Selective determination of alpha 2-plasmin inhibitor activity in plasma using chromogenic substrate

In determination of antiplasmin activity of alpha 2-plasmin inhibitor (alpha 2 PI) using an amidolytic method, alpha 2-macroglobulin (alpha 2 M) in concentrations present in normal plasma inhibits the cleavage of chromogenic substrates by plasmin even if the reaction time between alpha 2M and plasmin is shortened as much as possible. The purpose of this study was to develop a new method to assay alpha 2 PI selectively with the chromogenic substrate S-2251 using an end-point method. The antiplasmin activity of alpha 2 M was destroyed by the addition of methylamine hydrochloride. In a case of congenital alpha 2 PI deficiency, antiplasmin activity determined with the previous method without the addition of methylamine was 20% of normal plasma, whereas the results obtained using this newly developed method was 5%. A statistically significant correlation between alpha 2 PI levels determined with this method and with a single radial immunodiffusion method was observed in plasma samples collected from 40 healthy subjects and 11 patients with disseminated intravascular coagulation (DIC). In two cases of DIC, in which markedly depleted levels of alpha 2 PI and normal concentrations of alpha 2 M were observed, antiplasmin activities measured without the addition of methylamine were higher than the ones measured immunologically, whereas the results obtained with the method described here were in accordance with the antigenicities of alpha 2 PI.