Interaction between antimycin and cytochrome b560 in Chromatium chromatophores--The red shift of the absorption spectrum and photoreduction of cytochrome b560 in the presence of antimycin

Abstract

In the presence of dithionite, which reduced all the cytochromes present in Chromatium chromatophores, the addition of antimycin caused a shift (1-2 nm) of the α band in the absorption spectrum of reduced cytochrome b560 toward longer wavelength. The difference spectrum (antimycin-added-minus-no addition) had a trough at 556 nm, a peak at 564 nm and an isosbestic point at 561 nm. The locations of the trough, peak and isosbestic point did not depend on the concentration of antimycin, but the amount of shifted cytochrome b560 increased as the concentration of antimycin increased. As reported previously, a single-turnover flash reduced cytochrome b560 in the presence of antimycin. The extent of the reduction depended on the concentration of antimycin. Both titrations of the red shift and the photoreduction of cytochrome b560 with antimycin fitted a first-order sigmoid curve with a half-maximum value at 5 × 10(-7) m of antimycin concentration. The results indicated that antimycin molecules interacted with, or were bound to, cytochrome b560 molecules in the ratio of one to one with an apparent dissociation constant of 3.5-7.3 × 10(-7) m.