The effect of monensin on β-hexosaminidase transport in normal and I-cell fibroblasts

The carboxylic ionophore, monensin, blocks the migration of glycoprotein-containing vesicles from the Golgi region to the plasma membrane in fibroblast resulting in an accumulation of secretory products in the Golgi cisternae. Treatment of cultured I-cell fibroblasts with monesin (0.5 μM) decreased the abnormal excretion of β-hexosaminidase to 40% of untreated cultures within 15 min. A corresponding intracellular accumulation of the enzyme to >200% of untreated cultures by 24 h was also observed. A small intracellular accumulation and slightly enhanced excretion of β-hexosaminidase occurred in treated normal fibroblast cultures. The intra- and extra-cellular distribution of newly synthesized β-hexosaminidase in both normal and I-cell cultures converged during monensin treatment. β-Hexosaminidase isoenzymes excreted by both monensin-treated normal and I-cell fibroblasts were electrophoretically indistinguishable from the four bands characteristic of I-cell intracellular β-hexosaminidase. The excreted enzyme for both cultures was found to be a low- or no-uptake form. This form of β-hexosaminidase may have been excreted from a secondary route preceding the site of the monensin effect. The similar findings in monensin-treated normal and I-cell cultures suggest that the subcellular site of the biochemical defect in I-cell disease is at a location after the site of the monensin effect i.e. late in the Golgi region or at a post-Golgi-region location.