Evidence that two forms of bovine erythrocyte cytochrome b5 are identical to segments of microsomal cytochrome b5.

Homogeneous preparations of two forms of soluble cytochrome b5 have been obtained from bovine erythrocytes by successive chromatography on DEAE-cellulose, Bio-Gel P-60, and DEAE-Sephadex. Although the two forms could be separated on disc gel electrophoresis, they appeared to have similar molecular weights of approximately 12,000 and identical visible absorbance spectra. The tryptic hemepeptides derived from the two forms of bovine erythrocyte cytochrome b5 are electrophoretically indistinguishable from each other and from the tryptic core hemepeptide derived from liver microsomal cytochrome b5. The bovine erythrocyte tryptic hemepeptide was purified to homogeneity; its amino acid composition was shown to be identical to that of tryptic hemepeptide from liver microsomal cytochrome b5. The amino acid compositions of the two isolatable forms of erythrocyte cytochrome b5 correspond well to the compositions of the 97- and 95-residue segments of native liver microsomal cytochrome b5 that begin at the NH2 terminus. These results agree with the hypothesis that soluble erythrocyte cytochrome b5 is derived from microsomal protein by proteolysis during erythroid maturation.