Myeloperoxidase is synthesized as larger phosphorylated precursor.

Synthesis and processing of myeloperoxidase were examined in metabolically labeled cells of the human promyelocyte line HL‐60 and in an in vitro rabbit reticulocyte lysate system directed with HL‐60 mRNA. Radioactivity labeled products were isolated by immunoprecipitation and analyzed by gel electrophoresis and fluorography. In vivo, myeloperoxidase was labeled initially as a 85‐K glycosylated polypeptide (75 K after treatment with endo‐beta‐N‐acetylglucosaminidase H). This polypeptide was soon processed to an 81‐K intermediate and to smaller mature fragments of 60 K and 13 K within approximately 1 day. A minor portion of the precursor was converted to fragments of 40 K and 43 K. The pattern of labeled polypeptides of mature myeloperoxidase was similar to that of the enzyme purified from human leucocytes. The modifications of the polypeptide and of the oligosaccharide side chains in myeloperoxidase resembled those known to occur during the processing of lysosomal enzymes. In the absence or presence of dog pancreas membranes, myeloperoxidase was synthesized in vitro as a 76‐K polypeptide or a 87‐K glycosylated polypeptide, respectively. In HL‐60 cells [32P]phosphate was incorporated into endo‐beta‐N‐acetylglucosaminidase H‐sensitive oligosaccharides. The presence of phosphorylated oligosaccharides was inferred from the fact that endocytosis of leucocyte myeloperoxidase in fibroblasts was sensitive to mannose 6‐phosphate. It is suggested that myeloperoxidase is synthesized in the rough endoplasmic reticulum as a precursor of larger molecular mass and that the oligosaccharide side chains in the precursor are modified to contain mannose 6‐phosphate residues which may be involved in the segregation and transport of the precursor.