Antiglycative and carbonyl trapping properties of the water soluble fraction of coffee silverskin

Carbonyl stress and accumulation of advanced glycation end-products (AGEs) in human tissues are involved in diabetic complications, atherosclerosis, Alzheimer's disease and aging. The objective of this study was to evaluate the in vitro protective effect of aqueous extracts of coffee silverskin (CS) in the formation of AGEs and trapping of carbonyl reactive species such as methylglyoxal (MGO). Aqueous extracts of CS from Arabica and Robusta coffee varieties were obtained under environment friendly extraction conditions. CS extracts were characterized by the analysis of dietary fiber, caffeine, chlorogenic acids (CGAs), total phenolic compounds, browning, melanoidins, and antioxidant capacity. CS extracts and CGA exhibited a dose-dependent anti-AGE capacity in the protein–glucose model system (37 °C/21 days) with an IC50 of 0.6 mg/mL and 0.4 mg/mL, respectively. Caffeine did not prevent AGE formation under the studied conditions. Regardless to protein–MGO assay (37 °C/14 days), the anti-AGE capacity of CS extracts and CGA was also dose-dependent with an IC50 of 1.3 mg/mL and 0.1 mg/mL, respectively. Caffeine weakly inhibited the reaction of protein and MGO. The MGO trapping capacity was established as a model for protection against carbonyl stress. Robusta CS was very effective for the direct trapping of MGO with an IC50 of 0.055 mg/mL as compared with Arabica CS (IC50 of 0.6 mg/mL). CGA and caffeine showed an IC50 for MGO trapping capacity of 0.14 mg/mL and N10 mg/mL, respectively. The highest CGA content in the Robusta CS extract could explain its higher MGO trapping activity as compared with the Arabica CS extract. The anti-AGE and MGO trapping capacities of CS may be associated to other chemical components besides CGA. In conclusion, aqueous CS extract may be considered as a natural source of inhibitors of in vitro formation of AGEs and carbonyl stress. The inhibitory effect of the coffee extracts may be associated to their carbonyl trapping capacity. INTRODUCTION AGEs (advanced glycation end-products) are the final products derived from the Maillard reaction or nonenzymatic glycation process produced in the human body. It is known that AGEs are involved in the development of several health disorders such as diabetes and its complications (Vlassara & Palace, 2002), atherosclerosis (Vlassara, 1996), Alzheimer's disease and normal aging (Münch, Thome, Foley, Schinzel, & Riederer, 1997). In addition, the increase in reactive carbonyls in tissues is known as carbonyl stress which leads to directly increase chemical modification of proteins (glycation) and lipids (lipoxidation) in diabetes. Reactive carbonyl species generated from carbohydrate, lipid and amino acid metabolism such as methylglyoxal (MGO), glyoxal, glyoxalaldehyde, dehydroascorbate, 3-deoxyglucosone and malondialdehyde, are potent precursors of AGE formation and protein cross-linking (Thornalley, Langborg, & Minhas, 1999). MGO derived AGE structures, including CEL (N-epsilon-(carboxyethyl)-lysine) and MOLD (methylglyoxallysine dimer), are increased in diabetes (Baynes & Thorpe, 1999). Thus, preventing AGE formation/accumulation may control significantly the pathogenesis of diabetes complications. The inhibition of AGE formation might follow several mechanisms involving, e.g., aldose reductase, antioxidant activity, reactive dicarbonyl trapping, sugar autoxidation inhibition and amino group binding, where the antiglycative activity of phytochemicals has been usually linked to oxidative reactions (Bousová et al., 2005). The inhibition of AGE formation by some synthetic compounds such as aminoguanidine (AG) has been well documented. However, this compound has been associated with several adverse effects in in vivo studies (Thornalley, 2003; Williams, 2004) since it is a highly reactive nucleophilic reagent that reacts with many biological molecules (pyridoxal phosphate, pyruvate, glucose, malondialdehyde, and others). Hence, the search for natural products which can inhibit AGE formation has recently been an objective of worldwide research (Peng, Cheng, et al., 2008; Povichit, Phrutivorapongkul, Suttajit, Chaiyasut, & Leelapornpisid, 2010; Wang, Sun, Cao, & Tian, 2009).