Activation of human plasma prekallikrein induced by polymeric materials

The contact of blood with foreing surfaces causes the activation of factor XII to Xlla, which, through the conversion of plasma prekallikrein (PKK) into kallikrein (KK), starts the coagulation c~scade leading to thrombus formation (1). Since plasma KK is easily detectable spectrophotometrically by the p-nitroaniline released in the KK-promoted hydrolysis of the chromogenic substrate H-D-Pro-Phe-Arg-NH-Ph-N02 (2), activation of plasma PKK to KK can be used as a test for the antithrombogenicity of materials to be employed in contact with blood. We have tested the KK-like activity of human plasma put in contact whith the following materials: borosilicate glass, as a high-activation reference; silicone, as a low-activation reference; two commercial polyurethanes, Biomer and Cardiothane 51, commonly employed in bioprosthetic devices; a composite of Cardiothane 51 and fibrin (1:1 ratio), recently obtained in our laboratories (3). Some preliminar results have been obtained by using the so-called end-point method. 0.5 ml of diluted plasma (107. volume of whole plasma in TRIS-HCI buffer) were placed in 6.7 mm internal diameter glass tested and test kept tubes as such or internally coated with the materials to be at 37°C under stirring at 1100 r.p.m. After different contact times samples of 0.1 ml were drawn and each was added to te chromogenic substrate (S-2302 Kabi, 0.1 ml of 2 roM solution in 0.3 ml of TRIS-HCI) in a polystyrene test tube at 37°C. After a reaction time of 10 min, the reaction was stopped by adding 0.2 ml of CH 3COOH (507. volume) and the absorbance value was read at 405 rum. The KK-like activity increases in the first minutes of plasma contact time and then decreases, likely owing to the formation of KK-inhibiting substances, settling at nearly constant levels after about 16 min. The maximum absorbance obtained with glass has been conventionally assumed as 1007. activation. As regards the polymeric materials tested, it is noteworthy the maximum activation shown by the fibrin-containing composite (ab.137.), lower than those of the two commercial biomaterials Biomer (ab.287.) and Cardiothane 51 (ab.257.) and near to that of silicone (ab.87.). This is another result in favour of the good haemocompatibility of such composites, already seen by other tests (4). In addition, the validity of plasma PKK activation as a haemocompatibility test has been confirmed.