EFFECT OF THE R569W MISSENSE MUTATION ON THE BIOSYNTHESIS OF MYELOPEROXIDASE

Human neutrophil microbicidal activity is largely mediated by reactive species generated by the oxygen-dependent myeloperoxidase (MPO) syste m. Peroxidase-negative neutrophils from many patients with hereditary MPO deficiency possess a 90-kDa MPO-related protein. We recently ident ified a missense mutation, R569W, in the MPO gene of many subjects wit h MPO deficiency. In these studies we examined the consequences of R56 9W on MPO biosynthesis and processing, using stably transfected K562 c ells expressing normal MPO or the R569W mutation, K562 cells expressin g normal MPO mimicked faithfully many features of MPO biosynthesis in myeloid cells, 1) apopro-MPO was synthesized; 2) a functional heme gro up was inserted into apopro-MPO, and enzymatically active pro-MPO was thereby generated; 3) pro-MPO underwent proteolytic processing to matu re MPO; and 4) hemin augmented the processing of pro-MPO, pREP-R569W c ells synthesized apopro-MPO, but heme was not inserted, Neither enzyma tically active pro-MPO nor mature MPO was synthesized by transfectants expressing mutated cDNA, confirming our hypothesis that the R569W mut ation results in a form of apopro-MPO which does not undergo posttrans lational processing to enzymatically active MPO species. In addition, these data support previous suggestions that heme insertion into apopr o-MPO is necessary for its subsequent proteolytic processing into matu re MPO subunits.