Purified prethrombin 1 converts to thrombin in the presence of purified Ac-globulin, purified Factor Xa, phospholipid and calcium ions (Baker and Seegers, Thromb. Diath. Haemorrh. 17: 205-213 (1967)). The yields are, however, low. By adding a protein fraction isolated from bovine prothrombin complex the yield becomes maximum. This fraction we call Protein M. It is obtained by fractionating bovine prothrombin complex (McCoy and Seegers, Thromb. Res. 1: 451-472 (1972)) using a DEAE-Sephadex A-50

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... olumn 1.5 χ 15 cm as previously described (Novoa, Seegers and Hassouna, Prep. Biochem, 6: 307-338 (1975)). Prothrombin and Factor IX are eluted with 0.25M sodium chloride followed by Protein M at 0.30M sodium chloride. Protein M is obtained as a single component from a Sephadex G-100 column (2.5 χ 180 cm). Until now we have not been able to identify Protein M with any previously known coagulation factor. Protein M is a single chain protein with an apparent molecular weight near 60,000 as determined by Polyacrylamide gel electrophoresis. Litte prethrombin 1, highly purified bovine prothrombin also requires Protein M for maximum thrombin production in the presence of purified Ac-globulin, purified Factor xa, phospholipid and calcium ions. In relatively large amounts Protein M shortened the prothrombin time Of normal bovine plasma as well as plasma samples drawn during Coumadin administration.