尿激酶原激活是其催化纤溶酶原激活的限速步骤

尿激酶原催化纤溶酶原的激活包括3个反应：（1）尿激酶原内在酶活性催化纤溶酶原转化为纤溶酶；（2）纤溶酶激活尿激酶原变成尿激酶；（3）尿激酶激活纤溶酶原，所以一直以来人们对尿激酶原激活纤溶酶原的过程知之甚少．研究发现EACA可以抑制尿激酶原催化的纤溶酶原激活。通过EACA对上述3个反应的影响进行逐个分析，观察到在EACA浓度为2．0mmol／L时，尿激酶原内在酶活性催化纤溶酶原激活的反应速度提高了4．5倍，尿激酶激活纤溶酶原的反应速度提高了6倍．相反地，纤溶酶激活尿激酶原变成尿激酶的反直被抑制了50％，此反应决定了EACA对尿激酶原催化纤溶酶原激活反应的抑制作用．该结果提示尿激酶原催化纤溶酶原激活反应的限速步骤是纤溶酶激活尿激酶原变成尿激酶，缘于纤溶酶和尿激酶原的赖氨酸结合位点的相互作用．

Prourokinase Activation is the Speed-limited Step of Its Catalyzed Plasminogen Activation

Prourokinase catalyzed plasminogen activation is poorly understood since it includes following three reactions and is complicate for kinetics analysis: (1) the intrinsic activity of prourokinase activates plasminogen to plasmin; (2) plasmin activates prourokinase to urokianse; (3) urokinase activates plasminogen. In this paper, EACA was found to inhibit prourokinase catalyzed plasminogen activation. To analyze this inhibitory effect, the three reactions were separately studied. It was found that in the presence of 2.0 mmol/L EACA, plasminogen activation by the intrinsic activity of prourokinase and that by urokinase were promoted by 4.5 and 6-fold, respectively. In contrast, prourokinase activation by plasmin was found inhibited by 50%, suggested that it was responsible for the overall inhibitory effect of EACA on prourokinase catalyzed plasminogen activation. This also indicated that the rate-limited step of prourokinase catalyzed plasminogen activation was prourokinase activation by plasmin that was assisted by a lysine binding site(s) dependent interaction between plasmin and prourokinase.