ABNORMAL LYSOSOMAL SORTING WITH AN ENHANCED SECRETION OF CATHEPSIN-D PRECURSOR MOLECULES BEARING MONOESTER PHOSPHATE GROUPS

It has been reported that besides defects in the phosphorylation such as in the I-cell disease, a failure in the uncovering of mannose 6-pho sphate residues may result in an increase of lysosomal enzyme activiti es in serum [Alexander et al., Hum. Genet. 73, 53-59 (1986)]. We exami ned fibroblasts that were derived from the original biopsy, observed a n enhanced secretion of lysosomal enzymes including cathepsin D, but f ound that both the phosphorylation and uncovering of mannose 6-phospha te residues were normal. The enhanced secretion of cathepsin D was cha racterized by an increase in the secretion of phosphorylated molecules that were sensitive to a treatment with alkaline phosphatase. The enh anced secretion of the phosphatase-sensitive form of procathepsin D wa s further increased in the presence of antibodies directed to cation-i ndependent mannose 6-phosphate receptors. In contrast, antibodies spec ific to cation-dependent mannose 6-phosphate receptors selectively inh ibited the secretion of the phosphatase-sensitive procathepsin D molec ules. A chromatographic analysis of oligosaccharides from the secreted procathepsin D confirmed that the cells secrete proenzyme molecules r ich in oligosaccharides with two uncovered phosphate residues. It is s uggested that the enhanced secretion of procathepsin D in the variant fibroblasts results from an abnormal sorting rather than processing of phosphorylated lysosomal enzymes.