Studies of the Tertiary Structure and Structural Requirements for Toxic Expression of Heat-stable Enterotoxins Produced by Enterotoxigenic Escherichia coli

产肠毒素大肠杆菌产生的耐热肠毒素（ST）可引起人类急性腹泻。以前，我们和其他人确定了肠毒素E.大肠杆菌中，由19和18个残基组成。本工作的目的是阐明ST的三级结构和表达其毒性的结构要求。结果表明：1）不仅在大肠杆菌的两个ST中，而且在大肠杆菌的两个ST中，三个分子内二硫键的位置也发生了变化。大肠杆菌、大肠耶尔森氏菌和非01型霍乱弧菌的sTs。二硫键的模式是相同的，在所有的ST检查。2)肠毒素E. 3）STp类似物（ST由一株猪产肠毒素E. coli产生）的表达产物，其活性测定结果表明，三个二硫键中的一个二硫键是表达ST毒性所必需的。coli）中合成了在单个位置被D-氨基酸残基取代的氨基酸残基。STp的5位D-Cys类似物具有与STp相同的活性，并且其毒性持续24 h以上，而STp的毒性在6-10 h后降低。

Heat-stable enterotoxin (ST) produced by enterotoxigenic Escherichia coli causes acute diarrhea in human. Previously, we and others determined the amino acid sequences of the STs of enterotoxigenic E. coli, which consist of 19 and 18 residues. The present work was carried out with the aim to elucidate the tertiary structure of ST and the structural requirement for expression of its toxicity. The following tesults were obtained: 1) The positions of three intramolecular disulfide linkages not only in two STs of enterotoxigenic E. coli but also in sTs of yersinia entero colitica and vibrio cholerae non-01 were determined. The mode of disulfide linkages were same in all of the STs examined. 2) Analogs of the ST of enterotoxigenic E. coli with one or two among three disulfide linkages were synthesized and their activities were examined, demonstrating tha the special one of three disulfide linkages was essential for expression of the toxicity of the ST. 3) Analogs of STp (ST produced by a porcine stratin of enterotoxigenic E. coli) replaced by D-amino acid residues at a single position were synthesized. The analog of STp with D-Cys at position 5 had the same activity as that of STp and moreover the toxicity of the analog lasted after its administration into suckling mouse more than 24 hours, while the toxicity of STp decreased after 6-10 hours.