Standardization of Plasminogen Assays

Like a number of the components of the fibrinolytic and coagulation systems, plasminogen (plgn) is a multifunctional molecule. As a proenzyme, a number of its activities such as its binding to fibrin, histidine-rich glycoprotein (HRGP) and α₂-antiplasimn (AP) are expressed while its major enzymatic activity remains unexpressed. This latter activity has been used as a yardstick of plasminogen potency, despite the fact that no such activity resides in the native plasminogen molecule. Assay procedures usually involve the activation of the plasminogen to plasmin using an activator such as streptokinase (SK) or urokinase (UK) and a major problem involves the establishment of a properly-timed plasminogen-activator ratio to fully express the plasminogen as the active enzyme plasmin (Gaffney, P.J. et al. Activation of plasminogen as a feature of its assay. Haemostasis 1977, 6, 72–78). Substrates such as casein, fibrinogen and fibrin have been used to assess the plasmin activity developed while more recently the tripeptide chromogenic substrate S-2251 has been successfully used. These assays have been standardised using a reference preparation of the active enzyme, plasmin, and both a 1st and 2nd International Reference Preparation (IRP) have been established. These IRP’s differed in that the fibrin binding kringle-structures were missing in the 1st IRP yielding differing fibrinolytic and chromogenic activities (Philo, R.D. and Gaffney, P.J. Plasmin potency estimates. Influence of substrate used in assay. Thrombosis and Haemostasis 1981, 45, 107–109). Activation procedures of plasminogen and subsequent assays of plasmin using a variety of substrates have been recently superseded by an assay which involves the formation of a plgn-SK complex which complex has an active site which hydrolyses the chromogenic substrate S-2251. This avoids the problems highlighted above involved in measuring plasminogen activity at the optimum stage during activation. While plasmin standards have been suitable for the standardisation of plasminogen when it is measured by activation-based procedures, a British Standard for glutamic acid-plasminogen has now been established in order to standardise the plgn-SK assay (Gaffney, P.J. and Curtis, A.D. The establishment of a standard for plasminogen (glu-type). Thrombosis and Haemostasis 1984, 51, 376–378). The calibration of this standard using the 2nd IRP for plasmin and the value of this standard in the measurement of plasminogen in plasma is discussed.