Szmola Richárd, Sahin-Tóth Miklós
Department of Molecular and Cell Biology, Goldman School of Dental Medicine, Boston University, Boston, MA 02118, USA.
Proc Natl Acad Sci U S A. 2007 Jul 3;104(27):11227-32. doi: 10.1073/pnas.0703714104. Epub 2007 Jun 25.
Digestive trypsins undergo proteolytic breakdown during their transit in the human alimentary tract, which has been assumed to occur through trypsin-mediated cleavages, termed autolysis. Autolysis was also postulated to play a protective role against pancreatitis by eliminating prematurely activated intrapancreatic trypsin. However, autolysis of human cationic trypsin is very slow in vitro, which is inconsistent with the documented intestinal trypsin degradation or a putative protective role. Here we report that degradation of human cationic trypsin is triggered by chymotrypsin C, which selectively cleaves the Leu(81)-Glu(82) peptide bond within the Ca(2+) binding loop. Further degradation and inactivation of cationic trypsin is then achieved through tryptic cleavage of the Arg(122)-Val(123) peptide bond. Consequently, mutation of either Leu(81) or Arg(122) blocks chymotrypsin C-mediated trypsin degradation. Calcium affords protection against chymotrypsin C-mediated cleavage, with complete stabilization observed at 1 mM concentration. Chymotrypsin C is highly specific in promoting trypsin degradation, because chymotrypsin B1, chymotrypsin B2, elastase 2A, elastase 3A, or elastase 3B are ineffective. Chymotrypsin C also rapidly degrades all three human trypsinogen isoforms and appears identical to enzyme Y, the enigmatic trypsinogen-degrading activity described by Heinrich Rinderknecht in 1988. Taken together with previous observations, the results identify chymotrypsin C as a key regulator of activation and degradation of cationic trypsin. Thus, in the high Ca(2+) environment of the duodenum, chymotrypsin C facilitates trypsinogen activation, whereas in the lower intestines, chymotrypsin C promotes trypsin degradation as a function of decreasing luminal Ca(2+) concentrations.
消化性胰蛋白酶在人体消化道转运过程中会发生蛋白水解降解，一般认为这是通过胰蛋白酶介导的裂解反应（即自溶作用）发生的。自溶作用还被假定通过消除胰腺内过早激活的胰蛋白酶，对胰腺炎起到保护作用。然而，人阳离子胰蛋白酶的自溶在体外非常缓慢，这与已记录的肠道胰蛋白酶降解情况或假定的保护作用不一致。在此我们报告，人阳离子胰蛋白酶的降解是由糜蛋白酶C触发的，它选择性地裂解钙离子结合环内的Leu(81)-Glu(82)肽键。阳离子胰蛋白酶随后通过胰蛋白酶对Arg(122)-Val(123)肽键的裂解进一步降解并失活。因此，Leu(81)或Arg(122)的突变会阻止糜蛋白酶C介导的胰蛋白酶降解。钙离子可防止糜蛋白酶C介导的裂解，在1 mM浓度下可实现完全稳定。糜蛋白酶C在促进胰蛋白酶降解方面具有高度特异性，因为糜蛋白酶B1、糜蛋白酶B2、弹性蛋白酶2A、弹性蛋白酶3A或弹性蛋白酶3B均无效。糜蛋白酶C还能迅速降解所有三种人胰蛋白酶原同工型，并且似乎与1988年海因里希·林德克内希特描述的神秘胰蛋白酶原降解活性酶Y相同。结合先前的观察结果，这些结果确定糜蛋白酶C是阳离子胰蛋白酶激活和降解的关键调节因子。因此，在十二指肠的高钙离子环境中，糜蛋白酶C促进胰蛋白酶原激活，而在肠道下部，随着肠腔钙离子浓度降低，糜蛋白酶C促进胰蛋白酶降解。
