Lysosomal phospholipase activity is decreased in mucolipidosis II and III fibroblasts

Mucolipidosis (ML) II and III are rare autosomal recessively inherited diseases characterized by deficiency of multiple lysosomal enzymes and, as a result, a generalized storage of macromolecules in lysosomes of cells of mesenchymal origin. In ML II and ML III fibroblasts, most, but not all, newly synthesized lysosomal enzymes are secreted into the medium instead of being targeted correctly to lysosomes. Defects in the enzyme UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase underlie this effect. It is unknown how lysosomal phospholipases are targeted to the lysosomes of fibroblasts. In the present study lysosomal phospholipase activity was determined in delipidated fibroblast homogenates and plasma from ML II and ML III patients and controls using a [3H]choline-labeled phosphatidylcholine. After incubation, residual phosphatidylcholine and its labeled degradation products (lysophosphatidylcholine, glycerophosphorylcholine and choline phosphate) were quantified. We found that ML II and ML III fibroblasts are deficient in lysosomal phospholipase A and C activity. These enzymes were present in elevated amounts in plasma of ML II and ML III patients. These data indicate that phospholipases, like most other lysosomal enzymes in these diseases, are secreted into the blood instead of being targeted specifically to lysosomes. Thus, the mannose-6-phosphate receptor pathway is needed for proper delivery of lysosomal phospholipases to lysosomes. We also found that production of labeled choline phosphate was mainly due to the activity of acid sphingomyelinase instead of phospholipase C under the assay conditions used. Other active lipolytic enzymes were phospholipase A and lysophospholipase. No evidence for phospholipase D activity was found.