Mechanism of the Gossypol Inactivation of Pepsinogen

Previous experiments have shown that pepsinogen reacts with the cottonseed pigment gossypol in the presence of ethanol to form a stable complex with little or no potenitial pepsin activity. We now find the reaction is both temperature- and alcohol-dependent, suggesting a conformational change in the zymogen prior to reaction with gossypol. A yellow, ninhydrin-positive material has been recovered from the acid hydrolysate of borohydride-reduced, gossypol-inactivated pepsinogen. This material is identical with synthetic lysine-gossypol prepared by the reduction of the gossypol-lysine Schiff base. These results confirm that inactivation is due to Schiff base formation between the aldehyde functional groups of the gossypol and lysines 18 and 358 of the zymogen. Pyridoxal phosphate, another Schiff base-forming reagent, was found to protect pepsinogen from inactivation by gossypol. Gossypol has been found to be a good competitive inhibitor of pepsin with a Ki of 4.7 x 10-5 m when Z-Gly-Gly-Phe-Phe-3-(4-pyridyl)propyl-1-oxy was used as a substrate. Under the same conditions, pyridoxal phosphate does not inhibit pepsin at 1.25 x 10-3 m.