Forms and intracellular distribution of α-d-mannosidases in murine liver and spleen

Abstract 1. 1. The intracellular distribution of α- d -mannosidase in homogenates of murine liver and spleen was investigated by differential and gradient density centrifugation. 2. 2. In both tissues an enzyme with a neutral pH optimum was found in the cytosol together with an α- d -mannosidase with optimal activity between pH 5.5 and 6.0 which was also partially membrane-bound. 3. 3. In liver the acidic α- d -mannosidase was obtained almost entirely in a particulate form distributed equally between a heterogeneous low density region and heavy density lysosomes. 4. 4. The lysosomal form of the liver enzyme was purified to electrophoretic homogeneity and shown to be a glycoprotein composed of four identical subunits of molecular weight 65 kDa. 5. 5. Antibody raised against the purified liver α- d -mannosidase immunoprecipitated a polypeptide from spleen which had the same molecular size. This acidic enzyme was the predominant type of α- d -mannosidase in spleen, but in contrast to liver, it was obtained mainly in a cytosoluble form, the remaining activity being present in the heterogenous light density compartment. 6. 6. Although both tissues contain the same molecular form of the acidic α- d -mannosidase, in murine spleen this enzyme does not appear to be associated with stable heavy density lysosomes.