Bovine milk inhibits proteolytic degradation of epidermal growth factor in human gastric and duodenal lumen

Degradation of epidermal growth factor (EGF) in human gastric and duodenal lumen was analyzed by incubating ¹²⁵I-labeled or unlabeled human recombinant EGF with human gastric or duodenal luminal fluids in vitro. Degradation of EGF was assessed by measuring the generation of acid soluble radioactivity or by reversed-phase high-performance liquid chromatography (HPLC). Incubation with gastric luminal fluids resulted in a time- and dose- dependent degradation of labeled and unlabeled EGF at pH 2.5 but not at pH 7.5. Duodenal luminal fluids, on the other hand, degraded EGF at pH 7.5 but not at pH 2.5. The rate of degradation of unlabeled EGF in gastric luminal fluids was nearly 12-fold higher than the rate of degradation of labeled EGF, whereas only a slight difference in rates of degradation of labeled and unlabeled EGF was observed in duodenal luminal fluids. High-performance liquid chromatography analysis detected three major degradation products that eluted with retention time of 17.5 min, 20.0 min, and 22.5 min that was associated with a reduction of intact EGF (retention time 23.5 min). Defatted and decaseinated supernatant of bovine milk effectively inhibited the degradation of EGF in both gastric and duodenal luminal fluids. Dietary derived protease inhibitors, such as soya bean trypsin inhibitor, lima bean trypsin inhibitor, egg white protease inhibitor, and Bowman-Birk protease inhibitor prevented EGF degradation in duodenal luminal fluids but failed to inhibit EGF degradation in gastric luminal fluids. These results suggest that bovine milk may contain specific inhibitors that protect EGF from proteolytic degradation in human gastric lumen.