Free radicals are atoms, molecules or ions with an unpaired electron in the outer orbital. Although they are capable to exist independently, free radicals are unstable, i.e., chemically highly reactive, because the unpaired electron in their outer orbital tends to stabilize by pairing with another electron. Therefore, reactions of radicals with non-radicals leads to the formation of new, so-called secondary free radicals, i.e., a whole series of radicals are propagated and formed, which are also very toxic. In other words, "radicals give birth to radicals". Free radicals are unstable and highly reactive molecules, which, after entering into chemical reactions with parts of the cell (proteins, lipids, carbohydrates, DNA molecules) in the body, cause biochemical, structural, and functional imbalances.
Free radicals or reactive oxygen species (ROS) and reactive nitrogen species (RNS) are highly reactive molecules that occur naturally as products of normal cellular oxidative processes. In some pathological processes, concentrations of reactive oxygen species (ROS) can increase to high levels, but also react with cellular elements, causing damage, dysfunction or cell degradation.
Free radicals in the conditions of everyday life
The production of free radicals is stimulated by exogenous and endogenous oxidants, to which the body is constantly exposed. The exogenous oxidants are: ozone, smog, ionizing radiation, excess ions of transition metals, metabolism of toxic compounds and drugs, xenobiotics, excess
oxygen (i.e., its increased concentration), etc. The main endogenous source of free radicals is the respiratory chain of the electron transfer in the mitochondria, i.e., the incomplete reduction of oxygen during the oxidative phosphorylation, when 1-3% of the oxygen can be
converted to superoxide-anion radicals. This way of producing reactive oxygen species (ROS) is particularly important during physical activity, because over 40 times more oxygen is used than at rest.
The second most important endogenous source of reactive oxygen species (ROS) are enzymes that physiologically produce oxidants.
The third most important endogenous source of reactive oxygen species (ROS) is the autooxidation of small molecules during the respiratory burst in activated phagocytes during the inflammatory response. The intensity of the production of free radicals is well explained by the fact that, out of about 250 g of oxygen inhaled by a healthy person per day, 2-5% is converted into toxic superoxide-anion radicals, and nearly 5% of the total normal mitochondrial oxygen consumption releases singlet oxygen : 1