Inactivation of thrombin by antithrombin III on a heparinized biomaterial

Heparin covalently bonded to polyvinyl alcohol (PVA) is potentially useful as a nonthrombogenic coating in the preparation of small diameter vascular prostheses and blood sampling catheters. PVA-heparin is highly stable: the elution rate of 35S-heparin from the polymer was determined to be negligible (approx. 2 × 10−11 g/cm2 min) when washed with either buffered saline (pH 7.4) or citrated human plasma. The inactivation of thrombin by antithrombin III was studied on PVA-heparin. Using small columns of PVA-heparin beads eluted by 0.14M NaCl buffered at pH 7.4 both thrombin and antithrombin III bound to the immobilized heparin. If thrombin was loaded before an excess of antithrombin III, significant inactivation of thrombin was observed; however, loading antithrombin III before thrombin did not measurably inactivate thrombin. The results suggest that the covalently-bound heparin effectively participates in the inactivation of thrombin through the formation of surface-bound heparin-thrombin, which then reacts with antithrombin III to yield a surface-bound thrombin-antithrombin III complex. The fate of this surface-bound complex has yet to be clarified.