FHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 359 



Reversible oxidation of Chromatium cytochromes by light 



LIGHT 

 t 



400 450 500 550 600 , 



Fig. 7. Successive oxidation by light and reduction in the dark of cytochromes 

 in cell-free preparations of Chromatium. The reaction mixture included in a final 

 volume of 3-0 ml., chromatophores (P) containing o-o6 mg. bacteriochlorophyll 

 and supernatant fluid (S) corresponding to 0-5 mg. bacteriochlorophyll. A small 

 amount of Xa.,So04 was previously added to S, which was then dialyzed against 

 0-2 M tris buffer, pH 7-8 prior to use. The reaction was carried out at room 

 temperature. Gas phase argon. Difference spectra, using the dark treatment as 

 control, were made in Thunberg type cuvettes, with a Gary recording spectro- 

 photometer. Illumination was by a tungsten lamp (35 000 Lux). (Nozaki, Ogata, 

 and Arnon [i 14]). 



EFFECT OF VITAMIN K 



In fresh preparations of chromatophores the reduction of oxidized 

 cytochrome in the dark was not influenced by the additions of added 

 cofactors (compare Table I). However, as shown in Fig. 8, in aged prepara- 

 tions, the reduction of the oxidized cytochromes was greatly accelerated by 

 the addition of vitamin K, either in the oxidized form (vitamin K3) or in 

 the reduced form (vitamin K5). The effect of vitamin K, as an electron 

 carrier in accelerating the reduction of oxidized cytochrome depended on 

 the presence of chromatophores. Without chromatophores, using a 

 purified cytochrome Co, a hundred-fold greater concentration of reduced 

 vitamin K was required to reduce the oxidized cytochrome. 



The observed effects of vitamin K in catalyzing the reduction of cyto- 

 chromes which had been oxidized in light, support the electron flow- 

 theory [94] for cyclic photophosphorylation. This theory assigns to- 

 vitamin K, or some analogous quinone, a role of an intermediate electron 

 carrier in the electron transport chain associated with photophosphory- 

 lation. 



