626 LIGHT AND LIFE 



TABLE 3 

 GSH-ACTivATED Photohydrolysis OF ATP 



A /xmoles Pi/hr/mg 

 chlorophyll 



Complete 184 



No light 42 



No PMS 69 



Complete + 5 X IQ-^ M 37 



HO— <f \— COO— butyl 



Reaction mixture. Each tube contained the following in /nmoles/2.7 ml: 1 ATP, 

 20 phosphoenol-pyruvate (PEP), 20 MgClj, 40 GSH, 200 Tris buffer />H 7.7, 0.4 PMS, 

 5 jug pyruvic kinase, and Anabaena fragments containing approximately 0.1 mg chloro- 

 phyll. The incubation was for 20 minutes at 30°C in nitrogen. The reaction was 

 stopped with 0.3 ml 50% TCA, and the Pi was measured. 



to the light-induced reaction. It probably should be subtracted as a 

 blank. 



PMS may be replaced by either FMN or menadione, but then de- 

 phosphorylation is slower. As shown in Table 3, photohydrolysis 

 is inhibited by 5 X 10"^ M of the butyl ester of diiodohydroxybenzoic 

 acid, a concentration that also inhibits oxidative and photophos- 

 phorylation. In contrast to photophosphorylation, the rate of hydroly- 

 sis is the same in air as in nitrogen. The high SH concentration used 

 for hydrolysis seems to protect the system against photooxidation. 

 Under optimum conditions the rate of ATP hydrolysis is 184 /^moles 

 of inorganic phosphate per hour per mg chlorophyll in light, and 42 

 in dark. This particular preparation synthesized 280 yumoles ATP 

 per hour per mg chlorophyll in the light. 



Cysteine may replace GSH and is even slightly more effective. Fig. 

 2 describes the effect of cysteine concentration on the rate of ATP 

 hydrolysis. Its effect on jihotophosphorylation is also shown, and 

 one may see the inverse relationship of SH concentration to the two 

 light-induced reactions. SH concentrations that stimulate photophos- 

 phorylation only slightly affect photohydrolysis, whereas SH con- 

 centrations that inhibit photophosphorylation produce maximum hy- 

 drolysis. Since tlie {^reparation contains a potent inorganic pyro- 

 phosphatase, it was necessary to consider the possibility that the ATP 

 might i)c iiydroly/ed to AMP and inorganic pyrophosphate which 



