Binding of cholera toxin B subunit to intestinal epithelial cells

We have prepared ¹²⁵I-labeled cholera toxin B subunit (¹²⁵I-labeled CT-B, a specific activity of 98Ci/mmol) and found that it binds to rat IEC-6 and human Caco-2 intestinal epithelial cells with high affinity (K_d 3.6 and 3.7nM, respectively). The binding of labeled protein was completely inhibited by unlabeled thymosin-α₁ (TM-α₁), interferon-α₂ (IFN-α₂), and the synthetic peptide LKEKK that corresponds to residues 16-20 in TM-α₁ and 131-135 in IFN-α₂, but was not inhibited by the synthetic peptide KKEKL with inverted amino acid sequence (K_i>10μM). Thus, TM-α₁, IFN-α₂, and the peptide: LKEKK bind with high affinity and specificity to the cholera toxin receptor on IEC-6 and Caco-2 cells. It was found that CT-B and the peptide: LKEKK at concentrations of 10-1000nM increased in a dose-dependent manner the nitric oxide production and the soluble guanylate cyclase activity in IEC-6 and Caco-2 cells.