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

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Abstract
The carboxylic ionophore, monensin, blocks the migration of glycoprotein-containing vesicles from the Golgi region to the plasma membrane in fibroblasts resulting in an accumulation of secretory products in the Golgi cisternae. Treatment of cultured I-cell fibroblasts with monensin (0.5 .mu.M) decreased the abnormal excretion of .beta.-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 .beta.-hexosaminidase occurred in treated normal fibroblast cultures. The intra- and extra-cellular distribution of newly synthesized .beta.-hexosaminidase in both normal and I-cell cultures converged during monensin treatment. .beta.-Hexosaminidase isoenzymes excreted by both monensin-treated normal and I-cell fibroblasts were electrophoretically indistinguishable from the 4 bands characteristic of I-cell intracellular .beta.-hexosaminidase. The excreted enzyme from both cultures was a low- or no-uptake form. This form of .beta.-hexosaminidase may have been excreted from a secondary route preceding the site of the monensin effect. The subcellular site of the biochemical defect in I-cell disease may be at a location after the site of the monensin effect, i.e., late in the Golgi region or at a post-Golgi-region location.