Comparison of Chlorobium thiosulphatophilum Cytochrome c-555 with c-Type Cytochromes Derived from Algae and Nonsulphur Purple Bacteria*

Spectral and enzymatic properties of cytochrome c-555 obtained from Chlorobium thiosulphatophilum were studied in comparison with those of algal and nonsulphur purple bacterial c-type cytochromes. Chlorobium cytochrome c-555 had an asymmetric a-peak, the molar extinction co-efficient of the peak was considerably low, and the ratio of.^reduced)/ Aa(reduced) was very high. The cytochrome reacted fairly rapidly with Pseudomonas cytochrome oxidase [EG 1.9.3.2]. In these respects, the cytochrome c-555 was very similar to algal c-type cytochromes (the /-type cytochromes) but different from nonsulphur purple bacterial c-type cyto-chromes (the Cj-type cytochromes). The c-type cytochromes which participate in the photosynthesis in algae are called the /-type cytochromes and those in nonsulphur purple bacteria the Cj-type cytochromes. They are very similar to each other in that they have high midpoint redox potential; Eo', about +0.35V (1—3). However, the /-type cyto-chromes differ from the Cj-type cytochromes in several respects. The former has an asym-metric a-peak and a high ratio (about 7) of.4r(reduced)/A(reduced),* * and the millimolar extinction coefficient at the a-peak is con-siderably low (about 22) (1—7). On the contrary, the cs-type cytochrome has a sym-metric a-peak, and the ratio of Adduced)/A. (reduced) is about 5, and the millimolar ex-tinction coefficient at a-peak is comparable to that of mammalian cytochrome c (about 28) (2,8,9). From the green sulphur bacterium, Chloro