Lysosomal sialidase deficiency in sialidosis with partial beta-galactosidase deficiency

We analyzed the subcellular localization of sialidases in human lymphocytes from a patient with adult type sialidosis with partial beta-galactosidase deficiency and normal controls. Sialidase activities were measured with alpha,2 leads to 3 NeuAc-lactitol, 4-methylumbelliferyl-NeuAc and GM3 ganglioside as substrates. Sialidases in the lysosomes were sonication-labile and hydrolyzed mainly hydrophilic substrates such as NeuAc-lactitol and 4-methylumbelliferyl-NeuAc, but hydrolyzed subsidiarily GM3 ganglioside. On the other hand, sialidases in the plasma membrane were sonication-stable and hydrolyzed both hydrophilic substrates and GM3 ganglioside. In sialidosis with partial beta-galactosidase deficiency, the sialidases of the lysosomes showed 3-5% activity toward hydrophilic substrates and 25% activity toward GM3 ganglioside as compared with sialidase activities of the controls. However, there are no differences in the activities of the sialidases in the plasma membrane. These results demonstrate that the essential defect in this disease is the deficiency of a lysosomal sialidase.