Amidolytic assay of thrombin bound to alpha2-macroglobulin in plasma

A method for the determination in plasma of alpha2-macroglogulin-bound thrombin is described. Alpha2-Macroglobulin-bound thrombin is precipitated from plasma by 13% polyethyleneglycol, and its amidolytic activity is assayed by using the chromogenic substrate benzoyl-Phe-Val-Arg-p-nitroanilide (S 2160). After thrombin addition to plasma, only about 1.7% of the added thrombin activity was recovered in the alpha2-macro-globulin precipitate. It is suggested that the contribution of alpha2-macroglobulin to anti-thrombin activity of normal plasma is of little relevance.