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The intracellular pathway of lipoprotein lipase (LPL) has been examined in human monocyte-derived macrophages in culture. These cells were previously shown to synthesize and constitutively secrete LPL. The secretion is dependent on new enzyme synthesis. 6-d-old human monocytes have stores of mRNA for linear release of LPL up to 24 h. Enzyme activity in cells and in culture medium was almost completely inhibited by 24 h treatment with tunicamycin, an inhibitor of glycosylation. In monensin-treated cells a pronounced increase in enzyme activity was found, whereas the secreted activity was markedly reduced. This indicates that LPL in human monocytes is processed through a pH sensitive part of the Golgi complex and that the terminal glycosylation is not needed for the expression of its catalytic activity. Our results suggest that lysosomal function is not important in secretion of the enzyme, whereas vesicular transport seem to be involved in regulating LPL in human monocyte-derived macrophages in culture.
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