Oxidative stress is defined as a disturbance in the prooxidant-antioxidant balance in favor of the former (1) and has been suggested be a causative factor in aging and degenerative diseases such as heart attack, diabetes, and cancer. The ubiquinol/ubiquinone ratio should therefore be a good marker of oxidative stress because ubiquinol is very labile in the oxidation of low-density lipoprotein (LDL) or plasma. In fact, a decrease in ubiquinol/ubiquinone ratio has been reported in patients with adult respiratory distress syndrome (2) and in patients with hepatitis, cirrhosis, and hepatoma (3). Interestingly, newborn babies are under oxidative stress as judged by plasma ubiquinol/ubiquinone ratio (4). Here we describe a simple and reproducible high-performance liquid chromatography (HPLC) method for the detection of ubiquinol and ubiquinone using an on-line reduction column and an electrochemical detector (ECD), and its application to human plasma (5).
