Biosynthesis of the Phagocyte NADPH Oxidase Cytochrome b558 ROLE OF HEME INCORPORATION AND HETERODIMER FORMATION IN MATURATION AND STABILITY OF gp91 and p22 SUBUNITS*

The NADPH oxidase cytochrome b558 is a membrane heterodimer comprised of a glycosylated 91-kDa subunit, gp91, and a nonglycosylated 22-kDa subunit, p22. The role of heme in cytochrome b558 biosynthesis was studied using succinyl acetone, an inhibitor of heme synthesis, in PLB-985 myeloid cells undergoing granulocytic differentiation. Succinyl acetone markedly reduced expression of p22 and the mature 91-kDa form of gp91 but not its 65-kDa high mannose precursor, in association with a profound reduction in NADPH oxidase activity. Expression of non-heme-containing cytosolic oxidase components was unaffected. The reduction in cytochrome b558 expression and NADPH oxidase activity was prevented by adding exogenous heme and was reversible upon removal of succinyl acetone. Transgenic expression of gp91 in monkey COS-7 and murine 3T3 cells, both of which lacked endogenous p22 mRNA, demonstrated that p22 was not required for maturation of gp91 carbohydrate to complex oligosaccharides. However, coexpression of transgenic p22 increased the abundance of the mature gp91 glycoprotein. These results suggest that heme incorporation plays an important role in cytochrome b558 assembly and provide further support for the concept that stability of p22 and the mature gp91 subunit is increased by heterodimer formation.