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.

Insensitivity of factor V and factor Va to diisopropylfluorophosphate and antithrombin III

Bovine plasma Factor V and Factor Va, the latter prepared by thrombin or venom activator action on Factor V, are not inactivated by diisopropylfluorophosphate or antithrombin III nor do they form identifiable complexes with either of these reagents. On the basis of these data, it is concluded that bovine Factor V and Factor Va are not serine proteases.

The measurement of human plasma antiplasmin activity by radial diffusion assay

An assay of human antiplasmins has been developed utilizing radial diffusion of plasma from wells cut in plasmin-enriched, fibrinogen-agarose plates. After diffusion the fibrinogen is clotted. Zones of fibrin protected from background fibrinolysis develop as the result of plasma antiplasmin activity. A pooled plasma standard was taken to contain 100% antiplasmin activity. Antiplasmin activity of 52 normal subjects varied from 64 to 132%. Washed platelets contained 1-5% antiplasmin activity. Using antisera to precipitate individual inhibitors, physical methods of separation, and electrophoresis of plasma in agarose, several different proteins were found to have antiplasmin activity in this assay. Thus, alpha2-macroglobulin contributed 56%, alpha1-antitrypsin 20%, antithrombin III 2%, and other proteins 22% of the total antiplasmin activity. 1 ml of whole plasma neutralized 7.0 CTA units of plasmin.

Decreased heparin sensitivity of cycholhexanedione-modified thrombin

Modification of 5--6 arginine residues of thrombin with 1,2-cyclohexanedione has resulted in the selective abolition of the heparin sensitivity of the enzyme's reaction with antithrombin-III, whereas the antithrombin-III sensitivity of native and modified thrombin was indistinguishable. It is suggested that heparin accelerates the thrombin antithrombin-III reaction by interacting with thrombin.

Automated antithrombin III assay with a centrifugal analyser

The factor Xa inactivating function of antithrombin III is measured automatically by an amidolytic method, adapted to a centrifugal analyser. Plasma is diluted in buffer with heparin. In stage I, diluted plasma is incubated with excess factor Xa. Heparin accelerates the saturation of antithrombin with factor Xa. In stage II, remaining factor Xa is determined with the chromogenic substrate Bz-Ile-Glu-Gly-Arg-pNA. The precision of the present assay compares favourably with that of the clotting assays and immunoassay. There is a close correlation (r = 0.82) between the results obtained with this assay and the immunoassay of antithrombin III.

Chromogenic substrates for horseshoe crab clotting enzyme. Its application for the assay of bacterial endotoxins

An endotoxin-activated hemocyte lysate from the horseshoe crab (Tachypleus and Limulus) was found to hydrolyze Bz-Ile-Glu-(gamma-OR)-Gly-Arg-p-nitroanilide (PNA), Bz-Val-Gly-Arg-PNA, Boc-Val-Leu-Gly-Arg-PNA, and Boc-Leu-Gly-Arg-PNA, all of which have the COOH-terminal Gly-Arg sequence. This amidase activity was due to a clottting enzyme contained in the lysate. Furthermore, the amidase activity increased by increasing the concentration of bacterial endotoxin (E. coli, 0111-B4) added to the lysate. Therefore, the measurement of the endotoxin-induced amidase activity made it possible to determine the concentration of the endotoxin.

Quenching of tryptophan fluorescence in human antithrombin III by iodide ion

Iodide is an efficient quencher of antithrombin III intrinsic tryptophan fluorescence. The quenching pattern indicates that about 60% of the tryptophyl fluorescence originates from exposed residues in the multitryptophan-containing protein. In denaturing media all of the tryptophyls are solvent-exposed. The binding of heparin to antithrombin III influences the number of solvent-exposed tryptophan residues. By studying the dependence of the quenching on pH, information regarding the presence of charged residues adjacent to tryptophyls was obtained.

Coagulation and the hypertensive diseases of pregnancy

The coagulation and fibrinolytic systems were investigated in a group of patients with essential hypertension during pregnancy, and the findings were compared with those of normal gravid women. Patients with essential hypertension exhibited the following significant differences: shortened partial thromboplastin times, thrombocytopenia, and decreased antithrombin III levels. Euglobulin lysis times and assays for fibrin breakdown products suggest that essential hypertension is not associated with changes in the fibrinolytic mechanism. Until more sophisticated studies can be performed on such patients, it cannot be concluded that the increased coagulability observed in pregnant patients with essential hypertension represents a state of intravascular coagulation.

The effects of operative stress on the coagulation profile

There is a tendency toward hypercoagulability in the postoperative period. This is manifested by changes in a number of coagulation parameters, and if not offset by some protective mechanism, thrombosis may occur. This protection appears to be mediated more through the fibrinolytic mechanism than through the action of antithrombin 3 (AT-3) because AT-3 activity diminishes in the early postoperative period. The introduction of variables such as invasion of the vascular system, intraoperative heparin administration, administration of whole blood, and insertion of a Dacron prosthesis does not appreciably affect the response of the coagulation profile to operative stress.

The effect of conjugated estrogens on coagulability in menopausal women

To investigate the effect of conjugated equine estrogens on the coagulability in menopausal women, blood samples were drawn from 69 menopausal females--35 receiving estrogens and 34 receiving no replacement therapy. Using tests for antithrombin and heparin-antithrombin activities, 14.7% of the control patients were found to be hypercoagulable. This is a 289% increase in the incidence of hypercoagulability in the group treated with estrogens. The particular relevance of this finding to menopausal women and the importance of the use of the coagulation profile in managing such patients is discussed.