Purification and Characterization of the Photoreducible b-type Cytochrome from Dictyostelium discoideum.

The photoreducible cytochrome (Cyt) b from Dictyostelium discoideum was purified by differential precipitation with ammonium sulfate and chromatography over Sephadex G-100, diethylaminoethyl-cellulose, and calcium phosphate. The purified Cyt is composed of a single subunit of 15,000 daltons including a noncovalently bound protohaem, and exhibits in the reduced form alpha absorption bands at 555.5 and 560 nm at room temperature and 551 and 558.5 nm at 77 K. This Cyt is similar in some respects to Cyt b(557.5) from complex II of beef heart mitochondria, and to Cyt b(555) from the microsomal fraction of mung bean seedlings. Photoreduction by blue light of the purified Cyt b requires the addition of flavin; flavoprotein isolated from D. discoideum was the most active of four flavoproteins tested in catalyzing the photoreduction while diaphorase and l-amino-acid oxidase were inactive.