Several strategies have been used to obtain recombinant (r) human plasminogens (HPg) containing different oligosaccharide side chains on its sole N-linked glycosylation site, present at Asn289. The approaches included expression of the cDNA for HPg in insect cell lines under various conditions, addition of glycosidase inhibitors during expression, and purification of specific glycoforms of HPg using affinity chromatography on an insolubilized lectin column. The activation kinetics for urokinase (UK) of each of the purified HPgs, as well as their relative abilities to bind to the ligand, epsilon-aminocaproic acid (EACA), were then determined. Removal of both N- and O-linked oligosaccharide from HPg resulted in a slight increase in the Kcat/Km for its activation, while a glycoform containing tetrasialyl-tetra-antennary complex oligosaccharide on Asn289 was a slightly poorer substrate for UK than plasma HPg, which contains bisialyl-biantennary complex carbohydrate on Asn289. The most dramatic differences were observed for HPgs with high mannose-type glycans on Asn289. (Man9GlcNAc2)-HPg possessed only approximately 6% of the kcat/Km of plasma HPg, whereas (Glc3Man9GlcNAc2)-HPg did not undergo activation at a significant rate by UK. Differences were also found in the relative abilities of the HPg glycoforms to interact with EACA. The most effective interactions were observed with HPgs containing complex-type glycans, and the least effective binding was found for HPgs with high mannose-type oligosaccharides. The full range of the binding effects is represented by a fourfold difference between HPg containing tetrasialyl-tetra-antennary glycan and HPg with (Glc3Man9GlcNAc2) assembled on Asn289. These results clearly demonstrate that the nature of the N-linked glycan assembled on HPg dramatically influences its ability to be activated by UK and to bind to omega-amino acid effector molecules.
D J Davidson, F J Castellino