590 



Lester Packer 



presence of cysteine (O.08 M). Accordingly the action of ATP and 

 also of ITP on the scattering responses in fresh chloroplasts was 

 tested in the presence and absence of cysteine. Parallel deter- 

 minations were made of nucleoside triphosphatase activity. The 

 results (Table l) confirm the existence of the light activated 

 ATPase observed by Petrack and Lipmann, and also show photohydro- 

 lysis of ITP. Under conditions where nucleoside triphosphatase 



Table I 

 CORRELATION OF PHOTOHYDROLYSIS OF ATP AND ITP BY SPINACH 

 CHLOROPLASTS WITH CHANGES IN CHLOROPLAST STRUCTURE 



^ scattering change jumoles phosphate foimed/nig 

 chlorophyll/l^ min 



Condition 



Light 



Dark 



Light 



Dark 



ATP 



ITP 



Cysteine+ITP 



Cysteine+ATP 

 Llpoic + ITP 



Lipoic + ATP 



+22 



+83 



-f-69 

 +70 



+76 



0.38 



0.22 

 n 10 



~9 

 -69 



tr 



-18 0.37 



-21 0.10 



-21 1.11 



0.39 

 0.52 



20 5-5 0.6 

 activity is maximum (i.e. in the presence of cysteine), it may be 

 seen that the scattering increases induced by red light under con- 

 ditions of cyclic photophosphorylation are considerably larger as 

 compared with those in the absence of cysteine. The effects of a 

 second thiol compound, reduced lipoic acid, are also sho\m; its 

 action on scattering responses and nucleoside triphosphatase 

 activity are comparable to those observed with cysteine. Other 

 experiments verified that lipoic acid is more effective on a con- 

 centration basis than cysteine for the activation of scattering 

 and nucleoside triphosphate hydrolysis. The scattering changes in 

 the absence of cysteine or lipoic acid, when the triphosphatase 

 activity is lower, were largely reversed in the dark. However in 

 the presence of cysteine or lipoic acid and ATP, the scattering 

 increases were only slightly reversed when the red light was 

 turned off. This was consistently observed with ATP, but less 

 frequently when ITP or GTP were added. 



The nature of this incomplete return of the scattering response 

 occurring in the presence of thiol compounds and ATP upon removal 

 of actinic light, \ras further examined as illustrated by figure 3- 

 Figure 3 shows that in the presence of cysteine, ATP and basic 

 reaction mixture, chloroplasts manifested an incomplete return of 

 scattering after removal of actinic light. Assuming that this 

 incomplete return was a consequence of light activated hydrolysis 

 of nucleoside triphosphates, it seemed of interest to examine the 

 action of a number of substances which have been reported to inhi- 

 bit this activity in chloroplasts. NHi^Cl and ADP, added at con- 



I 



