Kunitz抑制剂家族成员抑肽酶对纤溶酶原活化的抑制作用

抑肽酶属Kunitz抑制剂家族成员，能够抑制激肽释放酶、纤溶酶及胰蛋白酶的蛋白水解活性．研究表明，抑肽酶能够抑制尿激酶型-纤溶酶原激活刹（u—PA）和组织型纤溶酶原激活剂（t—PA）对纤溶酶原的激活，但不影响u—PA和t—PA对小分子底物的酰胺水解活性．用u—PA研究了上述作用的机制，发现抑肽酶与u—PA的丝氨酸蛋白酶功能区特异性结合，而与纤溶酶原没有相互作用．抑肽酶与uPA的结合并不阻断u—PA的活性位点，因为u—PA对小分子底物的水解活性仍然保持．上述发现提示抑肽酶可能存在另一种抑制作用模式，该模式不同于以前报道的关于Kunitz抑制剂或纤溶酶原激活酶抑制剂的作用．由于人体内的Kunitz抑制剂与抑肽酶在结构上非常相似，根据研究结果，推测体内纤溶酶原的激活作用并非仅受丝氨酸蛋白酶抑制剂的控制。

Inhibition of Plasminogen Activation by a Kunitz Inhibitor, Aprotinin

Aprotinin, a natural inhibitor belonging to the Kunitz family, is known to inhibit proteolysis of kallikrein, plasmin and trypsin. In the present study, aprotinin was shown to inhibit plasminogen activation by urokinase-type plasminogen activator (u-PA) and tissue-type plasminogen activator (t-PA). By contrast, the amidolytic activities of these activators against their synthetic substrates were not inhibited by aprotinin. The mechanism of action was investigated using u-PA, and it was shown that aprotinin bound specifically to u-PA but not to plasminogen. Since aprotinin was also found to bind to low molecular weight u-PA, the binding site was therefore concluded to be the protease domain. However, it did not directly block the active site of u-PA, since the u-PA activity against the synthetic substrate was preserved. The findings suggested a model of action which has not been previously described for either Kunitz inhibitors or plasminogen activator inhibitors. Since the structure of aprotinin closely resembles certain human Kunitz inhibitors, these studies prompted the hypothesis that the pathway of plasminogen activation in vivo may not be solely under the control of serpins, as is currently believed.