Characteristics of lysosomal phosphomannosyl-enzyme receptors in the rat heart

The receptor system recognizing mannose 6-phosphate groups of lysosomal enzymes has been characterized, e.g. in fibroblasts and liver cells. The purpose of this study was to demonstrate the presence of a phosphomannosyl receptor system in rat heart muscle. The characterization of receptors was accomplished with beta-N-acetylglucosaminidase (beta-GA) secreted by rat embryo fibroblasts after ammonium chloride stimulation. The receptor binding of ligand enzymes was saturated by adding increasing concentrations of beta-GA and the binding increased linearly when the content of membrane protein was increased. The binding of beta-GA was inhibited by mannose and glucose phosphates, especially mannose 6-phosphate. Mannose itself did not inhibit binding of the enzyme, showing that the binding was not mediated by mannose receptors. Alkaline phosphatase treatment of beta-GA decreased the binding of ligand enzymes to receptors. Alkaline conditions increased the dissociation of receptor-ligand complexes, whereas the dissociation was minimal between pH 5.5 and 6.5. The proportion of endogenous beta-GA activity in membranes, probably representing receptor-bound location, varied between 40 and 55% of the total activity in various parts of rat cardiac muscle. The differences in the content of phosphomannosyl receptors, however, were insignificant between various cardiac muscle samples. At the organelle level the highest specific binding capacity, as well as the highest endogenous beta-GA activity, was in the sarcolemmal fraction. These results suggest that phosphomannosyl receptors also function in the endocytosis and transport of lysosomal enzymes in cardiomyocytes, as well as in several other cell types studied.