BARBARA PET RACK AND IR/TZ LIFMANN 029 



with the jjhosphate cktermination. The highest rate of photoliy- 

 drolysis observed in spinach chlorophists is 100 /^moles Pj/hr/mg 

 chlorophyll, corresponding to a specific activity for photophosphoryla- 

 tion ranging from 250 to 700. The inuisually high SH requirement 

 may well be a limiting factor, but this needs to be clarified. However 

 that may be, the essential similarity between the photohydrolysis of 

 ATP in chloroplasts and algae is apparent. 



Conclusions 



A photophosphorylating system was demonstrated in the blue- 

 green alga, Anabaena variabilis. It was hoped that the relatively 

 greater simplicity of this preparation in comparison to chloroplasts 

 might reveal some characteristics of the mechanism that would other- 

 wise be masked. This view proved to be useful in finding a photo- 

 hydrolysis of ATP, which could subsequently be demonstrated in 

 chloroplasts only by forcing conditions. 



The photophosphorylating activity in Anabaena has been localized 

 in a phycocyanin-free preparation which consists of lamellar struc- 

 tures appearing in vesicular forms. This contrasts with the notion 

 that phycocyanin may be essential for photosynthesis. The present 

 study suggests that phycocyanin may function by protecting the sys- 

 tem against photooxidation. The experiments reported by Thomas 

 (14), in which loss of the Hill reaction was correlated with a loss of 

 phycocyanin, were not carried out under anaerobic conditions. 

 Furthermore, the release of oxygen during the Hill reaction may even 

 be sufficient to inactivate the system by photooxidation in the absence 

 of phycocyanin. 



A dephosphorylation of ATP has been described in both spinach 

 and algal preparations. Its relationship to photophosphorylation is 

 seen in its dependence on light and on phenazine methosulfate, as 

 well as in the inhibitory effects of diiodohydroxybenzoic acid on the 

 two activities. Altogether, it seems likely that photohydrolysis is the 

 reverse of photophosphorylation, and it is hoped that it may provide 

 some new approaches to the understanding of the phosphorylating 

 mechanism. 



REFERENCES 



1. Anion, D. L., "The Photochemical Apparatus," Brookhaveu Symposia in Biol., 



11, p. 181 (1958). 



2. Avron, M., Jagendorf, A. T., and Evans, M., Biochiiu. et Biophys. Ada, 26, 



262 (1957). 



