Mechanism of inhibition of protein glycation by polyphenols

We have observed that protocatechuic (PC) and 3,4-dihydroxyphenylacetic (DHPA) acids, found in human plasma after the consumption of foods rich in anthocyanins and flavanols, inhibit the formation of advanced glycation endproducts (AGEs). We have investigated the mechanisms of inhibition incubating for 7 days bovine serum albumin (BSA) (50 mg/mL) and glucose (0.8 mol/L) in the presence and absence of these polyphenols at various concentrations. Fluorescence emission at 405 nm was used to evaluate AGEs formation. Carboxymethyl-lysine (CML), Amadori products and quinoproteins were determined by ELISA, NBT assay and electrophoresis, respectively. Both PC and DHPA were able to decrease the formation of Amadori products, fluorescent AGEs and CML. These decreases were concentration dependent. Quinoproteins formation increases with the increase of polyphenol concentration. BSA was also incubated with 0.5 and 1.0 mmol/L of phenols in the absence of glucose. After 7 days of incubation the unbound polyphenols were removed by ultrafiltration. The phenols-treated BSA was incubated with glucose for 7 days. No significant differences are observed in the inhibition of AGEs formation between phenol pre-incubated and non pre-incubated BSA. These results and the formation of quinone modified BSA suggest that polyphenols are pre-Amadori inhibitors of AGEs formation. Phenols bind BSA and decrease glucose binding. They are oxidized (pH 7.5, 37°C) in the reactive quinones intermediates and react with BSA amino groups forming quinone modified BSA and decreasing the AGEs formation.