Investigation of the role of an arginine esterase in EGF processing

Abstract

The epidermal growth factor (EGF) can be isolated from the male mouse submaxillary gland as part of a high molecular weight complex (HMW*EGF). The complex is composed of two molecules of EGF and two molecules of EGF*BP. The proteolytic activity of EGF*Binding Protein was demonstrated by its self-proteolysis in moderate (3-7 M) concentrations of urea, and, its inhibition by formation of a complex with pancreatic trypsin inhibitor (Kunitz). This complex was characterized by its isoelectric point and by its ability to yield PTI and EGF*BP in SDS/urea gel electrophoresis. The dissociation equilibrium constant was determined to be 2 .8 x 10^-8 M by inhibition studies of the esteropeptidase. The EGF*BP is able to catalyze the conversion of virgin STI (soybean trypsin inhibitor) to modified STI. The forms of STI are separated by polyacrylamide gel electrophores is and the EGF*BP modified STI possess a new carboxy-terminal arginine residue. These results, which indicate that EGF*BP is capable of auto digestion, that it forms a stable complex with a macromolecular inhibitor of trypsin, and that it converts virgin STI to modified STI, lend strong support to the hypothesis that EGF*BP is capable of cleaving a larger precursor by its proteolytic action.