DCCD inhibits the reactions of the iron-sulfur protein in Rhodobacter sphaeroides chromatophores

N,N'-dicyclohexylcarbodiimide (DCCD) has been reported to inhibit proton tr anslocation by cytochrome bc(1) and b(6)f complexes without significantly a ltering the rate of electron transport, a process referred to as decoupling . To understand the possible role of DCCD in inhibiting the protonogenic re actions of cytochrome bc(1) complex, we investigated the effect of DCCD mod ification on flash-induced electron transport and electrochromic bandshift of carotenoids in Rb. sphaeroides chromatophores. DCCD has two distinct eff ects on phase III of the electrochromic bandshift of carotenoids reflecting the electrogenic reactions of the bc(1) complex. At low concentrations, DC CD increases the magnitude of the electrogenic process because of a decreas e in the permeability of the membrane, probably through inhibition of FoF1. At higher concentrations (>150 muM), DCCD slows the development of phase I II of the electrochromic shift from about 3 ms in control preparations to a bout 23 ms at 1.2 mM DCCD, without significantly changing the amplitude. DC CD treatment of chromatophores also slows down the kinetics of flash-induce d reduction of both cytochromes b and c, from 1.5-2 ms in control preparati ons to 8-10 ms at 0.8 mM DCCD. Parallel slowing of the reduction of both cy tochromes indicates that DCCD treatment modifies the reaction of QH(2) oxid ation at the Q(o), site. Despite the similarity in the kinetics of both cyt ochromes, the onset of cytochrome c re-reduction is delayed 1-2 ms in compa rison to cytochrome b reduction, indicating that DCCD inhibits the delivery of electrons from quinol to heme c(1). We conclude that DCCD treatment of chromatophores leads to modification of the rate of Q(o)H(2) oxidation by t he iron-sulfur protein (ISP) as well as the donation of electrons from ISP to c(1), and we discuss the results in the context of the movement of ISP b etween the Q(o), site and cytochrome c(1).