Acid hydrolases from Dictyostelium discoideum contain phosphomannosyl recognition markers

Acid hydrolases from mammalian sources contain phosphorylated oligosaccharides which function as recognition markers for their receptor-mediated endocytosis by human fibroblasts. The discovery that glycopeptides derived from the slime mold, Dictyostelium discoideum, contain mannose 6-phosphate led to the suggestion that acid hydrolases from this source might also bear the marker. To test this hypothesis, the binding and endocytosis of purified beta-D-glucosidase (EC 3.2.1.21), beta-N-acetyl-D-hexosaminidase (EC 3.2.1.52), and alpha-D-mannosidase (EC 3.2.1.24) by human fibroblasts were investigated. These enzymes underwent endocytosis with efficiencies of 8.6 to 60%/mg/h, and 1 mM mannose 6-phosphate markedly inhibited their uptake (80 to 100%). The specificity of inhibition by sugar phosphates, the saturation kinetics of endocytosis, and the binding properties of D. discoideum acid hydrolases were similar to those reported for enzyme preparations derived from mammalian sources. In addition, 95 to 100% of the beta-D-glucosidase or alpha-D-mannosidase molecules from D. discoideum preparations were competent for in vitro clearance. Furthermore, the three purified acid hydrolases contain 5 to 7 mol of mannose 6-phosphate/mol of enzyme. This indicates that, unlike many mammalian enzyme preparations, most if not all of these enzyme molecules from D. discoideum contain the phosphomannosyl recognition marker.