158 M. O. KAMEN 



(6) note that the hght-induced oxidation is not affected by cyanid(% 

 and in addition is insensitive to CO. The isolated pigment has been 

 described previously as forming a CO-complex, but apparently this 

 compound may not be effective in preventing light oxidation because 

 of the well-known dissociation of such heme-CO compounds by light. 

 Both groups of workers postulate a competition between the photo- 

 lytic oxidant and oxygen, with the site of the competition in the 

 hematin chain. Chance and Smith (6) propose that the shift to the 

 oxidized state is the result of the preferential attack of the oxidase by 

 the photooxidant. 



Kamen and Vernon (18) have observed that the photooxidation in 

 air catatyzed by their photooxidase proceeds at a rate at light satura- 

 tion (which is attained at rather low light intensity) some five to ten 

 times faster than the rate of the dark oxidation under the same con- 

 ditions. They argue by analogy from this observation that competi- 

 tion occurs at the cytochrome c le\^el. In this scheme, the bacterial 

 cytochrome would act as the substrate for the two oxidative systems 

 and the light oxidase would compete successfully for the cytochrome 

 c during illumination. The shift from reduced to oxidized cytochrome, 

 on their viewpoint, results from the fact that while the dark oxidase 

 cannot keep up with the reductase in the absence of light, the in- 

 creased rate of oxidation possible with the light oxidase favors a 

 greater degree of oxidation in the light. 



The complexities which arise in the interpretation of shifts in oxi- 

 dation states in faculative tissues should be obviated by similar ex- 

 periments in tissues of strictly photoanaerobic character. A start has 

 been made by Olsen and Chance with Chromatium (28). Their pre- 

 liminary findings again indicate that practically the entire action spec- 

 trum found can be accounted for in terms of photooxidation of the 

 hematin system as isolated and characterized by Newton and Kamen, 

 described above (26). This finding, even though preliminary, provides 

 strong evidence that the hematin system of photosynthetic tissue 

 is coupled to the photochemical act, because no other function for 

 the hematin system is available in such a tissue. The bacterium can- 

 not use a chemosynthetic oxidation apart from photometabolism 

 to any useful purpose, and in fact, is inactivated and finally rendered 

 nonviable by air. 



A number of observations on chloroplasts and intact green plant 

 tissues have been reported which parallel those recorded for the bac- 



