Biosynthesis and processing of lactoferrin in bone marrow cells, a comparison with processing of myeloperoxidase

The processing and intracellular transport of lactoferrin of the neutrophil specific granules was investigated by biosynthetic labeling with (14C)leucine of bone marrow cells from healthy individuals and patients with chronic myeloid leukemia. Lactoferrin was precipitated with antilactoferrin serum and the immunoprecipitates were analyzed by sodium dodecyl sulfate (SDS), polyacrylamide gel electrophoresis (PAGE) followed by fluorography. In contrast to myeloperoxidase of azurophil granules, lactoferrin was not synthesized as a larger precursor, and it was not found to be phosphorylated. The transfer to granules of newly synthesized lactoferrin was demonstrated in pulse-chase labeling experiments followed by centrifugation of cell homogenate in a Percoll gradient. Monensin, which exchanges protons for Na+ and NH4+ cation, blocked the transfer completely, indicating a need for acidification mechanisms. Unlike myeloperoxidase, newly synthesized lactoferrin rapidly became resistant to endoglycosidase H, indicating a transport through the medial and transcisternae of the Golgi apparatus with conversion of "high mannose" to "complex" oligosaccharide side chains. Intracellular transfer of some major neutrophil azurophil and specific granule constituents is obviously regulated differently. Lactoferrin seems to be processed like proteins destined for secretion, while myeloperoxidase is processed more or less like lysosomal enzymes.