Effect of desialylation on the biological properties of human plasminogen

There are two major isoenzymes of plasminogen (Pg) in human plasma, designated Pg1 and Pg2. Both Pg forms have an identical primary structure, but differ in their extent of glycosylation. Removal of the oligosaccharide chains alters the normal physiological function of the zymogen and decreases the circulation time of both Pg glycoforms. Recent studies in our laboratory demonstrated that Pg2, with one carbohydrate chain, binds to the surface of U937 monocytoid cells considerably better than Pg1, with two carbohydrate chains, indicating a major role for the carbohydrate chains as determinants for differential binding to the cell surface [Gonzalez-Gronow, Grenett, Fuller & Pizzo (1990) Biochim. Biophys. Acta 1039, 269-276]. In this report we provide evidence that removal of terminal sialic acid from the Thr345-linked oligosaccharide chain of Pg2 is accompanied by the appearance of spontaneous amidolytic and fibrinolytic activity in the single-chain zymogen. Kinetic data demonstrate that asialo-Pg hydrolyses peptide substrates approximately 10% as efficiently as Pm. In addition, the change in carbohydrate content also alters Pg binding to U937 cells. Asialo-Pg binds to U937 cells with a decreased capacity but with a greater affinity than native Pg. Furthermore, asialo-Pg does not compete with native Pg for cell binding. These studies directly demonstrate that the oligosaccharide chains contribute to the heterogeneity observed in the physicochemical and biological properties of Pg1 and Pg2.