ⅥOxidative Stress What Is Oxidative Stress? JMAJ 45(7): 271–276, 2002 Toshikazu YOSHIKAWA* and Yuji NAITO** Professor* and Associate Professor**, First Department of Medicine, Kyoto Prefectural University of Medicine Abstract:Oxidative stress is well known to be involved in the pathogenesis of lifestyle-related diseases, including atherosclerosis, hypertension, diabetes mellitus, ischemic diseases, and malignancies. Oxidative stress has been defined as harm- ful because oxygen free radicals attack biological molecules such as lipids, pro- teins, and DNA. However, oxidative stress also has a useful role in physiologic adaptation and in the regulation of intracellular signal transduction. Therefore, a more useful definition of oxidative stress may be “a state where oxidative forces exceed the antioxidant systems due to loss of the balance between them.” The biomarkers that can be used to assess oxidative stress in vivo have been attracting interest because the accurate measurement of such stress is necessary for inves- tigation of its role in lifestyle diseases as well as to evaluate the efficacy of treat- ment. Many markers of oxidative stress have been proposed, including lipid hydro- peroxides, 4-hydroxynonenal, isoprostan, 8-hydroxyguanine, and ubiquinol-10. To prevent the development of lifestyle diseases, advice on how to lead a healthy life should be given to individuals based on the levels of oxidant and antioxidant activity assessed by pertinent biomarkers. Individual genetic information should also be taken into consideration. Key words:Oxidative stress; Free radicals; Active oxygen; Biomarkers Introduction and oxidative DNA damage, but also physi- ologic adaptation phenomena and regulation The close association between oxidative of intracellular signal transduction. From a stress and lifestyle-related diseases has become clinical standpoint, if biomarkers that reflect well known. Oxidative stress is defined as a the extent of oxidative stress were available, “state in which oxidation exceeds the antioxi- such markers would be useful for physicians to dant systems in the body secondary to a loss of gain an insight into the pathological features of the balance between them.” It not only causes various diseases and assess the efficacy of hazardous events such as lipid peroxidation drugs. This article is a revised English version of a paper originally published in the Journal of the Japan Medical Association (Vol. 124, No. 11, 2000, pages 1549–1553). JMAJ, July 2002—Vol. 45, No. 7 271 T. YOSHIKAWA and Y. NAITO Table 1 Major Active Oxygen Species true that the high reactivity of these oxygen metabolites is utilized to control various bio- Superoxide radical מO ⅐ 2 logical phenomena. H2O2 Hydrogen peroxide HO⅐ Hydroxyl radical From a biological viewpoint, various oxygen- 1 O2 Singlet oxygen derived free radicals have been attracting HOO⅐ Hydroperoxyl radical attention for the following reasons: Various LOOH Alkylhydroperoxide ⅐ active oxygen species are generated in the LOO Alkylperoxyl radical body during the process of utilizing of oxygen. LO⅐ Alkoxyl radical Hypochlorite ion Because the body is furnished with elaborate מCIO O Ferryl ion mechanisms to remove active oxygen speciesםFe4 O Periferryl ion and free radicals, these by-products of oxygenםFe5 NO⅐ Nitric oxide metabolism are not necessarily a threat to the body under physiological conditions. However, if active oxygen species or free radicals are generated excessively or at abnormal sites, the Free Radicals, Active Oxygen balance between formation and removal is lost, Species, and Oxidative Stress resulting in oxidative stress. Consequently, active oxygen species and free radicals can Usually, an atom is composed of a central attack molecules in biological membranes and nucleus with pairs of electrons orbiting around tissues, thus inducing various diseases. In other it. However, some atoms and molecules have words, oxidative stress is defined as a “state unpaired electrons and these are called free harmful to the body, which arises when oxida- radicals. Free radicals are usually unstable and tive reactions exceed antioxidant reactions highly reactive because the unpaired electrons because the balance between them has been tend to form pairs with other electrons. An lost.” oxygen molecule (O2) undergoes four-electron However, oxidative stress is actually useful reduction when it is metabolized in vivo. Dur- in some instances. For example, oxidative stress ing this process, reactive oxygen metabolites induces apoptosis to prepare the birth canal for are generated by the excitation of electrons delivery. Also, biological defense mechanisms secondary to addition of energy or interaction are strengthened by oxidative stress during with transition elements. The reactive oxygen appropriate physical exercise and ischemia. metabolites thus produced are more highly Therefore, a more useful definition of oxidative reactive than the original oxygen molecule and stress may be a “state where oxidation exceeds are called active oxygen species. Superoxide, the antioxidant systems because the balance hydrogen peroxide, hydroxyl radicals, and between them has been lost.” singlet oxygen are active oxygen species in the narrow sense. Active oxygen species in a broad Biomarkers of Oxidative Stress sense are listed in Table 1. Only active oxygen species having an unpaired electron, indicated The biomarkers that can be used to assess with a dot above and to the right of the chemi- oxidative stress have been attracting interest cal formula in the table, are free radicals. because the accurate assessment of such stress For aerobic organisms, a mechanism to is necessary for investigation of various patho- remove these highly reactive active oxygen logical conditions, as well as to evaluate the species is essential to sustain life. Therefore, efficacy of drugs. Assessment of the extent of various antioxidant defense mechanisms have oxidative stress using biomarkers is interesting developed in the process of evolution. It is also from a clinical standpoint. The markers found 272 JMAJ, July 2002—Vol. 45, No. 7 OXIDATIVE STRESS O2 · oxygen radical LOO been the most frequently used marker of oxi- · · IH dative stress partly because lipid peroxidation LH L LOO stable products (Fig. 1) is a very important mechanism of cell membrane destruction. Lipid peroxidation is a chain reaction by which unsaturated fatty acids LOOH LH (cell membrane components) are oxidized in various pathological conditions. membrane injury When a hydrogen atom is removed from a cellular injury fatty acid molecule for some reason, the free tissue injury radical chain reaction proceeds as shown in Fig. 1 The chain reaction causing lipid peroxidation Fig. 1. Thus, radicals that can be involved in the extraction of hydrogen atoms from lipids include the hydroxyl radical (HO⅐), the hydro- peroxyl radical (HOO⅐), the lipid peroxyl radi- in blood, urine, and other biological fluids may cal (LOO⅐), and the alkoxyl radical (LO⅐). provide information of diagnostic value, but it Metal-oxygen complexes, particularly iron- would be ideal if organs and tissues suffering oxygen complexes, are also important in vivo. from oxidative stress could be imaged in a man- The peroxidation chain reaction propagates ner similar to CT scanning and MR imaging. In itself once it has started. The process by which recent years, attempts have been made to use lipid radicals (L⅐) are generated from lipids electron spin resonance techniques for this pur- (LH) is called the chain initiation reaction. pose, but it will take time before such methods Lipid radicals (L⅐) thus generated react imme- can be applied to humans. diately with oxygen, resulting in the formation Because the body is not necessarily fully of LOO⅐, which attacks another lipid and protected against oxidative damage, some of removes a hydrogen atom from it, resulting in its constituents may be injured by free radicals, the formation of lipid hydroperoxide (lipid per- and the resultant oxidative products have oxide; LOOH) and another L⅐. This new L⅐ also usually been used as markers. Many markers reacts with oxygen and forms LOO⅐, which have been proposed, including lipid peroxides, attacks another lipid to generate lipid peroxide, malondialdehyde, and 4-hydroxynonenal as so lipid peroxide accumulates as the chain reac- markers for oxidative damage to lipids; iso- tion proceeds. prostan as a product of the free radical oxi- Gastric mucosal injury occurs in patients dation of arachidonic acid; 8-oxoguanine with extensive burns. Before the development (8-hydroxyguanine) and thymineglycol as indi- of mucosal lesions, the blood level of skin- cators of oxidative damage to DNA; and vari- derived substances that react with thiobarbi- ous products of the oxidation of protein and turic acid shows an increase. Then these sub- amino acids including carbonyl protein, stances also increase in the gastric mucosa, hydroxyleucine, hydrovaline, and nitrotyro- leading to the development of mucosal lesions. sine. Lipid peroxide was assessed in clinical The free radical peroxidation of lipids is an samples even in relatively early studies, and important factor in local injury to cell mem- the analytical methods for this substance have branes and impairment of the activity of improved. enzymes and receptors bound to the mem- The famous method of Yagi, which measures brane, and the lipid peroxide thus produced substances that react with thiobarbituric acid, can affect even remote organs. has been widely used in both