Characterization of the Binding Protein for Epidermal Growth Factor

Physical, chemical, and enzymatic properties of the protein which binds epidermal growth factor (EGF) have been examined. A molecular weight of 29,300 was determined from sedimentation equilibrium studies, in agreement with a value of 30,000 calculated from the s020, w (2.83 S) and D020, w (8.73 x 10-7 cm2 s-1). It has an E1%1 cm at 280 nm of 15.6 and the isoelectric point is 5.6. The circular dichroic spectrum indicates a major nonhelical character with a minor amount of α helical structure. The amino acid composition was determined. No free sulfhydryl groups were detected. Hexosamine analysis revealed the presence of glucosamine and galactosamine. EGF-binding protein was found to be an arginine esterase with a restricted substrate specificity. Kinetic studies of the catalytic hydrolysis of Nα-benzoyl-l-arginine ethyl ester revealed a Km of 130 µm and a pH optimum in the range of pH 7.8 to 8.5, where the specific activity is approximately 390 µmoles per min per mg. This arginine esterase is not identical, but is related antigenically, to the arginine esterases associated with the nerve growth factor complex. The EGF-binding protein is postulated to function in the enzymatic liberation of active EGF from an inactive precursor.