Free radicals in pathogenesis: Health-promoting functions of plant- and milk-derived antioxidants

Molecular atmospheric oxygen contains two unpaired electrons in the unreactive triplet state and thus has to be activated in order to react with other biological molecules in the singlet ground state. These processes of activation include very reactive intermediates. Therefore, aerobic cells must cope with - and to some extent also adapt to - oxidative stress provoked for example by infection or intoxication, where reactive intermediates of oxygen activation accumulate. Depending on the strength of these impacts, several symptoms indicate the deviation from normal, steady-state metabolism. Most of these visible or measurable symptoms are connected with oxygen activation where principally a transition from heterolytic reactions (two-electron transitions) to increased homolytic reactions (one-electron transitions) is observed. Homolytic reactions produce free radicals, which are generally counteracted by a parallel increase of intrinsic radical scavenging processes or by compounds administered with food, thus warranting metabolic control within certain limits. At advanced states of stress, control may be gradually lost and chaotic radical processes dominate. Finally, cellular decompartmentalizations yield apoptotic, lytic and/or necrotic processes. Every episode during this cascade is characterizable by the balance between pro- and antioxidative capacities. Antioxidants, which in many cases are free radical scavengers or quenchers of activated states, comprise a wealth of classes of organic molecules including phenolics, probably as the most prominent ones. In this article, mechanisms of oxygen activation and protection from oxidative damage are discussed. Furthermore, exam pies of antioxidative functions of a few important natural products are reported.