Human α-L-iduronidase. Catalytic properties and an integrated role in the lysosomal degradation of heparan sulphate

The kinetic parameters (K(m) and k(cat.)) of human liver α-L-iduronidase were determined with a variety of heparin-derived disaccharide and tetrasaccharide substrates. More structurally complex substrates, in which several aspects of the aglycone structure of the natural substrates heparin and heparan sulphate were maintained, were hydrolysed with catalytic efficiencies up to 255 times that observed for the simplest disaccharide substrate to be hydrolysed. The major aglycone structure that influenced both substrate binding and enzyme activity was the presence of a C-6 sulphate ester on the residue adjacent to the iduronic acid residue being hydrolysed. Sulphate ions and a number of substrate and product analogues were potent inhibitors of enzyme activity. Human liver α-L-iduronidase activity towards 4-methlylumbelliferyl α-L-iduronide at pH 4.8 had two K(m) values of 37 μM and 1.92 mM with corresponding k(cat.) values of 299 and 650 mol of product formed/min per mol of enzyme respectively, which may explain the wide range of K(m) values previously reported for α-L-iduronidase activity toward its substrate. Skin fibroblast α-L-iduronidase activity towards the heparin-derived oligosaccharides was influenced by the same substrate aglycone structural features as was observed for the human liver enzyme. A comparison was made of the effect of substrate aglycone structure upon catalytic activities of the enzymes which act to degrade the highly sulphated regions of heparan sulphate. A model was proposed whereby the substrate is directed from α-L-iduronidase to subsequent enzyme activities to ensure the efficient degradation of heparan sulphate.