The role of heparin in the thrombin-antithrombin III reaction

Studies on the mechanism of the rate-enhancing effect of heparin on the thrombin-antithrombin III reaction

The rate of the reaction between thrombin and antithrombin III is greatly increased in the presence of heparin. Several mechanisms for this effect are possible. To study the problems commercial heparin was fractionated into one fraction of high anticogulant activity and one of low anticoagulant activity by affinity chromatography on matrix-bound antithrombin III. The strength of the binding of the two heparin fractions to antithrombin III and thrombin, respectively, was determined by a crossed immunoelectrophoresis technique. As was to be expected, the high activity fraction was strongly bound to antithrombin III while the low activity fraction was weakly bound. In contrast, thrombin showed equal binding affinity for both heparin fractions. The ability of the two heparin fractions to catalyse the inhibition of thrombin by antithrombin III was determined and was found to be much greater for the high activity heparin fraction. A mechanism for the reaction between thrombin and antithrombin III in the presence of small amounts of heparin is suggested, whereby antithrombin III first binds heparin and this complex then inhibits thrombin by interaction with both the bound heparin and the antithrombin III.

The binding of low-affinity and high-affinity heparin to antithrombin. Fluorescence studies

The interaction between antithrombin and two forms of heparin, differing in their affinity for the matrix-linked protein, has been studied by fluorescence. The binding of the high-affinity heparin fraction to antithrombin leads to activation of the inhibitor, allowing it to react more rapidly with a number of serine proteases of the coagulation cascade. The interaction with the low-affinity heparin fraction, however, has considerably less influence on this inhibition rate. The binding of either fraction to antithrombin was found to result in an increase of the tryptophan fluorescence of the protein. This increase was much larger for high-affinity heparin than for low-affinity heparin, suggesting a different mode of binding of the two fractions to the protein. The fluorescence enhancement caused by high-affinity heparin is consistent with a conformational change of antithrombin related to its activation. Only the fluorescence enhancement observed on the binding of high-affinity heparin was of a sufficient magnitude to allow quantitative studies. These showed high-affinity heparin to bind to antithrombin with a stoichiometry of about one and with a binding constant at physiological ionic strength of about 8 X 10(7) M-1. At higher ionic strengths, however, the affinity decreased markedly.

A catalytic role for heparin. Evidence for a ternary complex of heparin cofactor thrombin and heparin

The interaction of heparin with chemically modified thrombin and heparin cofactor is studied. Amidinated heparin cofactor does not bind to heparin-agarose and the reaction rate of the amidinated inhibitor with unmodified thrombin is not affected by heparin. Likewise, thrombin modified with 1,2--cyclohexanedione does not bind to heparin agarose and the reaction rate of the modified enzyme with unmodified inhibitor is not affected by heparin. In the absence of heparin, the modified and unmodified proteins react at the same rate in all possible combinations. Affinity chromatography of diisopropylphosphoryl thrombin on heparin cofactor coupled to Sephadex G--50 is used to study the binding of heparin cofactor and thrombin to heparin. The thrombin for all experiments is tritium-labeled and then inactivated with diispropylfluorophosphate. Thrombin is not bound to heparin cofactor-Sephadex columns. However, after treatment of the columns with a heparin solution, thrombin binds tightly, and is eluted at high ionic strength. Bound thrombin can also be eluted with either excess non-radioactive thrombin or excess free heparin. Heparin-dependent binding of thrombin does not occur if the heparin cofactor-Sephadex is heat-denatured. The ability of heparin to couple solution-phase thrombin to solid-phase heparin cofactor indicates that a ternary complex is formed. Analysis of the binding of the proteins to heparin by a dye displacement method suggests that at least one site on heparin binds to thrombin but not to heparin cofactor. Further support for a catalytic role for heparin derives from the ability of catalytic concentrations of heparin to enhance the rate of hydrolysis of prothrombin by thrombin, another protein pair which bind mutually to heparin.

Effect of heparin on the extent of inhibition of thrombin by heparin cofactor

A heparin preparation obtained by gel chromatography is compared to unfractionated heparin with respect to the effects of heparin on the reaction between thrombin and heparin cofactor. Whereas both preparations enhance the rate of inhibition of thrombin by heparin cofactor, the extent of inhibition is decreased by the unfractionated, but not by the fractionated heparin. The decreased extent of inhibition is accounted for by residua of unreacted and undegraded heparin cofactor and thrombin, as demonstrated by gel electrophoresis in dodecyl sulfate. However both heparin preparations enhance the rate of degradation by thrombin of the thrombin-heparin cofactor complex.

The size and shape of human and bovine antithrombin III

Human and bovine antithrombin, purified by affinity chromatography on heparin-agarose, have been characterized with regard to chemical composition, size, shape and conformation. Both preparations were found to contain several active components of identical or similar size but different electrical charge. Amino acids and carbohydrate analyses revealed striking similarities between human and bovine antithrombin, while immunological analyses failed to demonstrate any cross-reactivity. The molecular weights were determined by sedimentation equilibrium to be 58 000 for human and 56 000 for bovine antithrombin. The small molecular weight difference suggested by these values was verified by several empirical methods of molecular weight estimation. Hydrodynamic measurements indicated that the two proteins have similar molecular shapes, both of which are slightly more extended that that of typical globular proteins. The internal folding of the two polypeptide chains is also similar, as evidenced by the identity of the far-ultraviolet circular dichroism spectra. Specifically, these analyses suggested a low alpha-helix content of both proteins. In conclusion, the marked structural similarity of human and bovine antithrombin indicates that the two proteins may also exhibit extensive functional similarities in the binding of heparin and the inhibition of various coagulation factors.