REDUCTION OF CYTOCHROMES WITH MENAQUINOL AND SULFIDE IN MEMBRANES FROM GREEN SULFUR BACTERIA

Reduction of cytochromes in chlorosome-free membranes of Chlorobia was studied anaerobically, with an LED array spectrophotometer. For Chlor obium tepidum these membranes contained 0.2 moles cytochrome per mole of bacteriochlorophyll a. The observed change upon complete reduction of oxidized membranes with dithionite could be satisfactorily fitted w ith three cytochrome components having absorption peaks at 553 (cyt c) , 558 and 563 nm (cyt b), in relative amounts of 5:1:2. About 20% of t otal cytochrome 553 were reducible by ascorbate. Menaquinol reduced al l of the 553-component, and this reduction was sensitive to stigmatell in, NQNO and antimycin A. The reduction was insensitive to KCN. Howeve r, it was transient at low concentrations of menaquinol in the absence of KCN, but permanent in its presence, demonstrating that electron tr ansport into an oxidation pool was blocked. The 563-component was only slightly reduced by menaquinol unless NQNO or antimycin were present. The stimulation of cytochrome 563-reduction by these inhibitors was m ore pronounced in the presence of ferricyanide. This phenomenon reflec ts 'oxidant-induced reduction' of cytochrome b and demonstrates that a Q-cycle is operative in Chlorobia. Also, sulfide fully reduced cytoch rome 553, but more slowly than menaquinol. KCN inhibited in this case, as did stigmatellin, NQNO and antimycin A. NQNO was a better inhibito r than antimycin A. Cytochrome 563 again was hardly reduced unless ant imycin A was added. The effect was more difficult to observe with NQNO . This supports the conclusion that sulfide oxidation proceeds via the quinone pool and the cytochrome bc-complex in green sulfur bacteria.