The influence of heparin and indol on the catalytic properties of alpha- and beta/gamma -thrombins

Reversible inactivation by dextran sulfate of bovine thrombin in dilute solution

Dextran sulfate (DS)-enhanced inactivation of alpha-thrombin was faster at pH 6 than at pH 8 in the absence of NaCl. Substrate protected only partly thrombin from inactivation by DS at pH 6. Some amidolytic activity of alpha-thrombin inactivated by DS was restored time-dependently by the addition of NaCl. Moreover, beta and gamma-thrombin, and other intermediates were not formed from alpha-thrombin with DS-enhanced inactivation. All these results indicate that autolysis of alpha-thrombin in dilute solution is not responsible for enhancement of reversible inactivation of alpha-thrombin by DS.

The non-Michaelian action of thrombin on peptide p-nitroanilide substrates

1. v([S]) kinetic data were obtained for the hydrolysis of the chromogenic substrates H-D-Phe-L-Pip-L-Arg-pNA (S-2238), H-D-Ile-L-Pro-L-Arg-pNA (S-2288), Tos-Gly-L-Pro-L-Arg-pNA (Tos-Ch-TH) and Cbz-Gly-L-Pro-L-Arg-pNA (Cbz-Ch-TH) by native human thrombin under different experimental conditions (Pip is pipecolyl, pNA is p-nitroanilide, Tos is tosyl and Cbz is benzyloxycarbonyl). 2. The data were fitted to rational functions of order 1:1, 2:2 and 3:3 by using non-linear regression. Discrimination between equations of different degree was made using the F-test. 3. In all, 24 curves were fitted. In 17 cases degree 2:2 was significantly better than degree 1:1 at a confidence level of 95%. In no case was any further significant improvement found with functions of degree 3:3. 3. Our results allow us to assert that native human thrombin is an enzyme that does not allow Michaelian kinetic behaviour when acting on chromogenic substrates. Instead, the empirically obtained steady-state data require mechanisms whose rate equation is at least of degree 2:2.

Heparin and the progressive anti-thrombin activity of alpha 2-macroglobulin

Interactions between alpha 2-macroglobulin (alpha 2M) and thrombin have been studied by spectroscopic, isotopic and electrophoretic methods in presence or in absence of heparin. It is shown that thrombin binds to alpha 2M in a 1:1 ratio. Fluorescamin labelled heparin of Mr 7 000 interacts with thrombin to form a 2:1 molar complex. This complex does not bind to alpha 2M and is unable to achieve any proteolytic cleavage of this protein. In contrast the interaction of alpha 2M with chymotrypsin is not significantly affected by the mucopolysaccharide. Moreover, heparin is unable to react with alpha 2M bound thrombin.

Role of macromolecular binding site of thrombin molecule in excitation of anticoagulation system

The reaction of anticoagulation system upon perfusion of humorally isolated (with retained innervation) carotid sinus of a rabbit by alpha-,beta/gamma-, DIP-alpha-thrombin and prethrombin I was studied. DIP-alpha-thrombin without clotting activity was shown to initiate like alpha-thrombin the reflex reaction of anticoagulation system characterized by a sharp increase in non-enzymatic fibrinolysis (by 225%) and total fibrinolytic activity of blood (by 51%). Prethrombin I (thrombin precursor) is also capable of exciting the function of anticoagulation system characterized by an increase in non-enzymatic fibrinolysis (by 82%) and total fibrinolytic activity (by 36%). Furthermore, perfusion of prethrombin I or alpha-thrombin at almost the same molar concentrations resulted in the similar degree of anticoagulation system effector reaction. Reflex response of anticoagulation system was not observed upon perfusion of carotid sinus by beta/gamma-thrombin that has high esterase but little if any clotting activity that appears to be due to molecular changes in the macromolecular binding site region. These data support the suggestion that the effect of anticoagulation system excitation is due to interaction of the macromolecular binding site in the structure of alpha-thrombin with anticoagulation system chemoreceptors.