Diabetes is one of the top 10 causes of death worldwide and approximately 90% of cases are characterized by hyperglycemia and insulin resistance (diabetes type 2). Diabetes is raising the concern for new treatments to prevent and control the disease. An alternative treatment, Psidium guajava leaves and fruits have been reported to improve diabetes condition by increasing insulin receptor substrate 1 phosphorylation, improving insulin sensitivity and glucose metabolism in vivo as well as in vitro, through the inhibition of Protein Tyrosine Phosphatase 1B (PTP1B) - a negative regulator in the insulin pathway. However, the inhibitory mechanism of the guava leaves extract on the PTP1B enzyme has remained unknown. This work aims to elucidate the inhibitory mechanism of the guava leaf extract on PTP1B. The enzyme reaction rate was determined by a fluorescence substrate, 6,8-Difluoro-4-Methylumbelliferyl Phosphate (Ex/Em: 360/460nm). The hydrogen peroxide production was determined by the oxidation of OxiRed probe. Total phenol content was determined by ferric ion reduction (FeCl3). Ion-exchange, size-exclusion and high-pressure liquid chromatography (HPLC) were used to isolate the compounds in the aqueous partition of guava leaf. The results suggest that the aqueous partition inactivates PTP1B through an oxidative mechanism, reversible by a reducing agent like dithiothreitol (DTT). We demonstrated that epigallocatechin gallate, gallic acid, and aqueous partition can produce hydrogen peroxide. Ion exchange chromatography followed by size exclusion chromatography were used to identify the active compound in the aqueous fraction, containing a gallyl moiety and flavonol structure. As a conclusion, PTP1B is inactivated by the oxidation of the Cys215 (active site) by hydrogen peroxide which is formed from the reduction of dissolved oxygen molecule reduced by phenolic compound from the aqueous partition of guava leaf extract.
