Purification and physical properties of a novel type of cytochrome b from rabbit peritoneal neutrophils

The main redox component of the O2- generating oxidase complex in neutrophils is believed to be a b-type cytochrome, named cytochrome b558. In the course of purification of cytochrome b558 from rabbit peritoneal neutrophils, another hemoprotein with an apparent molecular mass of 30 kDa, referred to as p-30, was isolated. Although the spectrum of p-30 was virtually identical to that of cytochrome b558, its redox potential, Em,7 = -4 +/- 10 mV, was much less negative than that of cytochrome b558 (-270 +/- 5 mV). The alkaline pyridine hemochrome from purified p-30 was typical of a b-type cytochrome. The 20 N-terminal amino acid residues and some tryptic peptides isolated from p-30 did not show any significant sequence homology to the human phagocyte cytochrome b558 or to mitochondrial and microsomal cytochromes, except for the N-terminal region which displayed some homology to that of rat liver P-450. After subcellular fractionation, p-30 was found to be located in the plasma membrane and the granule fractions, similarly to cytochrome b558. Upon neutrophil activation, part of p-30 was transferred from granules to the plasma membrane.