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Generally the concentration of an enzyme inhibitor in plasma is estimated as follows: plasma, in decreasing dilutions, is incubated with a fixed amount of enzyme. When the inhibitor has been irreversibly converted to its enzyme- inhibitor complex, the excess of enzyme is estimated as the initial rate (v) of its activity with a synthetic substrate. The plot of v against plasma dilution results in a straight line which intercepts the abscissa at the point of equivalency.  Contrary to the above method, for the estimation of α2-antiplasmin (A) in plasma, the excess of plasmin (Pi) is measured immediately after the attainment of a fast equilibrium between Pi, A and the reversible complex PlA (equilibrium constant Ki). Since this is attained almost two orders of magnitude faster than the irreversible reactions of PI with inhibitors in plasma other than A, the assay,thus, becomes more specific for A.  The results of the kinetic analysis are; 1. The involvement of the reversible equilibrium causes the plot of v against plasma dilution to be a hyperbola instead of a straight line. The hyperbolic shape of the plot becomes more pronounced at higher ratios of K, over Pi concentration. Moreover, the synthetic substrati (S) shifts the equilibriumto dissociation, thereby apparently increasing the Ki by a factor of (1 + S/Km). 2. Pi inhibitors in plasma other than A, interfere Relatively more at low plasma dilutions. Because, coincidently, this interference makes the hyperbola resemble a straight line, the contribution of the other inhibitors is largely overlooked.  It is concluded that the usual assay may have its merit as a routine bioassay, provided the conditions are chosen carefully. As an alternative approach it is recommended to titrate a fixed amount of plasma with increasing concentrations of Pi and to evaluate the initial velocities in a Scatchard plot. In this way a more reliable point of equivalency and in addition the Ki value can be obtained

The Estimation Of α2-Antiplasmin By Means Of Synthetic Substrates; Kinetic Analysis And Optimization Of The Method