Armstrong P B, Quigley J P
Marine Biological Laboratory, Woods Hole, MA 02543.
Biochem J. 1987 Dec 15;248(3):703-7. doi: 10.1042/bj2480703.
Intra-chain thiol ester bonds are present in a limited number of proteins. The thiol ester class of proteins includes vertebrate alpha 2-macroglobulin and the complement proteins C3 and C4. We report here the first instance of a thiol ester protein from an invertebrate, the alpha 2-macroglobulin proteinase-inhibitor homologue present in the plasma of the American horseshoe crab Limulus polyphemus. Our evidence is of three kinds: (1) the proteinase-binding activity of Limulus alpha 2-macroglobulin is inactivated by the low-molecular-mass primary amine methylamine; (2) the native protein is subject to autolytic fragmentation during mild thermal denaturation, yielding fragments of approx. 125 kDa and 55 kDa, whereas the methylamine-treated protein is stable under these conditions of thermal treatment; (3) new thiol groups are generated rapidly during reaction of the protein with trypsin. The demonstration of the thiol ester bond in a protein from an ancient invertebrate provides evolutionary evidence for the importance of this bond in the function of plasma forms of the alpha 2-macroglobulin-like proteinase inhibitors.
链内硫酯键存在于数量有限的蛋白质中。硫酯类蛋白质包括脊椎动物的α2-巨球蛋白以及补体蛋白C3和C4。我们在此报告了来自无脊椎动物的硫酯蛋白的首个实例，即存在于美洲鲎（Limulus polyphemus）血浆中的α2-巨球蛋白蛋白酶抑制剂同源物。我们的证据有三类：（1）鲎α2-巨球蛋白的蛋白酶结合活性可被低分子量伯胺甲胺灭活；（2）天然蛋白质在温和热变性过程中会发生自溶断裂，产生约125 kDa和55 kDa的片段，而经甲胺处理的蛋白质在这些热处理条件下是稳定的；（3）蛋白质与胰蛋白酶反应时会迅速产生新的巯基。在一种古老无脊椎动物的蛋白质中证明硫酯键，为该键在α2-巨球蛋白样蛋白酶抑制剂血浆形式功能中的重要性提供了进化证据。
