596 



LIGHT AND LIFE 



Reagents are as indicated. Note particularly the points marked higli 

 and low ATP. 



B. ATP-phosphate exchange in the presence of other factors. Table 

 1 illustrates the relative rates of Pj^- incorporation into added ATP 

 in the presence of other cofactors. 



C. Effect of ATP and TPN concentration on P^^^ incorporation. 

 The data presented in Fig. 1 indicate that the exchange reaction is 

 inhibited by higher ATP concentrations. A more detailed series of 

 experiments is illustrated in Fig. 2, in which the relative rate of in- 

 corporation of Pi^- into ATP is plotted against ATP concentration. 

 The percentage turnover of added ATP is likewise shown in Fig. 2, 

 as is the dependence of the reaction upon TPN concentration at a 

 constant ATP concentration of 1.9 X 10-^ M. It should be noted 

 that the inhibition of the Pj^^-ATP exchange at higher levels of ATP 

 has varied greatly in magnitude from experiment to experiment and 

 sometimes cannot be demonstrated. The experimental conditions 

 responsible for this variation have not been established. 



The data presented in Fig. 3 indicate that ATP both stimulates and 

 inhibits photophosphorylation of AMP, depending on its concentra- 

 tion. The stimulation at low levels of ATP is perhaps due to forma- 

 tion of ADP by myokinase. 



07t the mechanism of ATP-laheling. Evidence that the reaction is 

 largely made up of a dark ATPase activity followed by photophos- 

 phorylation is given by the experiments reported in Table 2. It is 

 apparent that incubation of ATP in the dark with chloroplasts li- 



TABLEll 



Factors added 



Relative Rate 



Exp. 1 Exp. 2 Exp. 3 Exp. 4 Exp. 5 



TPN 

 PMS 



FMN and ascorbate 



Menadione and ascorbate 



Fe+++ CN 



TPNH 



No oxidant 



Ascorbate 



3.9 

 9.0 



1.0 



1.0 

 5.\ 



2.8 



Relative rates of Pp — ATP exchange in the presence of various cofactors. In the 

 various experiments shown each tube contained: K2HPO4, 1.0 X 10~' M; NaCl, 

 1.0 X !<)"' ^^; MgClo. X lO"'' ^^' tiis-HCl, /;H 8.0. 1.2 x l^"' ^^^• a"*^' chloro- 

 plasts containing 60 to 132 Mg/ml. Where indicated each tube also contained the 

 following: TPN, 7 to 9 X 10"^ A/; PMS, 5 X 10-^ M ; FMN, 2 X lO-" M\ ascorbate, 

 .") X ">"" ^^'' menadione, saturated solution in HoO diluted 10 x: Fc (CN)8-^ 1 to 

 2 X 10-» M; TPNH, 1 X lO"' M. Incubations 20 min at 15°C. 



