The lysosomal storage disorders are a group of inherited
metabolic diseases each characterised by a relative or absolute deficiency of one or more of the
lysosomal proteins involved in the hydrolysis of
glycoconjugates or in the transport of the resulting product.
Enzyme replacement therapies are under consideration for a number of these disorders and are based on the in vitro observation that cells from affected patients can be corrected by addition of exogenous
enzyme. In this study, two glycosylation variants of the lysosomal
enzyme N-acetylgalactosamine-4-sulphatase (4S) (the deficiency of which causes
Mucopolysaccharidosis (MPS) type VI, (
Maroteaux-Lamy syndrome) were made by expression of 4S
cDNA in both wild type chinese hamster ovary (CHO-K1), and Lec1 (
N-acetylglucosaminyltransferase I deficient CHO-K1) cells. Differences in the glycosylation pattern of the two
enzyme forms were demonstrated with
endoglycosidase H and
N-glycosidase F digestions. The receptor mediated binding of these two forms of 4S to two cell types, human skin fibroblasts and rat alveolar macrophages, was then analysed. We have shown that both
enzyme forms bind to the
mannose-6-phosphate receptor on human skin fibroblasts with equal affinity demonstrating that the degree of phosphorylation of
mannose residues in the two forms is similar. However, using rat alveolar macrophages, we found that the binding/uptake of the two
enzymes differs considerably. These results show that differences in glycosylation of lysosomal
enzymes can be an important factor in altering
enzyme uptake by different cell types. Thus, producing
carbohydrate modification variants in this way may be useful for altering the distribution of exogenous
enzyme in vivo.