402 DANIEL I. ARNON 



Chromatiiim and R. riihnim. As shown in Fig. 35 the bacterial enzyme 

 catalyzed the photochemical reduction of pyridine nucleotides by chloro- 

 plasts that have been deprived of their own pyridine nucleotide reductase. 

 The bacterial PN-reductase was similar to the chloroplast PN-reductase 

 in reducing TPN preferentially to DPN (cf. [86, 148]). 



The reduction of TPN was coupled with oxygen evolution when the 

 bacterial enzyme was added to a chloroplast preparation that by itself 

 could not reduce TPN and thereby evolve oxygen (Table XIX). These 

 findings again support the conclusion that TPN reduction and oxygen 

 evolution are basically separate phenomena. The bacterial PN-reductase 

 cannot bring about a coupling of pyridine nucleotide reduction with 

 oxygen evolution in a bacterial system. 



TABLE XIX 



Photochemical Oxygen Evolution Catalyzed by Pyridine Nucleotide 

 Reductase from Chromatium 



(Losada, Tagawa, Nozaki, and Arnon [155]; Whatley, Dieterle, and Arnon [161] 



The reaction mixture contained in a final volume of 3 ml. : washed chloroplast 

 fragments containing 0-3 mg. chlorophyll; and the following in micromoles: tris 

 buffer, pH 7-8, 100; MgCl,, 5; ADP, 10; K.,H3"-P04, 10; TPN, 6; and a purified 

 pyridine nucleotide reductase preparation from Chromatium. The reaction was 

 run at 15° in the light. 



Non-cyclic photophosphorylation enabled green plants to form a CO2 

 reductant at the expense of light energy with the aid of an ubiquitous 

 substance, water, and in this way to invade and live autotrophically in 

 areas devoid of reduced sulphur compounds or of other electron donors of 

 restricted distribution. The resultant proliferation of plant growth was 

 responsible for releasing to the atmosphere the oxygen, locked in the water 

 molecule, by the only known important mechanism capable of accomplish- 

 ing this, photosynthesis of green plants [169, 170]. 



Once molecular oxygen became available, the way was open for bio- 

 chemical evolution to progress toward aerobic metabolism. The oxygen- 

 independent cyclic photophosphorylation by chlorophyll-containing 

 particles could now be paralleled by an efficient biological utilization of the 

 energy of chemical substrates through the mechanism of oxidative phos- 



