Antioxidant ability of caffeine and its metabolites based on the study of oxygen radical absorbing capacity and inhibition of LDL peroxidation.

Although caffeine has been suggested as an antioxidant at millimolar concentrations, little information is available about the antioxidant activity of caffeine and its metabolites at physiological concentrations. Antioxidant activity was measured by oxygen-radical absorbing capacity (ORAC) using 2, 2'-azobis(2-amidinopropane) dihydrochloride (AAPH) as a peroxyl radical generator. The antioxidant ability was tested at 40 micromol/l by determining the level of thiobarbituric acid reactive substances (TBARS) and conjugated dienes produced from the oxidation of human low density lipoprotein (LDL) by 45 mmol/l AAPH. There was neither antioxidant activity nor protective ability present with caffeine, 1,7-dimethylxanthine, 3,7-dimethylxanthine, or 1, 3-dimethylxanthine at this micromolar concentration. The antioxidant activity was however significant with 1-methylxanthine (1-X) and 1-methyluric acid (1-U), the main metabolites of caffeine in humans. The relative ORAC values of Trolox, ascorbic acid, uric acid, 1-X, and 1-U were 1:0.47:0.86:0.58:0.81, respectively, on a molar basis. These compounds also significantly reduced the level of TBARS and conjugated dienes produced from the LDL peroxidation. The antioxidant effect of 1-X is equivalent to ascorbic acid and 1-U is to uric acid.