Isolation of a Possible Primary Hydrogen Acceptor 

 from Photosynthesizing Barley Leaves* 



ALBERT R. KRALL,t Biology Division, Oak Ridge National Laboratory, 



Oak Ridge, Tennessee 



The identity of the primaiy acceptor for the electrons generated by 

 the photolysis of water in a photosynthesizing system has not yet 

 been estabUshed for any in vivo system. The most important point 

 estabhshed by Hill was that the isolated chloroplast must be given a 

 continuous supply of an exogenous electron acceptor before it can 

 evolve the oxygen at an appreciable rate. Until now, no naturally oc- 

 curring oxidant, not even diphosphopyridine nucleotide in the closely 

 coupled glutathione reductase system (3) , is capable of promoting oxy- 

 gen evolution at better than 5% of the rate at which the same quantity 

 of chlorophyll can promote its evolution in the whole cell under opti- 

 mum conditions. Calvin has suggested that 6,8-thioctic acid could act 

 to accept both the hydroxyl radical and the electron formed on pho- 

 tolysis, and (1) shown some stimulation of quinone reduction by algae 

 after preincubation with thioctic acid. 



In our studies on nitrogen and carbon monoxide inhibitions of P^^ 

 uptake by barley leaves, an unidentified phosphate ester was en- 

 countered, whose variations with changes in conditions to which the 

 leaves are subjected make it a good candidate for the title of "in vivo 

 Hill oxidant." This material apparently exists in the plant in two 

 forms that are rapidly converted from one to the other by changes in 

 illumination an . In the redox potential of the external environment. 

 By the anion-exchange chromatography method (4), modified to ana- 

 lyze P^Mabeled extracts, the unidentified material is eluted by 0.03il/ 

 NH4CI. The application of this eluting agent causes a drop in the pH 

 of the column effluent from 8.2 to 6.5. The bottom curve on Fig. 1 

 shows the P^^ level in the hquid during the course of this change, when 

 both components are present. The lower pH level is to the left. The 



* Work performed under U.S. Atomic Energy Commission Contract No. 

 W-7405-eng-26. 



t Present address, Rias, Inc., Baltimore, Md. 



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