Abstract
The phrases "free radicals" and "reactive oxygen species" (ROS) are frequently used interchangeably although this is not always correct. This article gives a brief description of two mentioned oxygen forms. During the first two-three decades after ROS discovery in biological systems (1950-1970 years) they were considered only as damaging agents, but later their involvement in organism protection and regulation of the expression of certain genes was found. The physiological state of increased steady-state ROS level along with certain physiological effects has been called oxidative stress. This paper describes ROS homeostasis and provides several classifications of oxidative stresses. The latter are based on time-course and intensity principles. Therefore distinguishing between acute and chronic stresses on the basis of the dynamics, and the basal oxidative stress, low intensity oxidative stress, strong oxidative stress, and finally a very strong oxidative stress based on the intensity of the action of the inductor of the stress are described. Potential areas of research include the development of this field with complex classification of oxidative stresses, an accurate identification of cellular targets of ROS action, determination of intracellular spatial and temporal distribution of ROS and their effects, deciphering the molecular mechanisms responsible for cell response to ROS attacks, and their participation in the normal cellular functions, i.e. cellular homeostasis and its regulation.
Highlights
If exi stence and role of low molecular mass antioxidants such as tocopherol or ascorbic acid were known at the time, information on the presence of enzymes dealing with free radicals directly as substrates ap peared only in the late 1960’s
superoxide dismutase (SOD) was found in virtually all aero bic organismsand, its activity was regulated at transcriptional level, and a small in crease in the level of reactive oxygen species (ROS) increased its activity
That the processes related to ROS homeostasis have been studied very inten sively for more than half a century, we cannot say that we have a clear understanding of their role in the processes which they participate in
Summary
Molecular oxygen by attaching four electrons and four protons is converted into water. Transformation of ROS occurs spontaneously with involvement of low-mo lecular mass electron donors (upper part of Fig. 1). In living organisms there are enzymatic systems significantly accelerating one-electron ROS conversion. Mentioned enzyme superoxide dismutase accelerates O2 ̅ dismutation transforming it into hydrogen peroxide and molecular oxygen (Fig. 1). Catalase dismutates H2O2 to water and molecular oxygen, whereas other peroxidases use as a cofactor various organic compounds capable of donating electrons to H2O2. I will focus only on the glutathionedependent peroxidases (GPx) They reduce hydrogen peroxide along with many other peroxides, in par ticular those of lipids, to alcohols and water. This results in simultaneous oxidation of glutathione. It is worthy to underline that the activity of an tioxidant enzymes is precisely regulated by the cell: on the one hand, it is enhanced at low ROS level increase due to upregulation of their expression, whereas on the other hand it is decreased at substan tial ROS level increase due to inactivation by ROS
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