Protocatechuic and 3,4-Dihydroxyphenylacetic Acids Inhibit Protein Glycation by Binding Lysine through a Metal-Catalyzed Oxidative Mechanism

The mechanism of inhibition of advanced glycation end product (AGE) formation by protocatechuic acid and 3,4-dihydroxyphenylacetic acid (DHPA) has been studied using a widespread applied in vitro model system composed of bovine serum albumin (BSA) and supraphysiological glucose concentrations. Protocatechuic acid and DHPA inhibited the formation of Amadori compounds, fluorescent AGEs (IC50 = 62.1 ± 1.4 and 155.4 ± 1.1 μmol/L, respectively), and Nε-(carboxymethyl)lysine (IC50 = 535.3 ± 1.1 and 751.2 ± 1.0 μmol/L, respectively). BSA was pretreated with the two phenolic acids, and the formation of BSA–phenolic acid adducts was estimated by nanoflow liquid chromatography–electrospray ionization–quadrupole time-of-flight mass spectrometry. Results showed that the tested phenolic acids bound key sites of glycation in BSA through a metal-catalyzed oxidative mechanism. The antiglycative activity mechanism involved the formation of BSA–phenolic acid adducts, and it is unlikely that this occurs in vivo. These results raise the problem to design in vitro models closer to physiological conditions to reach biologically sound conclusions.