Studies on the mechanism of action of colloidal bismuth subcitrate. II. Interaction with pepsin

The effects of colloidal bismuth subcitrate (CBS) on porcine pepsin have been studied in vitro. CBS inhibited pepsin activity in a pH-dependent manner. CBS was not active at pH 4.0 but inhibited pepsin activity at pH 1.0 (IC50: 2.3 +/- 0.09 mmol/l) and pH 2.0 (IC50: 8.9 +/- 0.7 mmol/l). This inhibition was reversible. In the presence of the sulfhydryl ligand mercaptoethanol, which prevents precipitation of CBS, the inhibitory potency of CBS increased. CBS bound to both positively (Amberlite) and negatively charged (Dowex) ion exchangers in a pH-dependent manner. With increasing acidity, binding to Amberlite increased, whereas binding to Dowex decreased. From these data we conclude that negatively charged bismuth salts derived from CBS bind at pH 2.0 and 1.0 via an ionic interaction to positively charged groups of pepsin, thereby inactivating the enzyme.