Role of charged groups in factor XI/XIa activity.

To elucidate the role of charged groups in expression of factor XI coagulant activity, the charged groups of purified human blood coagulation factor XI/XIa containing 125I-XI/XIa were derivatized: free amino groups by succinylation, guanido groups of arginine by reaction with phenylglyoxal hydrate, and free carboxyl groups by reaction with ethylenediamine. The modified proteins were tested for: 1) ability to adsorb to glass, 2) ability to be cleaved by trypsin or factor XII-high molecular weight kininogen, 3) coagulant activity. The amino group-modified factor XI had a significantly decreased ability to bind to glass; modification of arginine or carboxyl groups did not affect adsorption. Trypsin cleaved factor XI with modified free amino, guanido, or carboxyl groups. Factor XII-high molecular weight kininogen could cleave only the arginine-modified factor XI. Amino group-modified factor XI and carboxyl group-modified factor XI lost all their factor XI assay activity, whereas arginine-modified factor XI retained 50% of the original activity. Amino group-modified factor XI could not be activated by trypsin, but arginine-modified and carboxyl group-modified factor XI could be activated by trypsin to 50% of the original activity. Succinylation of the amino groups of factor XIa destroyed all its factor XIa activity. Arginine-modified and carboxyl group-modified factor XIa retained 50% of their factor XIa activity. We conclude that epsilon-amino groups are essential for adsorption; activation by factor XII-high molecular weight kininogen requires free amino and carboxyl but not guanido groups; free amino, carboxyl, and guanido groups in factor XIa all appear to be critical for interaction of factor XIa with factor IX.