Despite structural similarities between gp91phox and FRE1, flavocytochromeb(558) does not mediate iron uptake by myeloid cells

Superoxide (O-2(-)) generated by the phagocyte reduced nicotinamide adenine dinucleotide phosphate oxidase is dependent on electron transfer by flavoc ytochrome b(558) (flavocytochrome b), a transmembrane heterodimer that form s the redox center of the oxidase at the plasma or phagosomal membrane. The larger of its two subunits, gp91phox, is homologous to the yeast iron redu ctase subunit FRE1, and these two proteins share many structural and functi onal characteristics. Because FRE1 is required for iron uptake in yeast, we hypothesized that flavocytochrome b might serve a similar function in huma n phagocytes and thus provide a mechanism for the transferrin-independent i ron acquisition observed in myeloid cells. To determine whether flavocytoch rome b was required for iron uptake, we compared iron acquisition by polymo rphonuclear neutrophils (PMNs) or Epstein-Barr virus (EBV)-transformed B ly mphocytes derived from individuals with X-linked chronic granulomatous dise ase (CGD) with iron acquisition by normal cells. Our results indicate that all cells acquired iron to the same extent and that uptake could be signifi cantly enhanced in the presence of the trivalent metal gallium. The gallium enhancement of iron uptake observed in PMNs or in EBV-transformed B lympho cytes derived from healthy individuals was mirrored by those derived from i ndividuals deficient in flavocytochrome b. Furthermore, both normal and CGD -derived EBV-transformed B lymphocytes had similar iron reductase activity, suggesting that flavocytochrome b is not a biologically significant iron r eductase. In contrast to previously suggested hypotheses, these results sho w conclusively that flavocytochrome b is not necessary for cellular iron ac quisition, despite structural and functional similarities between yeast iro n reductases and flavocytochrome b.