ANTHONY SAN riETRO 637 



of the chlorophyll concentrations tested. In separate experiments, it 

 has been possible to demonstrate that the enzyme is completely in- 

 hibited by p-chloromercuribenzoate at a concentration of 10-=^ M; 

 this same concentration of mercurial is without effect on the dye re- 

 duction activity of the chloroplasts. 



Recently, Arnon, Whatley, and Allen (4) have reported that the 

 TPN-reducing factor in chloroplast extract is not required for the 

 reduction of ferricyanide. This factor is most probably the same as 

 the enzyme described here. 



Effect of Phosphorylation Cofactors 



It has recently been reported by Davenport (7) that the reduction 

 of TPN, in the presence of illuminated chloroplasts, was stimulated 



TABLE 3 



Lack of Dependence of Dye Reduction on 



Photosynthetic Pyridine Nucleotide Reductase 



Each reaction mixture contained 150 /jmoles of Tris buffer, /?H 7.3, 0.08 Mmole 

 2,3,6-trichlorophenol-indophenol, chloroplasts equivalent to varying amounts of 

 chlorophyll and, where indicated, 1 unit of PPNR. Final volume was 3 ml. Illumination 

 for 45 seconds at room temperature. 



— A£'620 



Chlorophyll, Mg - PPNR + PPNR 



1.4 0.04 0.04 



2.8 0.09 0.09 



4.2 0.13 0.12 



5.6 0.16 0.15 



7.0 0.19 0.18 



2V2-told by the presence of the phosphate acceptor system (ADP, 

 Mg++, and phosphate) and that all the components of this system 

 are necessary for appreciable stimulation. (See also the paper of Hill, 

 in this symposium.) A similar effect was reported several years ago 

 by Duysens (II); however, in a more recent report by Amesz and 

 Duysens (1) this stimulation was not observed. 



In our experiments, we have observed a stimulatory effect of Mg++ 

 alone (19) . This effect was not further enhanced by the addition of 

 ADP and phosphate. We have reinvestigated this effect as well as that 

 reported by Davenport and have confirmed both of them (Keister and 

 San Pietro, unpub. obs.) . The effect we reported is observed when 

 the rate of TPN reduction is proportional to enzyme concentration; 

 with excess enzyme, one observes the effect reported by Davenport (7) . 



