Properties and actions of heat-stable enterotoxin of Escherichia coli

The heat-stable enterotoxins (STs) produced by enterotoxigenic Escherichia coli are classified into two groups, methanol-soluble (STI) and methanol-insoluble (STII) enterotoxins. These are distinct toxins with unique properties. Their features in common include heat-stability, low molecular weight, secretion from the bacteria, and ability to induce fluid secretion from the intestine. STI is an 18- or 19-amino acid extracellular peptide with three intramolecular disulfide bonds, which is produced by proteolytic cleavage of 72 amino acid precursor. The STI in the lumen of the intestine binds to specific protein receptors (guanylate cyclase C) located in the brush border membrane and leads to elevation of intracellular cyclic GMP level. Several factors involved in the activation of guanylate cyclase by STI have been identified. Elevation of cyclic GMP level induces intestinal fluid secretion by stimulation of chloride secretion. Cystic fibrosis transmembrane conductance regulator, which is a chloride channel, might be involved in chloride secretion. In contrast, STII is a 48-amino acid peptide with two intramolecular disulfide bonds, which results from 71 amino acid precursor. Compared with STI, the steps that lead to intestinal fluid secretion by STII are not well established. It has been proposed that sulfatide in the brush border is a receptor for STII and that the STII bound to the receptor opens GTP-binding regulatory protein-linked calcium channels. These actions of STII induce not only stimulation of the production of secretagogues such as prostaglandin E₂ and serotonin, but also activation of the calcium-calmodulin-dependent protein kinase II in the cells.