Plasma membrane reductase

This mechanism is now considered to be of importance for the protection of LDL against oxidation stress, Chapter 25.) The antioxidant effect of ubiquinones on lipid peroxidation was first shown in 1980 [237]. In 1987 Solaini et al. [238] showed that the depletion of beef heart mitochondria from ubiquinone enhanced the iron adriamycin-initiated lipid peroxidation whereas the reincorporation of ubiquinone in mitochondria depressed lipid peroxidation. It was concluded that ubiquinone is able to protect mitochondria against the prooxidant effect of adriamycin. Inhibition of in vitro and in vivo liposomal, microsomal, and mitochondrial lipid peroxidation has also been shown in studies by Beyer [239] and Frei et al. [240]. Later on, it was suggested that ubihydroquinones inhibit lipid peroxidation only in cooperation with vitamin E [241]. However, simultaneous presence of ubihydroquinone and vitamin E apparently is not always necessary [242], although the synergistic interaction of these antioxidants may take place (see below). It has been shown that the enzymatic reduction of ubiquinones to ubihydroquinones is catalyzed by NADH-dependent plasma membrane reductase and NADPH-dependent cytosolic ubiquinone reductase [243,244]. 

Figure 7 Mixld for iron (Fe) deficiency induced changes in root physiology and rhizo-sphere chemistry associated with Fc acquisition in strategy I plants. (Modified froin Ref. 1.) A. Stimulation of proton extru.sion by enhanced activity of the plasnialemma ATPase —> Felll solubilization in the rhizospherc. B. Enhanced exudation of reductanls and chela-tors (carhoxylates. phenolics) mediated by diffusion or anion channels Pe solubilization by Fein complexation and Felll reduction. C. Enhanced activity of plasma membrane (PM)-bound Felll reductase further stimulated by rhizosphere acidificalion (A). Reduction of FolII chelates, liberation of Fell. D. Uptake of Fell by a PM-bound Fell transporter.

W. Briiggemann, P. R. Moog, H. Nakagawa, P. Janiesch, and P. J. C. Kuiper, Plasma membrane-bound NADH-Fe -EDXA reductase and iron deficiency in tomato. (Lycopcrsicon escidentiim). Is there a turbo reductase Physiol. Plant 79 339 (1991). 

M. J. Holden, D. G. Luster, R. L. Chaney, T. J. Buckhout. and C. Robinson, Fc -chelate reductase activity of plasma membranes isolated from tomato (Lyco-persicon escidentiim Mill.) roots. Comparison of enzymes from Fe-deficient and Fe-sufficient roots. Plant Physiol. 97 531 (1991). 

As we saw in the previous section, Strategy 1 plants utilize ferric reductases, with NADPH as electron donor, coupled to proton extrusion and a specific Fe(II) transport system localized in the root plasma membrane. Saccharomyces cerevisiae also uses cell surface reductases to reduce ferric iron, and in early studies (Lesuisse et ah, 1987 ... 

The yeast-mediated enzymatic biodegradation of azo dyes can be accomplished either by reductive reactions or by oxidative reactions. In general, reductive reactions led to cleavage of azo dyes into aromatic amines, which are further mineralized by yeasts. Enzymes putatively involved in this process are NADH-dependent reductases [24] and an azoreductase [16], which is dependent on the extracellular activity of a component of the plasma membrane redox system, identified as a ferric reductase [19]. Recently, significant increase in the activities of NADH-dependent reductase and azoreductase was observed in the cells of Trichosporon beigelii obtained at the end of the decolorization process [25]. 

Ramalho PA, Paiva S, Cavaco-Paulo A et al (2005) Azo reductase activity of intact Saccharomyces cerevisiae cells is dependent on the Frelp component of plasma membrane ferric reductase. Appl Environ Microbiol 71 3882-3888... 

From the biological point of view, the effect of anaerobiosis has been characterized in purely anaerobic, facultative anaerobic, and aerobic bacteria, in yeasts, and in tissues from higher organisms [6-12], From these studies it can be deduced that almost every azo compound can be biologically reduced under anaerobic conditions [4]. Reduced flavins are produced by cytosol flavin-dependent reductases [6, 13], while quinone reductase activity located in the plasma membrane [14] and extracellular azo reductase activities [9, 15] were also observed. 

To reach the reductive step of the azo bond cleavage, due to the reaction between reduced electron carriers (flavins or hydroquinones) and azo dyes, either the reduced electron carrier or the azo compound should pass the cell plasma membrane barrier. Highly polar azo dyes, such as sulfonated compounds, cannot pass the plasma membrane barrier, as sulfonic acid substitution of the azo dye structure apparently blocks effective dye permeation [28], The removal of the block to the dye permeation by treatment with toluene of Bacillus cereus cells induced a significant increase of the uptake of sulfonated azo dyes and of their reduction rate [29]. Moreover, cell extracts usually show to be more active in anaerobic reduction of azo dyes than whole cells. Therefore, intracellular reductases activities are not the best way to reach sulfonated azo dyes reduction the biological systems in which the transport of redox mediators or of azo dye through the plasma membrane is not required are preferable to achieve their degradation [13]. 

Also ascomycetes yeast strains showed decolorizing behaviors due to extracellular reactions on polar dyes. The process occur when an alternative carbon and energy source is available. The involvement of an externally directed plasma membrane redox system was suggested in S. cerevisiae, the plasma membrane ferric reductase system participates in the extracellular reduction of azo dyes [25]. 

In 1986, the antioxidant effects of thioredoxin reductase were studied by Schallreuter et al. [81]. It has been shown that thioredoxin reductase was contained in the plasma membrane surface of human keratinocytes where it provided skin protection against free radical mediated damage. Later on, the reductive activity of Trx/thioredoxin reductase system has been shown for the reduction of ascorbyl radical to ascorbate [82], the redox regulation of NFkB factor [83], and in the regulation of nitric oxide-nitric oxide synthase activities [84,85],... 

The marker enzymes used in this experiment are as follows vanadate-sensitive H+-ATPase (plasma membrane), nitrate-sensitive H+-ATPase or pyrophosphatase (tonoplast), TritonX-100 stimulated-UDPase or IDPase (Golgi complex), antimycin A-insensitive NADPH cytochrome c reductase (ER), and cytochrome c oxidase (mitochondria inner membrane). NADH cytochrome c reductase activity is found to be 10 times higher than NADPH cytochrome c reductase activity. Chlorophyll content can be measured as the chloroplast marker. The chlorophyll content is calculated by the following equation. Before measurement, auto zero is performed at 750 ran. 

A second example of a membrane-bound arsenate reductase was isolated from Sulfurospirillum barnesii and was determined to be a aiPiyi-heterotrimic enzyme complex (Newman et al. 1998). The enzyme has a composite molecular mass of 100kDa, and a-, P-, and y-subunits have masses of 65, 31, and 22, respectively. This enzyme couples the reduction of As(V) to As(III) by oxidation of methyl viologen, with an apparent Kra of 0.2 mM. Preliminary compositional analysis suggests that iron-sulfur and molybdenum prosthetic groups are present. Associated with the membrane of S. barnesii is a h-type cytochrome, and the arsenate reductase is proposed to be linked to the electron-transport system of the plasma membrane. 

The recently isolated Desulfotomaculum strain Ben-RB is able to grow using lactate as a substrate and arsenate as the sole electron acceptor (Macy et al. 2000). It has been proposed that arsenate reductase is associated with the respiratory chain of this organism, because >98% of the arsenate reductase bound to the plasma membrane. 

The answer is D. This patient s tests indicate that he has severe hypercholesterolemia and high blood pressure in conjunction with atherosclerosis. The deaths of several of his family members due to heart disease before age 60 suggest a genetic component, ie, familial hypercholesterolemia. This disease results from mutations that reduce production or interfere with functions of the LDL receptor, which is responsible for uptake of LDL-cholesterol by liver cells. The LDL receptor binds and internalizes LDL-choles-terol, delivers it to early endosomes and then recycles back to the plasma membrane to pick up more ligand. Reduced synthesis of apoproteins needed for LDL assembly would tend to decrease LDL levels in the bloodstream, as would impairment of HMG CoA reductase levels, the rate-limiting step of cholesterol biosynthesis. Reduced uptake of bile salts will also decrease cholesterol levels in the blood. 

As many as 1 in 10,000 persons may inherit such prob-lems.48 50a Tire proteins that may be defective include a plasma membrane carnitine transporter carnitine palmitoyltransferases camitine/acylcamitine trans-locase long-chain, medium-chain, and short-chain acyl-CoA dehydrogenases 2,4-dienoyl-CoA reductase (Eq. 17-1) and long-chain 3-hydroxyacyl-CoA dehydrogenase. Some of these are indicated in Fig. 17-2. 

Bacterial assimilatory nitrate reductases have similar properties.86/86a In addition, many bacteria, including E. coli, are able to use nitrate ions as an oxidant for nitrate respiration under anaerobic conditions (Chapter 18). Tire dissimilatory nitrate reductases involved also contain molybdenum as well as Fe-S centers.85 Tire E. coli enzyme receives electrons from reduced quinones in the plasma membrane, passing them through cytochrome b, Fe-S centers, and molybdopterin to nitrate. The three-subunit aPy enzyme contains cytochrome b in one subunit, an Fe3S4 center as well as three Fe4S4 clusters in another, and the molybdenum cofactor in the third.87 Nitrate reduction to nitrite is also on the pathway of denitrification, which can lead to release of nitrogen as NO, NzO, and N2 by the action of dissimi-latory nitrite reductases. These enzymes873 have been discussed in Chapters 16 and 18. 

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