The purpose of this study was to characterize the inhibitory effects of choline on the terminal enzymatic complex of the blood coagulation cascade. Reports in the literature have indicated that choline lengthens whole plasma clotting time with a likely site of inhibition being the step that converts prothrombin (II) to thrombin (IIa). In order to quantitate the effects of choline on prothrombinase activity, bovine factor V and human factor II were isolated in this laboratory by established methods, and other prothrombinase components were obtained from commercial sources. The activity of venom-activated bovine factor Xa was assayed in the presence of factors II and Va, phospholipid, Ca2+, and chromogenic substrate (Kabi S-2238). The release of p-nitroaniline from S-2238 was measured, by continuous spectrophotometric monitoring of incubation mixtures at 400 nm, as an index of IIa production. Under these assay conditions, choline acted as a mixed-type inhibitor of the prothrombinase complex with an apparent KI of 25 mM and an apparent KI' of 41 mM. At similar concentrations, choline also increased the activated coagulation time of whole-blood. The inhibition of prothrombinase activity was independent of [Ca2+], factor Va, and micelle phospholipid composition, suggesting an interaction between choline and factors Xa and/or II.
Biochemistry, Blood Coagulation Tests; Choline
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