Oxidation-reduction potentials and absorption spectra of two b-type cytochromes from the halophilic archaebacterium, Halobacterium halobium

The oxidation-reduction midpoint potentials were determined for two b-type cytochromes, which had been solubilized from the membrane of Halobacterium halobium and partially purified. The two b-type cytochromes have oxidation-reduction midpoint potentials of 175 and 7 mV, respectively. These b-type cytochromes could also be resolved by difference absorption spectroscopy, which revealed one b-type cytochrome with absorption maximum (α-peak) at 558 nm, reducible by ascorbate-tetramethyl- p-phenyl-enediamine, and the other with absorption maximum (α-peak) at 560 nm, reducible by dithionite. Different substrates such as succinate, NADH, and α-glycerophosphate were used to study the b-type cytochromes in situ when bound to the membrane in a functional state. Reducing equivalents from succinate and α-glycerophosphate appear to enter the respiratory chain at the 175 mV b-type cytochrome. Cytochrome a 3 is spectrophotometrically shown to be present in the membrane of H. halobium.