Studies on electron transfer systems in the marine diatom Phaeodactylum tricornutum. I. Isolation and characterization of cytochromes.

Two cytochromes of the C type, c-550 and c-553, were isolated from the marine diatom, Phaeodactylum tricornutum, and purified by ammonium sulfate fractionation and DEAE-cellulose column chromatography. The cytochrome c-550 had absorption maxima at 550, 522, and 417 nm in the reduced form and at 524, 407, 351, and 277 nm in the oxidized form. It was an autoxidizable acidic protein with an isoelectric point of 5.1 and had a low redox potential of about -0.20 V at pH 7.0. The molecular weight of this cytochrome was close to 17,000. This cytochrome combined with CO and CN-. The CO complex was dissociated reversibly by light. The cytochrome c-553 had absorption maxima at 553, 522.5, and 417 nm in the reduced form and at 528, 410, and 356 nm in the oxidized form. The protein had an acidic isoelectric point of 3.7 and had a high mid-point redox potential of +0.36 V at pH 7.0. Its molecular weight was approximately 10,500. The cytochrome may be considered to be a photosynthetic cytochrome of the f type. Cytochromes of the B type were also found in Phaeodactylum tricornutum; one in soluble form, and the other in bound form. The soluble form had absorption maxima at 560, 529, and 427 nm in the reduced state and at 413 nm in the oxidized state.