Purification and characterization of intact cytochrome b5 from yeast microsomes.

A method for purification of detergent-solubilized cytochrome b5 to gel electrophoretic homogeneity from yeast (Saccharomyces cerevisiae) microsomes is described. The purified preparation shows the same absorption spectra as the trypsin-solubilized cytochrome (Y. Yoshida, H. Kumaoka, and R. Sato J. Biochem. 75, 1211-1219 (1974)) in the visible and Soret regions. The detergent-solubilized cytochrome is an amphipathic protein having a monomeric molecular weight of about 18,000 and exists as a hexa- or heptameric aggregate (Mr 122,000) in aqueous media. In the presence of low concentrations of Triton X-100, it interacts effectively with the intact form of NADH-cytochrome b5 reductase purified either from yeast microsomes or from rabbit liver microsomes. Upon trypsin digestion, it is converted to a heme-containing, hydrophilic fragment (Mr 13,000) which retains the spectral characteristics of the original cytochrome, does not form aggregates, and interacts with the reductase only poorly. It is concluded that the preparation purified in this study represents the intact form of yeast cytochrome b5 consisting of a hydrophilic, heme-containing moiety (Mr 13,000) and a hydrophobic, membrane-binding tail (Mr 5000).