Nonenzymatic antioxidants

Antioxidants are snbstances that are able to protect organisms from oxidative stress. A distinction is drawn between three types of antioxidants enzymatic antioxidants, nonenzymatic antioxidants, and repair enzymes. 

Manzocco L, Calligaris S, Mastrocola D, Nicoli MC and Lerici CR. 2001. Review of nonenzymatic browning and antioxidant capacity in processed foods. Trends Food Sci Technol 11 340-346. 

The antioxidant system in humans is a complex network composed by several enzymatic and nonenzymatic antioxidants. In addition to being an antioxidant, lycopene also exerts indirect antioxidant properties by inducing the production of cellular enzymes such as superoxide dismutase, glutathione S-transferase, and quinone reductase that also protect cells from reactive oxygen species and other electrophilic molecules (Goo and others 2007). 

A large number of nonenzymatic compounds, including tocopherols, caroti-noids, vitamins C and D, steroids, ubiquinones, thiols, uric acid, bilirubin, ino-sine, taurine, pyruvate, CRP, and so on, demonstrate qualitative antioxidant properties under experimental conditions. However, the quantitative relevance of most findings remains unclear. 

The second maximum is riboflavin-independent (Fig. 1). In this case, luminol obviously plays a double role it is the chemiluminogenous detection compound for free radicals and photosensitizer as well. It is a remarkable characteristic of this system that the signal intensity decreases only very slowly, giving an opportunity for detection of nonenzymatic antioxidants. 

In the last decade numerous studies were dedicated to the study of biological role of nonenzymatic free radical oxidation of unsaturated fatty acids into isoprostanes. This task is exclusively difficult due to a huge number of these compounds (maybe many hundreds). Therefore, unfortunately, the study of several isoprostanes is not enough to make final conclusions even about their major functions. F2-isoprostanes were formed in plasma and LDL after the treatment with peroxyl radicals [98], It is interesting that their formation was observed only after endogenous ascorbate and ubiquinone-10 were exhausted, despite the presence of other antioxidants such as urate or a-tocopherol. LDL oxidation was followed by... 

Belkner et al. [32] demonstrated that 15-LOX oxidized preferably LDL cholesterol esters. Even in the presence of free linoleic acid, cholesteryl linoleate continued to be a major LOX substrate. It was also found that the depletion of LDL from a-tocopherol has not prevented the LDL oxidation. This is of a special interest in connection with the role of a-tocopherol in LDL oxidation. As the majority of cholesteryl esters is normally buried in the core of a lipoprotein particle and cannot be directly oxidized by LOX, it has been suggested that LDL oxidation might be initiated by a-tocopheryl radical formed during the oxidation of a-tocopherol [33,34]. Correspondingly, it was concluded that the oxidation of LDL by soybean and recombinant human 15-LOXs may occur by two pathways (a) LDL-free fatty acids are oxidized enzymatically with the formation of a-tocopheryl radical, and (b) the a-tocopheryl-mediated oxidation of cholesteryl esters occurs via a nonenzymatic way. Pro and con proofs related to the prooxidant role of a-tocopherol were considered in Chapter 25 in connection with the study of nonenzymatic lipid oxidation and in Chapter 29 dedicated to antioxidants. It should be stressed that comparison of the possible effects of a-tocopherol and nitric oxide on LDL oxidation does not support importance of a-tocopherol prooxidant activity. It should be mentioned that the above data describing the activity of cholesteryl esters in LDL oxidation are in contradiction with some earlier results. Thus in 1988, Sparrow et al. [35] suggested that the 15-LOX-catalyzed oxidation of LDL is accelerated in the presence of phospholipase A2, i.e., the hydrolysis of cholesterol esters is an important step in LDL oxidation. 

Similar to lipids the oxidation of proteins has already been studied for more than 20 years. Before discussing the data on protein oxidation, it should be mentioned that many associated questions were already considered in previous chapters. For example, the oxidation of lipoproteins, which is closely connected with the problems of nonenzymatic lipid peroxidation was discussed in Chapter 25. Many questions on the interaction of superoxide and nitric oxide with enzymes including the inhibition of enzymatic activities of prooxidant and antioxidant enzymes are considered in Chapters 22 and 30. Therefore, the findings reported in those chapters should be taken into account for considering the data presented in this chapter. 

ROS are maintained at tolerable levels through the combined efforts of antioxidant mechanisms, which include both enzymes and nonenzymatic molecules. Important antioxidant enzymes include superoxide dismutases (SODs), catalases (CATs), peroxidases, and those maintaining reduced glutathione (GSH) levels. Nonenzymatic antioxidants include GSH, a tripeptide containing a cysteinamino acid, and vitamins, such as vitamin A, E, and C. 

A19. Atsumi, T., Iwakura, I., Kashiwagi, Y., Fujisawa, S., and Ueha, T., Free radical scavenging activity in the nonenzymatic fraction of human saliva A simple DPPH assay showing the effect of physical exercise. Antioxid. Redox Signal. 1, 537-546 (1999). 

J5. Jozwik, M., Jozwik, M., Kuczynski, W., and Szamatowicz, M., Nonenzymatic antioxidant activity of human seminal plasma. Fertil. Steril. 68, 154-157 (1997). 

OS is caused by excess of oxidation and/or lack of antioxidant defense. Since it can damage all the constituents of the body (proteins, lipids, DNA, etc.), OS has to be a temporary condition, under strict control by the antioxidant defense network, which is represented by a variety of enzymatic and nonenzymatic systems,... 

OS can seriously damage molecules such as lipids, DNA, proteins, etc. following an imbalance between production/ presence of RS and antioxidant defense. This consists of pool of nonenzymatic antioxidants and antioxidants enzymes, which have to be present and efficient in that part of the body where the oxidation is underway. Some examples may help clarify the concept. 

The lung also possesses nonenzymatic antioxidants such as vitamin E, beta-carotene, vitamin C, and uric acid. Vitamin E is lipid-soluble and partitions into lipid membranes, where it is positioned optimally for maximal antioxidant effectiveness. Vitamin E converts superoxide anion, hydroxyl radical, and lipid peroxyl radicals to less reactive oxygen metabolites. Beta-carotene also accumulates in cell membranes and is a metabolic precursor to vitamin A. Furthermore, it can scavenge superoxide anion and react directly with peroxyl-free radicals, thereby serving as an additional lipid-soluble antioxidant. Vitamin C is widely available in both extracellular and intracellular spaces where it can participate in redox reactions. Vitamin C can directly scavenge superoxide and hydroxyl radical. Uric acid formed by the catabolism of purines also has antioxidant properties and primarily scavenges hydroxyl radical and peroxyl radicals from lipid peroxidation. 

The oxidants responsible for initiating LDL oxidation have been under intense investigation, and several possible mechanisms have been suggested. For example, Oj has been implicated as a major contributor to LDL oxidation mediated by macrophages and smooth muscle cells (H8). Here, O " is converted to H2O2 by SOD, which in turn is acted upon by a transition metal ion with the formation of HO. Another possible role for Cff is its reaction with NO to form ONOCT, which is capable of oxidizing lipids and sulfhydryl groups, even in the presence of plasma antioxidants (VI). Moreover, in vitro studies have shown that ONOCT can induce the formation of F2-isoprostanes, nonenzymatic products of the free radical-catalyzed oxidation of arachidonic acid (M13). 

Both in MCI and AD patients, mean plasma levels of nonenzymatic antioxidants and activity of antioxidant enzymes appeared to be lower than in controls, with no parallel induction of antioxidant enzymes (Keller et al., 2005). In order to explain these results it has been suggested that the increased free radical production in MCI might lead to a rapid consumption of plasma antioxidants without a simultaneous activation of new molecules of antioxidant enzymes. Individuals with MCI, and subsequently with AD, are likely to have an inadequate antioxidant enzymatic activity, unable to counteract the increased production of free radicals during the pathogenesis of the disease. 

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