Purification and characterization of cytochrome f-556.5 from the blue-green alga Spirulina platensis.

The membrane-bound cytochrome f-556.5 from the blue-green alga Spirulina platensis was purified to apparent homogeneity. Most of its properties are comparable to cytochrome f isolated from higher plants and green algae. It is clearly distinguishable from soluble cytochrome c-554, also present in Spirulina, which probably replaces the function of plastocyanin in photosynthetic electron transport. 1. The reduced form of cytochrome f exhibits an asymmetrical alpha-band with a maximum at 556.5 nm, and a pronounced shoulder at 550 nm. The beta-, gamma and delta-bands coincide with those described for Scenedesmus cytochrome f-553, with maxima at 524 (532), 422, 331 and a protein peak at 276 nm. The maximum of ferricytochrome f is at 410.5 nm; there is no indication of a weak 695 nm band, described for soluble c-type cytochromes. The purest preparations had a delta/protein-peak ratio of 0.8; the gamma/alpha ratio was 7.3. Formation of a pyridine hemochromogen with a maximum at 550 nm indicated a c-type cytochrome. The molar extinction coefficient at 556.5 nm is 30200, the differential extinction coefficient 21 500. 2. The molecular weight determined by gel filtration or SDS-polyacrylamide gel electrophoresis is 33 000 and 34 000, respectively. 3. The redox properties differ from those described for other cytochromes f isolated from green algae and higher plants: the midpoint redox potential is significantly more negative (+318 mV, pH 7.0) and from pH 6 to 10 no pH dependence is observed. 4. The isoelectric point was determined at pH 3.95, which is more acidic as compared to other cytochromes f. 5. Comparison of the amino acid composition indicated a distant relationship to higher plant cytochrome f and a closer relationship to cytochrome f from green algae.