Secretion by mononuclear phagocytes of lysosomal hydrolases bearing ligands for the mannose-6-phosphate receptor system of fibroblasts: Evidence for a second mechanism of spontaneous secretion?

β-Hexosaminidase secreted by peritoneal macrophages in response to stimulation by zymosan or NH₄Cl, or spontaneously by a macrophage-like cell line (P388D₁), is susceptible to receptor-mediated endocytosis by human fibroblasts. This endocytosis is almost completely blocked by exogenous mannose-6-phosphate and therefore seems to depend on a mannose-6-phosphate ligand on the enzyme. It is suggested that macrophage lysosomal enzyme packaging may involve mannose-6-phosphate recognition markers, and that a continuous hypersecretion mechanism may exist which does not depend on a defect in this ligand.