Purification and partial characterization of the b-type cytochrome from human polymorphonuclear leukocytes

Abstract

Polymorphonuclear leukocytes contain an oxidase system that can be activated to produce superoxide radicals and hydrogen peroxide. A nonmitochondrial b cytochrome, functioning in the generation of these oxygen species, has been purified to apparent homogeneity from human polymorphonuclear phagocytes. After solubilization of the cytochrome with Triton X-100, the cell extract was subsequently chromatographed on Blue Sepharose and Sephacryl S-300. The final preparation was maximally purified 170-fold with a specific content of 5.33 +/- 2.03 nmol mg-1 of protein (mean +/- S.D.; n = 7) and a yield of 21 +/- 13% (n = 5). The apparent molecular mass of the nondenatured cytochrome was estimated by gel filtration to be 235 kDa. Upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, a single polypeptide was found with a molecular mass of 127 kDa. From the pyridine hemochrome spectrum 1 protoheme IX/polypeptide was calculated. The light absorbance bands of the dithionite-reduced cytochrome were found to be at 558.5 (alpha), 529 (beta), and 426 nm (Soret), and that of the oxidized cytochrome at 413.5 nm. The difference absorbance coefficients are delta epsilon (426.5 - 440 nm) = 160.6 +/- 11 mM-1 cm-1 and delta epsilon (558.5 - 542 nm) = 29.3 +/- 2 mM-1 cm-1 (mean +/- S.D.; n = 5). Carbon monoxide binds to the cytochrome in a time-dependent fashion (maximum binding after 50-60 min). The midpoint potential of the solubilized nonpurified cytochrome is identical to the cytochrome in situ (Em7.0 = -218 +/- 7 mV (mean +/- S.D.; n = 5)). However, purified cytochrome b shows a significantly decreased midpoint potential, estimated at -407 +/- 18 mV (n = 4). The protein does not contain noncovalently bound FAD or FMN, and no spectral evidence was obtained for the presence of covalently bound flavin. Preliminary amino acid analysis of the cytochrome shows a high content of hydrophilic residues.