Phenolic Acids and Antioxidant Mechanisms

The free and conjugated phenolic acids tested in this study are listed in Figure 2; 3,4-dihydroxyphenylacetic acid (DHPA) and 3,4-dihydroxybenzoic acid (protocatechuic acid; PCA) were purchased from Sigma-Aldrich (San Louis, MO, USA). Dihydrocaffeic acid (3-(3′,4′-dihydroxyphenyl)propionic acid, DHCFA) and its conjugated forms [dihydrocaffeic acid 3-O-β-D-glucuronide (DHCFAg) and dihydrocaffeic acid 3-O-sulfate (DHCFAs)] were purchased from Toronto Research Chemicals, Inc. (TRC, Toronto, QC, Canada). N-acetyl-L-cysteine (NALC), which is a common antioxidant that can directly react with ROS, promote glutathione synthesis, and inhibit nerve cell apoptosis, was obtained from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) and was used as the antioxidant reference control [31 (link)]. Stock standard solutions of these compounds (1 mM) were freshly prepared and filtered (0.22 μm) in sterile Dulbecco’s phosphate buffered saline solution (DPBS, Lonza, Walkersville, MD, USA).
The bacterial lipopolysaccharide (LPS), a pro-inflammatory inducer, and tert-butyl hydroperoxide (tBHP), used to induce in vitro cellular oxidative stress, were purchased from Sigma-Aldrich (San Louis, MO, USA).