A detailed study of horseradish peroxidase (HRP) uptake by in vitro cultured bone marrow-derived macrophages was undertaken. Biochemical quantitations performed over a wide range of HRP concentrations, in the absence or presence of yeast mannan, showed that these macrophages pinocytose HRP by both fluid phase and mannose receptor-mediated uptake. The relative contribution of these two types of endocytosis varied with the concentration of enzyme in the extracellular medium. A morphological study at the light and electron microscope levels conducted in parallel confirmed the biochemical data.
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Lang et al. (1985) studied this question.