Free radicals occur constantly during metabolism and take part in numerous
physiological processes, such as: intra-cellular and inter-cellular
signalization, gene expression, removal of damaged or senescent cells, and
control of the tone of blood vessels. However, there is an increased quantity
of free radicals in situations of so-called oxidative stress, when they cause
serious damage to cellular membranes (peroxidation of their lipids, damage of
membrane proteins, and similar), to interior cellular protein molecules, as
well as DNA molecules and carbohydrates. This is precisely why the organism
has developed numerous mechanisms for removing free radicals and/or
preventing their production. Some of these are enzyme-related and include
superoxide-dismutase, catalase, glutathione-peroxidase, and others. Other,
non-enzyme mechanisms, imply antioxidative activities of vitamins E and C,
provitamin A, coenzyme Q, reduced glutation, and others. Since free radicals
can leave the cell that has produced them and become dispersed throughout the
body, in addition to antioxidative defense that functions within cellular
structures, antioxidant extra-cellular defense has also been developed. This
is comprised by: transferrin, lactoferrin, haptoglobin, hemopexin,
ceruloplasmin, albumins, extra-cellular isoform SOD, extracellular
glutathione-peroxidase, glucose, bilirubin, urates, and many other molecules.