348 DANIEL I. ARNON 



originally described [13] and even these high rates were exceeded by 

 Jagendorf and Avron [74]. The improved rates of photosynthetic phos- 

 phorylation were equal or greater than the maximum known rates of carbon 

 assimilation in intact leaves. It appeared, therefore, that the enzymic 

 apparatus for photosynthetic phosphorylation that is present in chloro- 

 plasts, can under appropriate experimental conditions, function outside 

 the organized cell without substantial loss of activity. 



Unlike the phosphorylating system, the enzymes catalyzing CO2 

 assimilation are water-soluble [38, 39, 37] and are therefore partly lost 

 during the isolation of chloroplasts. This results in lower rates of COg 

 assimilation by isolated chloroplasts than by the intact parent leaves. The 

 difference between the rate of CO., assimilation by isolated chloroplasts 

 and that of intact leaves may be made to appear greater, though less 

 relevant, if the comparison is made not between isolated chloroplasts 

 and their parent leaf tissue, but between isolated chloroplasts and un- 

 related leaf material that gave maximum rates of CO2 assimilation under 

 different experimental conditions. Nevertheless, the now known rates of 

 CO., assimilation in isolated chloroplasts (10 to 20'^ ^ of that in parent leaf 

 tissue [20, 35]) are substantial enough to strengthen the conclusion that 

 photosynthesis by isolated chloroplasts mirrors that in the intact leaf. This 

 conclusion is fortified by the identity of the photosynthetic products found 

 in both cases. 



5. Catalysts of photosynthetic phosphorylation 



Photosynthetic phosphorylation emerged as a major mechanism for 

 converting light into useful chemical energy independently of CO2 

 assimilation. It became important therefore to investigate systematically 

 the mechanism of this direct conversion of light into pyrophosphate bond 

 energy. The first question that received attention was the identity of the 

 catalysts. 



In searching for catalysts of photosynthetic phosphorylation by 

 isolated chloroplasts special attention was given to normal constituents of 

 chloroplasts and green leaves. The first factors which were found to 

 stimulate cyclic photophosphorylation without themselves being con- 

 sumed in the reaction were magnesium ions and ascorbate [13, 51]; the 

 next to be recognized were, as already mentioned, FMN and vitamin K 

 compounds [53, 52]. Magnesium and ascorbate have long been known to 

 be present in chloroplasts [75]. FMN is widely distributed in green 

 leaves [76]. Ohta and Losada in our laboratory (unpublished data) have 

 found FMN to be a regular constituent of chloroplasts. Of unusual interest, 

 however, was the antihaemorrhagic factor, vitamin K, which occupied, 

 since its discovery in plants, a unique position among other vitamins in 



