Does Epigallocatechin-3-Gallate-Insulin Complex Protect Human Insulin from Proteolytic Enzyme Action?

Insulin is a polypeptide hormone produced by the β cells present in Islets of Langerhans of the pancreas. Either failure to produce (type 1 diabetes) or utilize insulin (type 2 diabetes) causes diabetes mellitus. Insulin administration is used to treat type 1 diabetes. The common route for insulin administration is via subcutaneous injection. The oral insulin delivery has been proposed, however it suffers from poor bioavailability which is mainly due to the presence of proteolytic enzymes (pepsin, trypsin, and chymotrypsin) in the gastrointestinal (GI) tract. Protecting insulin from these enzymes when given orally might improve its bioavailability. In general, condensed tannins have been shown to reduce the activity of digestive enzymes. Epigallocatechin-3-gallate (EGCG) is the most abundant tannin component found in green tea. The present study investigated the ability of EGCG to protect insulin, through the formation of EGCGinsulin complex, from the proteolytic enzyme action by pepsin and trypsin/chymotrypsin, in vitro. The amount of insulin remaining in the presence and absence of EGCG following incubation with either simulated gastric fluid (SGF) containing pepsin or simulated intestinal fluid (SIF) containing trypsin/chymotrypsin at two different temperatures (25°C and 37°C) for 1 hour and 7 hours was determined using an HPLC technique. The results showed that the presence of proteolytic enzymes (pepsin or trypsin/chymotrypsin) and absence of EGCG in the sample negatively affected the stability of insulin in solution. In the presence of EGCG, insulin was partially protected from trypsin/chymotrypsin but it was not protected from the action of pepsin. Insulin degradation was more pronounced at 37°C than that at 25°C (p = 0.0188). The initial concentration of insulin present (10 IU/mL or 20 IU/mL) or the time of incubation (1 h vs. 7 h) had no influence on the stability of insulin in the sample (p = 0.2842 and p = 0.2114, respectively). In conclusion, EGCG was not able to protect insulin against the proteolytic activity of pepsin. However, EGCG was shown to have some protective effect on insulin against the degradative effect of trypsin/chymotrypsin at room temperature, in vitro. Furthermore, this protection was greatly weakened at 37°C, which suggested that the protective action of EGCG would not be present in vivo.