1. In a patient with I-cell disease the activities of several acid hydrolases were elevated in plasma and reduced in cultured fibroblasts when compared with normal values. Normal activities for the enzymes were found in leucocytes. These findings agree with reports on other cases.

2. N-Acetyl-β-d-glucosaminidase was resolved into its component forms by chromatography on microcolumns of DEAE-cellulose coupled with continuous automated assay of activity in the column effluent. Cultured skin fibroblasts from three patients showed a profound deficiency of glucosaminidase component A and a relative increase in the activity of a form eluted earlier than A.

3. In the one patient studied, the elution profile of plasma glucosaminidase was similar to that of normal plasma, but treatment with neuraminidase revealed a minor component which did not appear in control specimens.

4. Chromatographic resolution of glucosaminidase secreted by normal fibroblasts into the culture medium showed that component A comprised two forms, a serum-type and a tissue-type, whereas only a serum-type was found in I-cell medium.

5. Different forms of α-l-fucosidase were shown to occur in normal plasma and fibroblasts. This is the second lysosomal hydrolase for which differences between intracellular and extracellular forms have been described and might reflect a general phenomenon.

6. The major acidic component of fucosidase from normal fibroblasts was not detected in I-cell fibroblasts. Elution profiles of fucosidase activity in normal and I-cell plasma were indistinguishable, both before and after treatment with neuraminidase.

7. On the basis of the above findings, we suggest that for several acid hydrolases there is a common biosynthetic reaction, which produces forms of these enzymes destined for incorporation into primary lysosomes rather than secretion by the cell. In cultured fibroblasts from patients with I-cell disease, the enzyme catalysing the reaction leading to the production of intracellular forms is deficient or defective, whereas the synthesis of precursor and secreted forms is unaffected.

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