Antioxidant capacities of ascorbic acid, uric acid, alpha-tocopherol, and bilirubin can be measured in the presence of another antioxidant, serum albumin.

Human serum contains several antioxidants. The total antioxidant capacity (AOC) is the sum of all the antioxidant activities present in serum. The aim of our study was to investigate whether the AOC of bilirubin (BR), alpha-tocopherol (TOH), ascorbic acid (AA), and uric acid (UA) could be measured with good precision and recovery in the presence of human serum albumin (HSA). We measured the AOC of each antioxidant using a Cobas Mira S instrument (Roche Diagnostic Systems, Montclair, NJ) by measuring the inhibitory effect of a given compound on the oxidation of the radical cations of 2,2'-azino-di-(3-ethylbenzthiazoline 6-sulphonate) (ABTS) incubated with metmyoglobin and H(2)O(2). The assay had a linear AOC response range of 27-2,000 micromol/L. The within- and between-day coefficients of variation (CVs) did not exceed 3.4% and 4.2%, respectively. The AOC of albumin in serum is much greater than that of BR, TOH, AA, or UA owing to the substantially greater concentration of HSA in serum. An aliquot of a solution of AA, UA, BR, or TOH was added to HSA or distilled water, and the AOC was determined. The AOC of BR, TOH, AA, and UA increased in a linear way with increasing concentrations. However, we found that the magnitude of increase in the AOC of a mixture of HSA and any of these antioxidants was lower than the sum of the AOC of HSA and any one of the following: AA, UA, BR, or TOH (all expressed in micromol/L).