photochei\[k:al rkdhction of triphosphopyridine 

 nucileoi ide by illuminated chloroplasts 



Anthony San Pietro 



McCoUum-Pratl Institute 



The ] alius Hopkins University 



Baltimore, Maryland 



It is becoming increasingly apparent that the elucidation of the 

 mechanism whereby pyridine nucleotides are reduced by illimiinated 

 chloro])lasts is of prime importance for an understanding of the path 

 of hydrogen (or electrons) in photosynthesis. Furthermore, knowl- 

 edge of the mechanism by which hydrogen (or electrons) is trans- 

 ferred from the photolytic system to the pyridine nucleotides may well 

 further the understanding of the mechanism of photosynthetic phos- 

 phorylation. The relationship between these two processes has been 

 the subject of considerable discussion both at this symposium and 

 in the literature and will not be discussed here. (See the paper by 

 Arnon for a discussion of this relationship and references.) 



The ability of illuminated chloroplasts to reduce pyridine nucleo- 

 tides was first demonstrated in 1951-52 by Vishniac and Ochoa (27, 

 28) , Tolmach (24), and Arnon (2) . In these experiments the forma- 

 tion of reduced pyridine nucleotides was demonstrated indirectly by 

 coupling the photochemical reaction with a suitable dehydrogenase 

 and measuring the formation of the product of the dehydrogenase 

 system. No directly measurable reduction of pyridine nucleotides was 

 observed in the absence of the coupling system (17). It was, there- 

 fore, suggested that the inability of pyridine nucleotides to undergo 

 directly measurable reduction was a consequence of their low oxida- 

 tion-reduction potential (£„' at /?H 7 = —0.32 volt) , since most 

 substances that are effective as oxidants in the Hill reaction have high 

 oxidation-reduction potentials (£/ at pH 7 = -|-0.l to 0.4 v) . 



The rate of pyridine nucleotide reduction observed by Vishniac and 

 Ochoa (28) was 0.03 to 1.6 ^^moles per hour per mg chlorophyll. A 

 somewhat higher rate was reported by Arnon (2), who observed the 

 evolution of oxygen in the presence of TPN, malic enzyme, pyruvate, 

 and carbon dioxide at a rate of 7.5 /^moles per hour per mg chloro- 



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