ON THE REDUCTIVE DEPHOSPHORYLATION HYPOTHESIS 

 OF PHOTOSYNTHETIC REDUCTANT FORMATION 



D. D. Hendley and B. L. Strehler 



Gerontology Branch, National Heart Institute, National Institutes of Health, 

 Bethesda, Maryland, and the Baltimore City Hospitals, Baltimore, Maryland 



Introduction 



Among the possible functions of ATP in photosynthesis pointed out 

 in 1950 by Kandler (12) and proposed in 1951 at the Second Phos- 

 phorus Symposium in relation to the demonstration of a light-induced 

 ATP synthesis in intact Chlorella by Strehler (16), is the production 

 of a more reduced compound than the initial photoproduced re- 

 ductant through a process analogous to the reverse of oxidative phos- 

 phorylation. It was postulated that (1) a portion of the initial oxi- 

 dants and reductants produced by the photochemical apparatus re- 

 combined in a coupled phosphorylation, and (2) this was followed by 

 a utilization of a portion of the ATP thus formed to drive electrons 

 from the level of the photoproduced reductant to that of hydrogen 

 carriers such as the pyridine nucleotides at the reduction level of 

 carbohydrates. A critical test of this hypothesis was not feasible at 

 that time because of the probable concurrent utilization of ATP 

 in driving the carbon assimilation cycle (6) , although the steady- 

 state depression of ATP at high illuminating intensities even in the 

 absence of COo was suggestive- of a light-driven ATP utilization (17) . 

 Our report in 1956 (18) of a lack of light-induced pyridine nucleotide 

 reduction in the presence of CO2 as measured by fluorescence assay 

 in intact Chlorella, and its confirmation by Chance and Sager (7) 

 through sensitive spectrophotometric measurements in a Chlamy- 

 domonas mutant further challenged the hypothesis (20) of pyridine 

 nucleotide reduction as the primary photochemical reaction. 



Several years after the demonstration of photochemical formation 

 of ATP in intact Chlorella, Frenkel (8) and Arnon and co-workers 

 (2, 3, 1) , in a series of elegant experiments, established unequivocally 

 that this process occurred anaerobically in the photochemical appara- 

 tus rather than via a mitochondrial oxidative phosphorylation. 



The work from Arnon's laboratory and the parallel intensive studies 



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