Posttranslational processing of a human myeloid lysosomal protein, myeloperoxidase

Myeloperoxidase (MPO) is a lysosomal enzyme present in the azurophilic granules of human neutrophils and monocytes and is important for optimal oxygen-dependent killing of microorganisms. The native molecule is a heterodimer composed of a pair of heavy-light protomers, each containing a 59-kDa and 13.5-kDa subunit. The intracellular processing during biosynthesis of MPO was examined in the human promyelocytic cell line HL-60. Endoglycosidase H and F digestion of immunoprecipitated pro-MPO demonstrated the presence of five N-linked--high-mannose oligosaccharide side chains and no complex mannose units. Incorporation of the threonine analogue beta-hydroxynorvaline produced species approximately 2.5 kDa and approximately 5 kDa smaller than the fully glycosylated pro-MPO, suggesting that two of the glycans were in the asparagine-X-threonine tripeptide sequence. Processing of pro-MPO occurred very rapidly, within approximately five minutes, and was best identified using glucosidase inhibitors. The presence of such inhibitors resulted in synthesis of a 92-kDa glycoprotein rather than the usually identified 89-kDa peptide. Swainsonine, a Golgi mannosidase inhibitor, did not alter the size of the earliest synthesized protein, suggesting that pro-MPO exited the endoplasmic reticulum or cis-Golgi proximal to the site of mannosidases. Intracellular transport and proteolytic maturation of MPO was retarded by weak bases (NH4Cl, chloroquine) or monensin at concentrations shown to raise intralysosomal pH. However, these agents did not qualitatively alter transport nor increase secretion. Thus, although MPO biosynthesis resembled that of other lysosomal enzymes, significant differences exist, including only limited oligosaccharide processing and intracellular transport and proteolytic maturation of pro-MPO that was only retarded by alkalinizing lysosomes without affecting the products or the fraction of pro-MPO secreted. Characterization of the determinants for targeting and of the regulatory factors in processing lysosomal enzymes in myeloid cells will provide insight into the molecular mechanisms underlying common disorders such as myeloperoxidase deficiency.