286 ELECTRON TRANSPORT 



termined the concentrations of these compounds in chromatophores to 

 be 8.0 X 10-5 m for cytochrome c^ and 7.4 x 10-5 M for RHP. Little 

 work has been done on the isolation and characterization of &-type 

 cytochromes from photosynthetic bacteria (1,44), although Vernon and 

 Kamen (39) have detected the existence of a &-type cytochrome in R. 

 rubrum. The cytochrome b is autoxidizable and bound firmly to the 

 chromatophores (45). There are NADH-linked reductases which are 

 capable of reducing the Cyt 6(39). Geller (46) has fovind that the ratio of 

 Cyt b content of light-grown todark-growncellsis 1:3. Using chroma- 

 tophores freed of photosynthetic pigments by being well washed with 

 acetone containing ether, Cyt b was found to be a one-electron carrier 

 of Eq at pH 7 of +0.04 V (Horio and Kitahara, unpub). 



Horio and Kamen (47,18) have highly purified a flavoenzyme which 

 is capable of reducing RHP and cytochrome c? in the presence of 

 NADH. In addition, it was found recently that the flavoenzyme could 

 catalyze reduction of the cytochrome b present in chromatophores 

 which had been well washed with acetone. The reduction of RHP and 

 cytochrome b by the enzyme could be observed under anaerobic but 

 not aerobic conditions, the anaerobic reductions being nearly equivalent 

 in rate to both aerobic and anaerobic reductions of cytochrome cp. 

 Under anaerobic conditions the enzyme could also reduce externally 

 added flavins (FAD, FMN and riboflavin), and under aerobic conditions 

 it could oxidize NADH by molecular oxygen. These findings raise the 

 possibility that the cells consume molecular oxygen at the levels of 

 flavins, cytochrome b, and RHP, When cytochrome c? was present, the 

 flavoenzyme could not catalyze the NADH oxidation by molecular oxy- 

 gen until all the cytochrome c^ became reduced. This phenomenon 

 enables us to speculate how respiration of the light-grown cells are 

 inhibited under illumination; the flavoenzyme does not react with 

 molecular oxygen so long as cytochrome C9 is oxidized by the illumi- 

 nated photochemical apparatus and is partly in its oxidized form. It 

 may be rational to extend this speculation to the respiratory activity 

 of RHP. This may be strengthened by the findings of Vernon (48), who 

 noted that the aerobic NADH oxidation by chromatophores was re- 

 markably inhibited under illumination. 



Analogous to the mitochondrial electron transport system con- 

 structed by Green's school (4), cytochrome b has been placed outside 

 the photosynthetic, cyclic electron transport chain of the R. rubrum 

 chromatophores, but was located on a site connecting both the succinic 

 and NADH dehydrogenases with the cyclic chain in the hypothetical 

 scheme for the electron transport system in light-grown R. rubrum 

 (see below). 



Oxygen uptake with chromatophores from R. rubrum is faster in 

 light than in darkness, either in the presence or absence of externally 

 added substrate. The chromatophores, if dialyzed, lose most of the 



