Measurement of Fat-Soluble Vitamins and Antioxidants by HPLC With Electrochemical Array Detection

Fat-soluble vitamins and antioxidants (FSVAs) are a structurally diverse group of compounds (Fig. 1 ) that play important roles in a wide spectrum of biochemical and physiological processes, e.g., photoreception (vitamin A, retinol); plasma calcium homeostasis (vitamin D2, ergocalciferol; vitamin D3, cholecalciferol); and blood clotting (vitamin K1, phylloquinone). Of considerable interest is the involvement of some FSVAs in oxidative metabolism and the prevention of damage by reactive oxygen species (ROS) ( 1 , 2 ). For example, α-tocopherol (vitamin E) is the primary antioxidant species in the membrane. Here it intercepts lipid peroxyl radicals, thereby inhibiting lipid peroxidation, a self-perpetuating chain reaction, and preventing catastrophic membrane damage. α-Tocopherol is thought to be regenerated from the resulting α-tocopheryl radical by reaction with reduced coenzyme Q10 (CoQ10) (the ubiquinone/ubiquinol system) also located within the membrane, or with cytosolic ascorbic acid (or glutathione) at the cytoplasm-membrane interface ( 1 , 2 ). Another form of vitamin E, γ-tocopherol, readily reacts with reactive nitrogen species (RNS) such as peroxynitrite to form 5-nitro-γ-tocopherol, a marker of RNS production ( 3 ).   Fig. 1.  The chemical structures of some fat-soluble vitamins and antioxidants.