Studies on the Nature of the Inhibition by Gossypol of the Transformation of Pepsinogen to Pepsin

Pepsinogen reacts with gossypol to form gossypolpepsinogen, a zymogen which cannot be activated to pepsin. This modified protein was digested with Nagarse followed by partial acid hydrolysis with 0.03 n HCl at 110° for 40 hours. The gossypol-containing portion of the molecule was isolated and purified by gel filtration on Sephadex G-15 and LH-20 columns and countercurrent distribution by means of a 1-butanol, pyridine, and acetic acid system. The material thus purified was found to be a decapeptide originating from the NH2-terminal portion of the protein and a heptapeptide obtained from the COOH terminus. The two peptides were cross-linked by gossypol through the e-NH2 groups of Lys18 and Lys358. As indicated by gel filtration on Sephadex G-200 and by sodium dodecyl sulfate-polyacrylamide electrophoresis, it is apparent that intra- as well as intermolecular cross-linkages may occur. Furthermore, the optical rotatory dispersion and circular dichroism differ from those of pepsinogen.