The inhibition by heparin of the intrinsic pathway activation of factor X in the absence of antithrombin-III

Heparin was found to inhibit the activation of purified factor X by purified factors IXa and VIII (or VIIIa) in the presence of calcium and phospholipid but in the absence of antithrombin-III. In contrast, heparin did not inhibit the activation of factor X by either the extrinsic pathway or by the factor X activator isolated from Russell's Viper Venom (RVV-X). The heparin inhibition of the intrinsic pathway activation of factor X could be totally reversed by polybrene. These results suggest that even in the absence of antithrombin-III, heparin reversibly interferes with the interactions of one or more of factors IXa, VIII (or VIIIa) and X with calcium and phospholipid that result in the activation of factor X.