BARliARA FETRACK AM) IRITZ LIFMANN 



625 



at 1 X 10-^ M. A similar effect has been reported by Geller with 

 R. rubrum (5) . Antimycin A does not block photophosphorylation 

 with either PMS or .the combination of FMN and menadione. 



Photohvdrolysis of ATP 



As mentioned already, a stimulatory effect of GSH similar to the 

 one reported for spinach chloroplast fragments is found with this 

 system. Fig. 1 shows, however, that increasing the GSH concentration 

 beyond the maximum causes increasing inhibition. The stimulatory 

 effect of lower levels of GSH is somewhat variable and highly active 

 preparations may not show it. At higher levels, however, all prepara- 

 tions are inhibited. Frequently, 100% inhibition is seen with 0.015 

 M GSH, which appears to be a non-specific SH effect, since cysteine 

 has similar effects. 



From these observations a suspicion arose that high concentrations 

 of GSH inhibit by producing an ATPase-like activity in the prepara- 

 tion. This suspicion could, indeed, be verified. With 0.015 M GSH, 

 inorganic phosphate was released from ATP under the usual condi- 

 tions of photophosphorylation. In these experiments, ATP was added 

 catalytically together with phosphoenol-pyruvate -|- pyruvic kinase 

 (PEP-PK) as the ATP-generating system. 



Table 3 shows the GSH-activated photohydrolysis of ATP. This 

 activity requires Mg++, light, and PMS, and is lost by boiling the 

 preparation for 3 minutes. PEP itself is not hydrolyzed. Lower rates 

 are found when substrate levels of ATP replace the generating sys- 

 tem. The rate of hydrolysis is optimal at pH 7.7, the same pH shown 

 to be optimal for photophosphorylation. The dark hydrolysis of ATP 

 is not stimulated by either GSH or BSA and appears to be unrelated 



I, f feet of GSH on Photophosphorylation 



^ 250 



^^ 







10 20 30 



/^K GSH/3ml. 



Fig. 1. The effects of glutathione on photophosphorylation in cell-free extracts of 

 Anabaena variabilis. Stimulation by low and inhibition by high concentrations. 



