Neuraminidase activities in sialidosis and mucolipidosis

Cellular localization and substrate specificity of isoelectric forms of human liver neuraminidase activity

The four major isoelectric forms of human liver neuraminidase (with pI values between 3.4 and 4.8) have been isolated by preparative isoelectric focusing and characterized with regard to their substrate specificity using glycoprotein, glycopeptide, oligosaccharide and ganglioside natural substrates. All forms exhibited a rather broad linkage specificity and were capable of hydrolyzing sialic acid glycosidically linked alpha 2-3, alpha 2-6 and alpha 2-8, although differential rates of hydrolysis of the substrates were found for each form. The most acidic form 1 (pI 3.4) was most active on sialyl-lactose, whereas form 2 (pI 3.9) and 3 (pI 4.4) were most active on the more hydrophobic ganglioside substrates. Form 4 (pI 4.8) was most active on the low-Mr hydrophilic substrates (fetuin glycopeptide, sialyl-lactose). Each form was less active on the glycoprotein fetuin than on a glycopeptide derived from fetuin. Organelle-enriched fractions were prepared from fresh human liver tissue and neuraminidase activity on 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid was recovered in plasma membrane, microsomal, lysosomal and cytosolic preparations. Isoelectric focusing of the neuraminidase activity recovered in each of these preparations resulted in significantly different isoelectric profiles (number, relative amounts and pI values of forms) for each preparation. The differential substrate specificity of the isoelectric forms and the different isoelectric focusing profiles of neuraminidase activity recovered in subcellular-enriched fractions suggest that specific isoelectric forms with broad but defined substrate specificity are enriched at separate sites within the cell.

Lysosomal enzymes in ataxia: discovery of two new cases of late onset hexosaminidase A and B deficiency (adult Sandhoff disease) in French Canadians

We have measured in leukocytes the following lysosomal enzymes in 11 Friedreich disease cases, 11 "atypical" recessive ataxias, 13 neurological controls and 16 normal controls: hexosaminidase A and B; beta-galactosidase and neuraminidase (labile and cold stable, or A and B). The lysosomal enzyme deficiencies known to produce certain forms of spinocerebellar degeneration were not present in Friedreich's disease or the Charlevoix-Saguenay syndrome. The very small scale survey of "atypical" recessive ataxias revealed 3 cases of severe deficiencies in hexosaminidase activity. Two adult brothers presenting with the clinical phenotype of Kugelberg-Welander disease (one also with ataxia), were shown to have a severe deficiency of both HEX A and HEX B activity (Sandhoff biochemical pattern). This is the first such report. A further adult female patient, unrelated to the others, had a severe isolated deficiency of HEX B and presented with a very slowly progressive and mild ataxia with severe internal strabismus. These patients and their families are being studied clinically and biochemically in greater detail and will be reported elsewhere. However these preliminary findings justify screening for such lysosomal defects in all cases of "atypical" recessive ataxia.

Carrier detection of sialidosis with partial beta-galactosidase deficiency by the assay of lysosomal sialidase in lymphocytes

Lysosomal and plasma membrane sialidase activities in lymphocytes were studied in four patients with sialidosis with partial beta-galactosidase deficiency, four obligate heterozygotes, and three siblings of a patient. Lysosomal sialidase activity in homozygotes was absent, and that in heterozygotes was significantly decreased to 70% of control level. The results indicate that carriers can be detected by the assay of lysosomal sialidase activity of lymphocytes.

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.