HEMATIN COMPOUNDS IN PHOTOMETABOLISM 157 



that for the cytochrome component were not reactive (37). Shortly, 

 thereafter, Chance and Smith (6,7) conducted a more exhaustive 

 spectrophotometric analysis of the action spectra exhibited by R. 

 ruhrum cell suspensions under a variety of experimental conditions. 

 In agreement with the other workers, they found that the net effect 

 of illumination was an overall oxidation of one or more hematin 

 compounds. In this facultative bacterium, which displays competi- 

 tion between dark aerobic oxygen uptake and light anaerobic metabo- 

 hsm, the effect of Ught under anaerobic conditions was qualitatively 

 the same as that in the dark when oxygen was admitted. 



Chance and Smith (6) have proposed a scheme in which the hema- 

 tin compounds {R. ruhrum cytochrome c and yellow pigment) act as 

 a bridge for electron transfer between the products of photolysis and 

 the systems concerned with reduction of CO2 and other substrates. 

 Their basic assumption is that the hematin chain which reacts with 

 oxygen can also react with the products of photolysis. They place the 

 yellow pigment at the end of the respiratory chain. The redox po- 

 tential in the isolated form appears to be negative by more than 100 

 mv. compared to the cytochrome c component (38). This fact would 

 seem to disagree with the notion of the yellow pigment as a terminal 

 oxidase. However, it has not been established that the yellow pigment 

 has not undergone some modification during the procedures em- 

 ployed for extraction. 



Kamen and Vernon (18) have shown that, in R. ruhrum, the 

 reductase activity far exceeds the oxidase activity, using bacterial 

 cytochrome c or mammalian cytochrome c as substrate. Hence the 

 cytochrome c should be largely in the reduced state in the dark under 

 aerobic conditions, as well as anaerobic conditions. Chance and 

 Smith (6) find that addition of phenyl mercuriacetate causes a rapid 

 oxidation of the cytochrome c. This is in agreement with the known 

 inhibition of reductase activity by this agent. In the absence of this 

 inhibitor, they suppose that the photooxidant can react with the 

 yellow pigment and thus pull electrons away from the cytochrome 

 in a manner analogous to that observed by Vernon and Kamen for 

 the coupled oxidation of cytochrome c by air in the presence of yellow 

 pigment (38). 



These latter workers have also shown that the photoenzymic oxi- 

 dation of cytochrome c is not inhibited by cyanide as is the oxidase of 

 the aerobic system in the dark (37). Similarly, Chance and Smith 



