Studies on Thrombin Inactivation in Normal Plasma, Serum and Plasma Fractions, and its Relation to Heparin

Summary The antithrombin activity of plasma, serum and plasma fractions has been studied under different experimental conditions. Two different methods have been used for the assay of antithrombin activity. Under the experimental conditions described the following antithrombin activities have been found to be of importance in the inactivation of thrombin: 1. Progressive inactivation in the absence of heparin can take place in plasma and to a less extent in defibrinated plasma and serum. 2. Inactivation of thrombin in presence of fibrinogen and heparin in low concentration (about 0.03 I.U./ml). This effect was abolished at high heparin levels. 3. Inactivation of thrombin by plasma, defibrinated plasma and serum at high concentrations of heparin (about 2—15 I.U./ml). This activity is tentatively named the heparin co-factor.