Protein catabolism in fibroblasts cultured from the skin of normal individuals and of patients with
mucolipidosis II (
I-cell disease) and several other
lysosomal storage diseases was examined by metabolic labelling with [3H]
leucine and following the fate of radioactive
proteins in pulse-chase experiments. In
mucolipidosis II cells, overall
protein degradative rates were found to be distinctly lower than in normal control cells. To distinguish lysosomal from non-lysosomal degradation, labelling experiments were carried out in the presence and absence of 10 mM NH4Cl, an inhibitor of lysosomal function. It was found that
mucolipidosis II fibroblasts exhibited a markedly reduced rate of lysosomal protein degradation, whereas the rate of nonlysosomal degradation appeared normal. Serum and
amino acid starvation led to a marked increase in lysosomal protein degradation in normal cells, but had only a minimal effect on that in
mucolipidosis II fibroblasts. The specific activities of
cathepsins B, H and L were profoundly diminished in all
mucolipidosis II cell lines tested. Lysosomal protein degradation in a
mucolipidosis III cell line was impaired to a similar degree as in
mucolipidosis II cells, whereas it was decreased to a lesser extent in fibroblasts from patients with
mucopolysaccharidoses I and VI,
galactosialidosis and GM1-gangliosidosis. We conclude that fibroblasts from patients with
mucolipidosis II and III have a severely compromised capacity for endogenous lysosomal protein degradation that appears to result from multiple
cathepsin deficiency. This lysosomal defect is likely to have pathophysiological consequences.