180 L. SMITH 



washed particle suspension, there was no reduction of cytochrome c 

 in the dark in the presence of cyanide, which inhibits the dark oxidase, 

 but the cytochrome was rapidly reduced on addition of succinate. 

 Thus the soluble cytochrome seemed to be available for reaction with 

 the enzymes on the particles. 



Quantitative measurements were made to determine whether the 

 rate of oxidation of the cytochrome c could account for the observed 

 rate of oxygen uptake of the same suspension while oxidizing succinate 

 in the dark, as is observed to be the case with the mammalian respira- 

 tory sj^stem. Table I shows a comparison of the observed rate of 

 oxygen uptake of the particles oxidizing succinate and the rate of 

 oxygen uptake calculated from the rate of oxidation of cytochrome c, 

 assuming that one molecule of O2 is equivalent to four molecules of 

 cytochrome c. The data show that the rate of oxidation of 15 nM 

 cytochrome c could account for less than 10% of the respiration; 

 and the rate of oxidation of bacterial cytochrome C2 is even slower. 

 The data argue against the presence in the bacteria of a cytochrome d 

 dark oxidase system similar to that of mammalian tissues. 



TABLE I. Comparison of Observed Rate of Oxygen Uptake of Rhodospirillum 

 rubrum Extract with Oxj^gen Uptake Calculated from the Rate of Oxidation of 

 Reduced Mammalian Cytochrome c 



Oxygen uptake calcul 

 xidation of cytochrome 



jlated from the rate of 

 3me c, assuming 1 O2 <s 4 

 Observed oxygen uptake cytochrome c 



47 . 3 Ml./hour 3 . 4 Atl./hour 



The same amount of extract was used in the two experiments. The concentra- 

 tion of cytochrome c was 15 fiM; the mixtures were prepared in phosphate buffer, 

 pH 7.4, 0.1 M. In separate experiments, it was ascertained that there was no 

 measurable change in optical density at 550 m/x when the same concentration of 

 Rhodospirillum rubrum extract was illuminated in the absence of cytochrome c. 



LIGHT REACTIONS 



The optical density changes at 550 m/z (absorption peak of re- 

 duced cytochrome c) were followed when an aerobic mixture of re- 

 duced cytochrome c and rubrum extract were illuminated from the 

 side in a special Beckman cuvette designed by Keilin and Hartree 

 (7). The Hght was filtered to remove light of wavelength shorter than 

 700 m/i, and the Beckman IP28 phototube was shown not to respond 

 to the filtered light. The optical density changes are plotted in Fig, 1. 

 Illumination produces a rapid decrease in optical density at 550 m/i 



