Relationship between free radicals and vitamins: an overview.

Aerobic metabolism entails the generation of reactive oxygen species. These include the superoxide anion radical, hydrogen peroxide, the hydroxyl radical, and singlet molecular oxygen. Further, organic alkoxyl or peroxyl radicals may be formed. In the pattern of antioxidant defense, some vitamins play a prominent role and have therefore received much attention recently in the study of the physiological or pathophysiological imbalance in the prooxidant/antioxidant equilibrium. Prooxidant states are sometimes also referred to as oxidative stress. Vitamin E in the membrane compartment and vitamin C in the aqueous compartment exhibit direct reactivity with radicals, and there are repair pathways for the tocopheryl and ascorbyl radicals that are generated. In this regard, there can be an interaction between these two vitamins. Also, glutathione (GSH) can exhibit repair capacity as well as direct reactivity with radicals. Singlet molecular oxygen is an electronically excited state of oxygen with considerable chemical reactivity. It is a non-radical compound, generated by photochemical reactions or by the process of lipid peroxidation of biomembranes, i.e., by photoexcitation or by chemiexcitation. Singlet oxygen can be inactivated by a number of biological compounds known as quenchers. beta-Carotene is the most important vitamin in this respect.