter observation is regarded by Arnon and his 

 coworkers as further evidence that oxygen acts only catalytically in 

 photosynthetic phosphorylation; and they conclude that the anaerobic 

 situation, with optimal amounts of FMN c^r vitamin K, was the more 

 fundamental situation for photosynthesis in general, since it would 

 apjjly to bacterial photosynthesis, which is anaerobic. Frankel, also 

 in 1954, discovered photophosphorylation in Rhodospirilliim rubriun, 

 and cell-free preparatif:)ns from the obligate anaerobes Chromatium 

 and Chlorohiuin were likewise soon demonstrated to produce ATP 



