Rippel K M, Nielsen S S, Hayes K D
Department of Food Science, Purdue University, West Lafayette, IN 47907, USA.
J Dairy Sci. 2004 Aug;87(8):2344-50. doi: 10.3168/jds.S0022-0302(04)73356-1.
The plasmin system native to bovine milk consists of the caseinolytic serine proteinase plasmin; its inactive zymogen, plasminogen; plasminogen activators; and inhibitors. Evidence in the literature indicates that whey proteins may inhibit plasmin activity, but there is very little mention of their effect on plasminogen activators. The objective of this research was to determine the effect of both unheated and heat-denatured beta-lactoglobulin (beta-LG), alpha-lactalbumin (alpha-LA), and BSA on plasminogen activators. Plasminogen activator activity was significantly stimulated by non-heat treated and denatured alpha-LA as well as by denatured beta-LG. The stimulation effect by these whey proteins was kinetically characterized, which showed that all 3 significantly increased the rate of plasminogen activation. The stimulation effect was shown to be independent of any effect of the whey proteins on plasmin activity by testing 2 different substrates, d-Val-Leu-Lys p-nitroanilide (S-2251) and Spectrozyme PL (Spec PL), in a plasmin assay. Results using S-2251 confirmed the inhibitory effect of whey proteins on plasmin observed by several researchers. However, use of SpecPL did not suggest inhibition. Ligand binding studies showed this discrepancy to be due to significant interaction between S-2251 and the whey proteins. Overall, this study indicates that whey protein incorporation into cheese may not hinder plasmin activity and may stimulate plasminogen activation. Furthermore, the results indicate the need for careful consideration of the type of synthetic substrate chosen for model work involving whey proteins and the plasmin system.
牛乳中的纤溶酶系统由酪蛋白分解丝氨酸蛋白酶纤溶酶、其无活性的酶原纤溶酶原、纤溶酶原激活剂和抑制剂组成。文献中的证据表明，乳清蛋白可能会抑制纤溶酶的活性，但很少提及它们对纤溶酶原激活剂的影响。本研究的目的是确定未加热和热变性的β-乳球蛋白（β-LG）、α-乳白蛋白（α-LA）和牛血清白蛋白（BSA）对纤溶酶原激活剂的影响。未热处理和变性的α-LA以及变性的β-LG均能显著刺激纤溶酶原激活剂的活性。对这些乳清蛋白的刺激作用进行了动力学表征，结果表明，这三种蛋白均能显著提高纤溶酶原激活的速率。通过在纤溶酶测定中测试两种不同的底物d-缬氨酸-亮氨酸-赖氨酸对硝基苯胺（S-2251）和Spectrozyme PL（Spec PL），证明了这种刺激作用与乳清蛋白对纤溶酶活性的任何影响无关。使用S-2251的结果证实了几位研究人员观察到的乳清蛋白对纤溶酶的抑制作用。然而，使用SpecPL并没有显示出抑制作用。配体结合研究表明，这种差异是由于S-2251与乳清蛋白之间存在显著相互作用。总体而言，本研究表明，将乳清蛋白加入奶酪中可能不会阻碍纤溶酶的活性，反而可能刺激纤溶酶原的激活。此外，结果表明，在涉及乳清蛋白和纤溶酶系统的模型研究中，需要仔细考虑所选合成底物的类型。
