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A reaction-center fraction isolated from Rhodopseudomonas spheroides chromatophores exhibits light-induced changes in its optical and electron spin-resonance (ESR) spectra.

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I. A hemeprotein with properties similar to microsomal cytochrome b 5 has been detected in the supernatant fraction of hemolysates of human, beef, and rabbit erythrocytes. A method has been developed for determining the amount of this soluble cytochrome in small volumes of blood. The amount of the protein decreases during cell storage at 4 °C. Blood cells rich in retieulocytes contain more of the protein than do mature cells. 2. The cytochrome has been purified from human erythrocytes by a procedure which employs chromatography on Amberlite CG-5o and DEAE-cellulose, ultrafiltration, and gel filtration. The purified protein sedimented in the ultracentrifuge as a single peak with an Szo,w of 1.4o. However, minor impurities were detected by polyacrylamide disc electrophoresis. 3-The molecular weight of the purified protein has been calculated to be 14600 from sedimentation and diffusion measurements and 18400 as determined by gel filtration. The prosthetic group has been identified as protoheme IX. The spectral properties of the hemeprotein are those of a low spin heine complex. The EPR spectrum of the oxidized form shows g values of 3.03, 2.21, and 1.39 and the visible spectrum has a Soret absorbance maximum at 413 nm. The protein is reducible by dithionite or NADH plus cytochrome b 5 reductase and the reduced form shows absorbance maxima at 423,527, and 556 nm with a shoulder at 560 nm. 4. The cytochrome b 5 differs from the other B-type cytochrome of erythrocyte, S-protein (hemeprotein 559), and is not derived from this protein. The erythrocyte cytochrome b 5 is similar to the cytochrome b 5 solubilized from liver microsomes in terms of spectral properties, molecular weight, prosthetic group, and reactivity.