ENZYMATIC REMOVAL OF SIALIC-ACID FROM HUMAN FACTOR-IX AND FACTOR-X HAS NO EFFECT ON THEIR COAGULANT ACTIVITY

Factor IX and factor X have sialic acid in O-linked and N-linked oligo saccharides on their activation peptides, and a terminal sialic acid i s found on a recently described O-linked tetrasaccharide at Ser-61 in the light chain of human factor Ma. In studies presented here, the pot ential role of sialic acid residues in mediating activity of human coa gulation factors IX and X was tested after enzymatic removal of sialic acid residues. In contrast to previous reports, treatment of factor I X or factor Ma with recombinant sialidase did not decrease the rate of factor IX activation or proteolytic properties of human factor IXa. T he activation rates of factor IX and desialated factor IX were indisti nguishable when treated with factor XIa, with factor VIIa/tissue facto r complex, and with the factor X activating enzyme from Russell's vipe r venom. Desialated human factor Ma showed full activity in the non-ac tivated partial thromboplastin time assay and retained full ''tenase'' activity in a coupled amidolytic assay. Similar experiments with huma n factor X showed no detectable loss of clotting activity in the proth rombin time assay after desialation. Additionally, desialated human fa ctor X was cleaved by the factor X activating enzyme from Russell's vi per venom and intrinsic tenase at the same rate as untreated factor X when analyzed by SDS-polyacrylamide gel electrophoresis. These studies have shown that factor IX and factor X clotting activity are not depe ndent on sialic acid content. Further studies are needed to determine whether desialated factor IX binds to endothelial cells, and whether f actors IX and X are more rapidly cleared from circulation or have alte red susceptibility to proteolysis after enzymatic removal of sialic ac id.