Finn A L, Kuzhikandathil E V, Oxford G S, Itoh-Lindstrom Y
Department of Medicine, University of North Carolina School of Medicine, Chapel Hill, North Carolina, USA.
Gastroenterology. 2001 Jan;120(1):117-25. doi: 10.1053/gast.2001.20884.
BACKGROUND & AIMS: We previously isolated a monoclonal antibody against a Necturus gallbladder epitope that blocks native adenosine 3',5'-cyclic monophosphate (cAMP)-dependent chloride channels in intestine, gallbladder, urinary bladder, and airway epithelia in various animals.
Using this antibody, we purified a 200-kilodalton protein that, when reconstituted in lipid bilayers, forms 9-pS chloride channels that are blocked by the antibody.
Amino acid sequencing of the purified protein showed strong homology to rabbit sucrase-isomaltase, an abundant intestinal enzyme. Western blot analysis of the in vitro-translated sucrase-isomaltase was indistinguishable from that of the protein used in the lipid bilayer studies. Expression of this protein in Chinese hamster ovary cells and in Xenopus laevis oocytes yielded cAMP-dependent chloride currents that in the latter system were blocked by the antibody.
Because the monoclonal antibody blocks cAMP-dependent currents in epithelia as well as those produced both by the reconstituted and by the heterologously expressed protein, sucrase-isomaltase is a cAMP-dependent epithelial chloride channel. Thus an enzyme that can also function as an ion channel has been described for the first time.
我们之前分离出一种针对美西螈胆囊表位的单克隆抗体，该抗体可阻断多种动物肠道、胆囊、膀胱及气道上皮细胞中天然的3',5'-环磷酸腺苷（cAMP）依赖性氯离子通道。
利用该抗体，我们纯化出一种200千道尔顿的蛋白质，将其重构于脂质双分子层时，可形成被该抗体阻断的9皮安氯离子通道。
纯化蛋白质的氨基酸测序显示，其与兔蔗糖酶-异麦芽糖酶有高度同源性，兔蔗糖酶-异麦芽糖酶是一种丰富的肠道酶。体外翻译的蔗糖酶-异麦芽糖酶的蛋白质印迹分析与脂质双分子层研究中所用蛋白质的分析结果无法区分。该蛋白质在中国仓鼠卵巢细胞和非洲爪蟾卵母细胞中的表达产生了cAMP依赖性氯离子电流，在后者系统中该电流被该抗体阻断。
由于该单克隆抗体可阻断上皮细胞中的cAMP依赖性电流以及重构蛋白和异源表达蛋白所产生的电流，蔗糖酶-异麦芽糖酶是一种cAMP依赖性上皮氯离子通道。因此，首次描述了一种既能发挥酶功能又能充当离子通道功能的蛋白质。
