Thrombin-induced proteolysis of human antithrombin III: an outstanding contribution of heparin

Products obtained in reaction of excess antithrombin III (AT III) with human alpha-thrombin and heparin were found to contain markedly less of residual AT III than products of similar reactions without heparin. The increased utilization of AT III was primarily due to limited proteolysis of a portion of unbound inhibitor, associated with the release of a 50 000-dalton protein fragment. Thrombin-induced release of this fragment was promoted by polydispersed heparin preparation and, to a variable degree, by all heparin fractions obtained in gel filtration. The optimum amount of heparin required to facilitate AT III proteolysis was of 5 microgram/ml (0.8 u/ml). Excessive reduction of the residual inhibitory activity and changes in two-dimensional immunoelectrophoresis suggested that AT III in plasma is also subjected to nonproductive proteolysis and formation of a modified AT III derivative following addition of thrombin and heparin. These data indicate that the effect of heparin on AT III is more complex than generally recognized. On the one hand, heparin interacting with AT III and thrombin contributes to a rapid binding and neutralization of the enzyme; on the other, heparin facilitates proteolytic degradation of unbound inhibitor even in the presence of small quantities of thrombin accounting for excessive reduction of the overall inhibitory potential of AT III.