Aldose Reductase Inhibitory and Antiglycation Activities of Four Medicinal Plant Standardized Extracts and Their Main Constituents for the Prevention of Diabetic Complications

Abstract

The present study was intended to evaluate, highlight and explore the role of four medicinal plant (Morus alba L., Phyllanthus amarus Schum. & Thonn., Punica granatum L., and Stevia rebaudiana Bertoni) standardized extracts and their major constituents (morusin, phyllanthin, punicalagin and stevioside) in the treatment of long-term diabetic complications by inhibition of aldose reductase (AR) enzyme and advanced glycation end products (AGEs) formation. Rat lens and kidney homogenates, which contain rat lens AR (RLAR) and rat kidney AR (RKAR) crude enzymes, respectively, prepared in the laboratory and commercially available human recombinant AR (HRAR) were used to carry out in vitro bioassays. AR inhibitory activity and inhibition of AGEs formation was done by using ultraviolet-visible (UV-Vis) and fluorescence spectroscopy with aminoguanidine as a standard. In vivo AR inhibitory activity, which involves determination of rat lens galactitol levels in galactosemic condition by using reverse phase high pressure liquid chromatography (RP-HPLC) and gas liquid chromatography (GLC) was determined. In the in vitro bioassays, punicalagin, the major constituent of P. granatum was found to have the most potent AR inhibitory activity with IC₅₀ values of 5.28, 6.22 and 4.70 μM in RLAR, RKAR, and HRAR assays, respectively, among the tested standardized extracts and pure compounds. Punicalagin (IC₅₀ = 5.11 μM), also inhibited generation of AGEs at a much lower concentration than that of the reference standard aminoguanidine (IC₅₀ = 50.47 μM). The effect of punicalagin in suppressing rat lens galactitol levels in galactose-fed rat model was also superior to all other studied compounds including the positive control quercetin. Standardized extracts of the plants and their major components have the potential to be used as capable shielding agents in therapy of diabetic complications; however, further studies are warranted to explain and explore their use in the treatment of diabetic complications.

Keywords: Standardized Extracts, Aldose Reductase, Non-enzyme Maillard Reaction, Punicalagin, Galactitol Accumulation