The inactivation of hemostatic factors by hematin

Prolonged clotting times and reduced levels of clotting factors have been reported in hematin-treated patients. This effect persists for up to 5 hr after hematin infusion, associated with plasma levels ranging from 0.01 to 0.04 mg/ml. Therefore we performed in vitro studies to investigate the effects of hematin on fibrinogen, thrombin, factor VIII:C, and plasmin. Hematin in a final concentration of 0.01 mg/ml inhibited the clotting of bovine fibrinogen (1.3 to 2.6 mg/ml) by bovine thrombin (0.12 U/ml) and inhibited the hydrolysis of a synthetic substrate by human thrombin. However, if the hematin was first mixed with albumin (25 mg/ml), fourfold higher concentrations were required to prolong the thrombin clotting time. Hematin, 0.035 mg/ml, reduced VIII:C activity from 0.88 to 0.40 U/ml as measured by two-stage assay. Hematin (0.05 mg/ml) also inhibited the activation of VIII:C by thrombin (0.04 U/ml): baseline activity, 0.84 U/ml; thrombin-activated, 2.94 U/ml; with hematin added, 1.33 U/ml. Hematin also inhibited clot lysis. The inclusion of hematin (0.03 mg/ml) in the diluting buffer reduced the lysis of whole blood clots from 86% ± 5 to 23% ± 5 (p < 0.001, mean ± S.D. of four determinations) and decreased the lysis of ¹²⁵I-fibrin clots induced by plasmin (0.02 CTA U/ml) from 100% to 27%. In concentrations as low as 0.09 μg/ml, hematin inhibited the hydrolysis of a synthetic substrate by plasmin. Hematin was mixed with fibrinogen, albumin, or thrombin, and the mixtures applied to Sephadex G-200 columns. Adherence of the hematin to Sephadex was prevented by either prerinsing the column with albumin or using borate buffer at pH 9.2. Hematin co-eluted with each protein applied to the column and, in the case of fibrinogen, altered its electrophoretic mobility and markedly prolonged the thrombin clotting time of the eluted fibrinogen. We conclude that hematin binds to a variety of hemostatic proteins, inhibiting their biologic activity.