Three c-type cytochromes from the red alga Porphyridium cruentum

Three c-type cytochromes were isolated from the red alga, Porphyridium cruentum, and partially characterized. The cytochrome c553 was an extrinsic membrane protein which was easily released to the aqueous phase with a yield of 10 to 15 mg pure protein/100 g cells. The properties of this cytochrome were similar to other algal and cyanobacterial cytochromes which function as Photosystem I electron donors. Absorption maxima in reduced cytochrome c553 were at 552.8, 521.8, and 415.6 nm. The molecular mass was 10 kDa, the redox potential was +340m V, and the pI was 4.3. The near-infrared absorption band of ferricytochrome was found to titrate with a single pK of 9.5. In addition to the loss of the near-infrared band at alkaline pH, other spectral changes were noted in the ferrocytochrome. The most obvious change was that of the alpha-band shifting to 550 nm in the dithionite-reduced cytochrome at alkaline pH. Small quantities of a cytochrome c550 were detected in some preparations of P. cruentum. This cytochrome had absorption maxima in the reduced form at 550, 522, and 416 nm. The pI was 4.1. This cytochrome was isolated in the oxidized form and was not ascorbate-reducible. The third c-type cytochrome isolated from P. cruentum was an intrinsic membrane protein analogous to higher plant cytochrome f. The absorption maxima in the reduced cytochrome were at 553.8, 522.5, and 421.0 nm. Cytochrome f was always isolated in the reduced form with a yield of 1 to 1.5 mg/100 g of algal membranes. The cytochrome f was unstable at 4 or -20 degrees C. The molecular mass was 26 kDa, and the pI was 6.5. Photosynthetically active vesicles from P. cruentum were prepared and exogenous cytochrome c553 stimulated Photosystem I-dependent activity but not reactions dependent on both photosystems. Antibody to cytochrome c553 had no effect on photochemical activity.