ReviewFree radicals and antioxidants in normal physiological functions and human disease
Introduction
The causes of the poisonous properties of oxygen were obscure prior to the publication of Gershman's free radical theory of oxygen toxicity in 1954, which states that the toxicity of oxygen is due to partially reduced forms of oxygen (Gerschman, Gilbert, Nye, Dwyer, & Fenn, 1954). In the same year, observations of a weak electron paramagnetic resonance (EPR) signal attributable to the presence of free radicals in a variety of lyophilised biological materials were reported by Commoner, Townsend, and Pake (1954). The world of free radicals in biological systems was soon thereafter in 1956 explored by Denham Harman who proposed the concept of free radicals playing a role in the ageing process (Harman, 1956). This work gradually triggered intense research into the field of free radicals in biological systems. A second epoch of the research of free radicals in biological systems was explored in 1969 when McCord and Fridovich discovered the enzyme superoxide dismutase (SOD) and thus provided convincing evidence about the importance of free radicals in living systems (McCord & Fridovich, 1969). A third era of free radicals in biological systems dates from 1977 when Mittal and Murad provided evidence that the hydroxyl radical, OH, stimulates activation of guanylate cyclase and formation of the “second messenger” cyclic guanosine monophosphate (cGMP) (Mittal & Murad, 1977). Since then, a large body of evidence has been accumulated that living systems have not only adapted to a coexistence with free radicals but have developed various mechanisms for the advantageous use of free radicals in various physiological functions.
Oxygen free radicals or, more generally, reactive oxygen species (ROS), as well as reactive nitrogen species (RNS), are products of normal cellular metabolism. ROS and RNS are well recognised for playing a dual role as both deleterious and beneficial species, since they can be either harmful or beneficial to living systems (Valko, Rhodes, Moncol, Izakovic, & Mazur, 2006). Beneficial effects of ROS occur at low/moderate concentrations and involve physiological roles in cellular responses to noxia, as for example in defence against infectious agents and in the function of a number of cellular signalling systems. One further beneficial example of ROS at low/moderate concentrations is the induction of a mitogenic response.
The harmful effect of free radicals causing potential biological damage is termed oxidative stress and nitrosative stress (Kovacic & Jacintho, 2001; Ridnour et al., 2005; Valko, Morris, Mazur, Rapta, & Bilton, 2001). This occurs in biological systems when there is an overproduction of ROS/RNS on one side and a deficiency of enzymatic and non-enzymatic antioxidants on the other. In other words, oxidative stress results from the metabolic reactions that use oxygen and represents a disturbance in the equilibrium status of pro-oxidant/antioxidant reactions in living organisms. The excess ROS can damage cellular lipids, proteins, or DNA inhibiting their normal function. Because of this, oxidative stress has been implicated in a number of human diseases as well as in the ageing process. The delicate balance between beneficial and harmful effects of free radicals is a very important aspect of living organisms and is achieved by mechanisms called “redox regulation”. The process of “redox regulation” protects living organisms from various oxidative stresses and maintains “redox homeostasis” by controlling the redox status in vivo (Dröge, 2002).
This review examines the available evidence for the involvement of cellular oxidants in the maintenance of “redox homeostasis” in the redox regulation of normal physiological functions as well as pathogenesis of various diseases, including cancer, diabetes mellitus, ischemia/reperfusion injury, inflammatory diseases, neurodegenerative disorders and ageing. A discussion is also devoted to the various protective pathways that may be provided by the antioxidant network against the deleterious action of free radicals.
Section snippets
Reactive oxygen species (ROS)
Free radicals can be defined as molecules or molecular fragments containing one or more unpaired electrons in atomic or molecular orbitals (Halliwell & Gutteridge, 1999). This unpaired electron(s) usually gives a considerable degree of reactivity to the free radical. Radicals derived from oxygen represent the most important class of radical species generated in living systems (Miller, Buettner, & Aust, 1990). Molecular oxygen (dioxygen) has a unique electronic configuration and is itself a
Reactive nitrogen species (RNS)
NO is a small molecule that contains one unpaired electron on the antibonding orbital and is, therefore, a radical. NO is generated in biological tissues by specific nitric oxide synthases (NOSs), which metabolise arginine to citrulline with the formation of NO via a five electron oxidative reaction (Ghafourifar & Cadenas, 2005). Nitric oxide (NO) is an abundant reactive radical that acts as an important oxidative biological signalling molecule in a large variety of diverse physiological
Oxidative damage to DNA, lipids and proteins
At high concentrations, ROS can be important mediators of damage to cell structures, nucleic acids, lipids and proteins (Valko et al., 2006). The hydroxyl radical is known to react with all components of the DNA molecule, damaging both the purine and pyrimidine bases and also the deoxyribose backbone (Halliwell & Gutteridge, 1999). The most extensively studied DNA lesion is the formation of 8-OH-G. Permanent modification of genetic material resulting from these “oxidative damage” incidents
Antioxidants
Exposure to free radicals from a variety of sources has led organisms to develop a series of defence mechanisms (Cadenas, 1997). Defence mechanisms against free radical-induced oxidative stress involve: (i) preventative mechanisms, (ii) repair mechanisms, (iii) physical defences, and (iv) antioxidant defences. Enzymatic antioxidant defences include superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT). Non-enzymatic antioxidants are represented by ascorbic acid (Vitamin C),
ROS and mechanisms of maintenance of “redox homeostasis”
Free radicals and reactive diamagnetic species derived from radicals operate at low, but measurable concentrations in the cells. Their “steady state” concentrations are determined by the balance between their rates of production and their rates of removal by various antioxidants. Thus each cell is characterised by a particular concentration of electrons (redox state) stored in many cellular constituents and the redox state of a cell and its oscillation determines cellular functioning (Schafer &
ROS, antioxidants and signal transduction—an overview
Cells communicate with each other and respond to extracellular stimuli through biological mechanisms called cell signalling or signal transduction (Poli, Leonarduzzi, Biasi, & Chiarpotto, 2004). Signal transduction is a process enabling information to be transmitted from the outside of a cell to various functional elements inside the cell. Signal transduction is triggered by extracellular signals such as hormones, growth factors, cytokines and neurotransmitters (Thannickal & Fanburg, 2000).
ROS and redox regulation of physiological functions
A great number of physiological functions are controlled by redox-responsive signalling pathways (Dröge, 2002). These, for example involve: (i) redox regulated production of NO; (ii) ROS production by phagocytic NAD(P)H oxidase (oxidative burst); (iii) ROS production by NAD(P)H oxidases in nonphagocytic cells; (iv) regulation of vascular tone and other regulatory functions of NO; (v) ROS production as a sensor for changes of oxygen concentration; (vi) redox regulation of cell adhesion; (vii)
ROS, human disease and ageing: pathophysiological implications of altered redox regulation
Oxidative stress has been implicated in various pathological conditions involving cardiovascular disease, cancer, neurological disorders, diabetes, ischemia/reperfusion, other diseases and ageing (Dalle-Donne et al., 2006; Dhalla, Temsah, & Netticadan, 2000; Jenner, 2003; Sayre, Smith, & Perry, 2001). These diseases fall into two groups: (i) the first group involves diseases characterised by pro-oxidants shifting the thiol/disulphide redox state and impairing glucose tolerance—the so-called
Free radicals-induced tissue injury: Cause or consequence?
From the discussion above, it is clear that free radials act as signalling species in various normal physiological processes. It is also clear that excessive production of free radicals causes damage to biological material and is an essential event in the etiopathogenesis of various diseases (Juranek & Bezek, 2005). However, the question was recently raised whether uncontrolled formation of ROS species is a primary cause or a downstream consequence of the pathological process. While the role of
Conclusions
Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are products of normal cellular metabolism. ROS/RNS are known to act as secondary messengers controlling various normal physiological functions of the organism and therefore the production of NO by NOS and superoxide by NAD(P)H is tightly regulated by hormones, cytokines, and other mechanisms. In addition, ROS and RNS participate in various redox-regulatory mechanisms of cells in order to protect cells against oxidative stress
Acknowledgements
We apologise to those authors whose work we have not cited for space reasons. MV thanks DAAD for a postdoctoral fellowship to work in Bremen University. The preparation of this paper was assisted in part by the Leverhulme Academic Exchange Fund (UK) and a NATO collaborative linkage grant. We also thank VEGA (#1/2450/05 and 1/3579/06) and APVT (#20-005702) for financial support.
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