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In most flow systems, the rate of protein transfer from bulk solution to a macroscopic surface is site-dependent. In studies on surface- mediated protein conversion, this hampers the comparison of a proposed expression for the conversion process, such as the Michaelis-Menten equation, which actually measured overall conversion rates. However, the rotating disc is a classical example of a uniformly accessible surface and therefore was used for a quantitative analysis of prothrombin conversion by the phospholipid-bound factor Xa/factor Va complex (prothrombinase complex). A simple design of a rotating disc, adapted for ellipsometric measurement of protein adsorption, is presented. Agreement between experiment and theory was obtained for the influence of rotation velocity on the initial, transport-limited, adsorption rates of lysozyme, prothrombin, and fibrinogen. After coverage of the disc with a 20% phosphatidylserine/80% phosphatidylcholine bilayer and preadsorption of factor Va, addition of excess factor Xa and prothrombin resulted in effective conversion of prothrombin. For high (10 fmol.cm-2) surface coverage of prothrombinase, the rate of conversion equals the transport limited adsorption rate of prothrombin. For low (0.1 to 0.5 fmol.cm-2) surface concentrations of prothrombinase, the conversion rate dropped below the transport limit and the intrinsic kinetic parameters could be estimated at Km = 7.1 +/- 1.2 nM and kcat = 25 +/- 1.0 s-1 (20 degrees C). At these low surface activities of prothrombinase, the effect of the rotation rate (6 to 225 rad.s-1) on prothrombin conversion could be explained by the rotation-rate dependent prothrombin transport. This indicates that the fluid shear rate has no drastic influence on the intrinsic kinetics of prothrombin conversion.

Generation and inhibition of activated factor IXa was studied in factor XIa-activated plasma containing 4 mmol/L free calcium ions and 20 mumol/L phospholipid (25 mol% phosphatidylserine/75 mol% phosphatidylcholine). Interference of other (activated) clotting factors with the factor IXa activity measurements could be avoided by using a highly specific and sensitive bioassay. Factor IXa generation curves were analyzed according to a model that assumed Michaelis-Menten kinetics of factor XIa-catalyzed factor IXa formation and pseudo first order kinetics of inhibition of factor XIa and factor IXa. In the absence of heparin, factor IXa activity in plasma reached final levels that were found to increase with increasing amounts of factor XIa used to activate the plasma. When the model was fitted to this set of factor IXa generation curves, the analysis yielded a rate constant of inhibition of factor XIa of 0.7 +/- 0.1 min-1 and a kcat/Km ratio of 0.29 +/- 0.01 (nmol/L)-1 min-1. No neutralization of factor IXa activity was observed (the estimated rate constant of inhibition of factor IXa was 0). Thus, in the absence of heparin, the final level of factor IXa in plasma is only dependent on the initial factor XIa concentration. While neutralization of in situ generated factor IXa in normal plasma was negligible, unfractionated heparin dramatically enhanced the rate of inactivation of factor IXa (apparent second order rate constant of inhibition of 5.2 min-1/per microgram heparin/mL). The synthetic pentasaccharide heparin, the smallest heparin chain capable of binding antithrombin III, stimulated the inhibition of in situ generated factor IXa, but sevenfold less than unfractionated heparin (k = 0.76 min-1 per microgram pentasaccharide/mL). We found that free calcium ions were absolutely required to observe an unfractionated heparin and pentasaccharide-stimulated neutralization of factor IXa activity. Factor XIa inhibition (psuedo first order rate constant of 0.7 min-1) was not affected by unfractionated heparin or pentasaccharide in the range of heparin concentrations studied.