892 LIGHT AND LIFE 



luminescence can be observed from the assay system as the illumina- 

 tion is turned on and off and the ATP concentration consequently 

 rises and falls. The rapid decay in the dark was attributed to the 

 presence of myokinase in the crude firefly extract. The addition of 

 TPN and other cofactors (TPNH, folic acid, cytochrome c, ferri- 

 cyanide, PMS, and FMN) was found to enhance phosphorylation, as 

 measured by the luminescent system. In some experiments a decrease 

 in the level of luminescence was found upon exposure to light, in 

 agreement with the observation reported by Petrack and Lipmann. 

 Hendley and Strehler examine this phenomenon in terms of a "re- 

 ductive dephosphorylation" hypothesis advanced independently by 

 Kandler and Strehler in 1950 and 1951. According to that hypothesis, 

 some portion of the ATP generated by phosphorylation coupled to 

 photosynthesis would be used to raise the energy of electrons from 

 the level of the photoreductant to that of pyridine nucleotides. Ex- 

 periments now reported indeed demonstrate that TPN enhances the 

 incorporation of inorganic phosphate into ATP, but the exchange 

 reaction is inhibited by higher concentrations of ATP. Activity of 

 ATPase in the dark and occurrence of photophosphorylation in the 

 light account for most of the incorporation of P32 from inorganic 

 phosphate. Preincubation in the dark, presumably by increasing the 

 concentration of ADP, greatly speeds up the subsequent incorpora 

 tion of P32 into ATP in the light. These data do not support the 

 hypothesis that some ATP produced in photophosphorylation is uti- 

 lized in the reduction of TPN during photosynthesis, but neither do 

 they rule it out. 



Anthony San Pietro has successfully purified the enzyme "photo- 

 synthetic pyridine nucleotide reductase" (PPNR), which catalyzes the 

 transfer of electrons in photosynthesis to TPN. About a 200-fold in- 

 crease in specific activity over the crude preparations has been at- 

 tained. The properties of the enzyme, as now reported, include a 

 linear relation of enzyme concentration to reduction of TPN. The 

 nucleotide specificity of the enzyme is limited to TPN and acetyl- 

 pyridine-TPN (no action on DPN, a-DPN, or acetylpyridine-DPN, 

 nicotinamide mononucleotide or nicotinamide riboside). The enzyme 

 lacks effect in the Hill reaction (reduction of indophenol dye or 

 ferricyanide). The reduction of TPN by the enzyme is not enhanced 

 by the photophosphorylating system (ADP and inorganic phosphate), 

 although the reverse effect, stimulation of photophosphorylation by 

 the enzyme, does occur. There is a linear proportionality in rate of 

 formation of TPNH to the chloroplast concentration. Mg++ ions 



