Enzymatic depolymerization of heparin-related polysaccharides. Substrate specificities of mouse mastocytoma and human platelet endo-beta-D-glucuronidases.

Two endo-beta-D-glucuronidases acting on heparin-related polysaccharides were investigated: a mouse mastocytoma enzyme previously implicated in the postbiosynthetic modification of the heparin proteoglycan and a human platelet-derived enzyme, capable of degrading heparan sulfate as well as heparin (with elimination of anticoagulant activity). The mastocytoma enzyme was found to depolymerize a heparin precursor polysaccharide containing both N- and O-sulfate groups to fragments somewhat smaller in molecular size than commercially available heparin. In contrast, another heparin precursor species, containing N- but no O-sulfate groups was resistant to degradation. Furthermore, incubation of the mastocytoma endoglucuronidase with a heparin octasaccharide having high affinity for antithrombin failed to cleave the beta-glucuronidic linkage in the antithrombin-binding region. Previous studies established that the platelet endoglucuronidase can degrade the exclusively N-sulfated as well as the N- and O-sulfated heparin precursor polysaccharides (Oldberg, A., Heldin, C.-H., Wasteson, A., Busch, C., and Höök, M. (1980) Biochemistry 19, 5755-5762). This enzyme has now been found to attack also the beta-glucuronidic linkage in the antithrombin-binding region of the heparin molecule. The loss of bio-affinity resulting from cleavage of this linkage in the antithrombin-binding heparin octasaccharide was utilized to construct a sensitive, specific, and simple assay method for the platelet endoglucuronidase. It is concluded that the platelet endoglucuronidase has a lower degree of substrate specificity than has the mastocytoma enzyme, in reflection of the surmised functional roles of the two enzymes.