Active oxygen detoxification

Figure 5 (A) Evidence that paraquat transiently affects chloroplasts of resistant Conyza and (B) that there are constitutively elevated levels of the Halliwell-Asada active oxygen detoxification pathway in the chloroplasts. A. Resistant and susceptible plants of Conyza bonariensis were sprayed to runoff with 0.1 mM paraquat and whole leaves were removed for measurement of photosynthesis at times thereafter as an estimation of paraquat arriving at, and affecting chloroplasts. Simultaneous measurements of stomatal aperture were made to ascertain that the stomates remained open. Source Data redrawn from (60). B. Normal enzyme levels (without paraquat treatment) in resistant and susceptible Conyza bonariensis. Source Collated and drawn from (57, 61).

Table 1. Herbicide cross tolerances to oxidative stresses and relations with enzymes of the Halliwell-Asada active oxygen detoxification pathway [cf. 36-39]...

Iron 5 X 104 3 X 10 3 Oxygen transport, storage, activation and detoxification, electron transfer, nitrogen fixation, ribose reduction, etc. 

Catalase (CAT) Therapeutically active protein (detoxification of reactive oxygen species)... 

Two different enzyme systems have been described, one uses cytochrome P-450 to activate oxygen whereas the other employs flavin adenine dinucleotide (FAD). Both the cytochrome P-450 and the flavin monooxygenase systems have broad substrate specificities and oxidize and oxygenate a variety of organic nitrogen or sulfur compounds. The enzymes have a widespread distribution and have been detected in animals, plants, fungi and bacteria. Their function appears to be primarily one of detoxification of xenobiotics. Microbial enzymes... 

Superoxide Reductase (see Iron Proteins with Mononuclear Active Sites). Detoxification of reactive oxygen species in anaerobic microorganisms has recently been shown to center around SOR, a novel mononuclear iron enzyme that reduces superoxide to hydrogen peroxide (see equation 4), rather than dismuting superoxide to oxygen and hydrogen peroxide as is the case for the superoxide dismutases found in aerobic organisms. 

Figure 4. Scheme of interactions between active oxygen generating xenobiotics and the oxygen detoxification system. Light intensity, the rate constants of photogeneration of active oxygen, dissipation of the xenobiotics and the levels and rate constants of the enzymes interact to determine whether a plant will be spared or killed. 

The mutagenic action of rubroskyrin (785) was examined by Mori and co-workers, who revealed that this activity resulted from the generation of active oxygen in the course of detoxification. Rubroskyrin (785) becomes reduced by NADH and is autoxidized by dissolved oxygen. Then, H2O2 is produced, which immediately decomposes to reactive oxygen by a catalase. Thereafter, superoxide dismutase produces the very reactive superoxide anion. Once this process is completed, rubroskyrin is transformed to stable products that are not toxic (548). 

Cytochrome P-450s are the best-known class of hydroxylation enzyme. Their active sites contain a heme iron that forms a highly activated oxygenating species that reacts by a radical mechanism. In higher animals, they function primarily in metabolite degradation as part of pathways that clear unnatural substances such as toxins and drugs. Hydroxylation inaeases polarity that facilitates further derivatization by other detoxification enzymes or excretion of the hydroxylated products. Other P-450 family members are involved in secondary metabolite biosynthesis, particularly in plants and microbial cells (Figure 1.6). 

Fig. 2. Schematic diagram delineating some of the multiple stages of mutagenesis and the interference by flavonoids. 1. Flavonoids induce apoptosis and enhance mutagen detoxification and extrusion from the cell. 2. Flavonoids interfere with the metabolic activation of mutagens and protect DNA by means of their antioxidative action. GST glutathione-S-transferase ROM reactive oxygen metabolites.

Besides the enzyme, the superoxide ion can also be an electron donor. The ion arises as a result of detoxification of xenobiotis (xenobiotics are outsiders, which are involved in the chain of metabolism). Xenobiotics yield anion radicals upon the neutralizing influence of redox proteins. Oxygen (inhaled with air) takes an unpaired electron off from a part of these anion radicals and forms the superoxide ion. The latter plays its own active role in biochemical reactions (see Sections 1.7.l.C. and 3.4.5). 

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