Reduction of nucleotides

Redox catalysis Zn, Fe, Cu, Mn, Mo, Co, V Se, Cd, Nl Enzymes (see Table 11.4 for more Information) Reactions with oxygen (Fe, Cu) Oxygen evolution (Mn) Nitrogen fixation (Fe, Mo) Inhibition of llpid peroxidation (Se) Carbonic anhydrase (Cd) Reduction of nucleotides (Co) Reactions with H2 (Nl) Bromoperoxidase activity (V)... 

Fig. 29. Mechanism of reduction of nucleotide into deoxynucleotide by RDPR [92].

Box HC (1977) Radiation effects ESR and ENDOR Analysis. Academic Press, New York Box HC, Budzinski EE (1975) Primary radiation damage in thymidine. J Chem Phys 62 197-199 Box HC, Potter WR, Budzinski EE (1975) The reduction of nucleotides by ionizing radiation Uridine 5 phosphate and cytidine 3 phosphate. J Chem Phys 62 3476-3478 Box HC, Budzinski EE, Freund HG, Evans MS, Patrzyc HB, Wallace JC, Maccubbin AE (1993) Vicinal lesions in X-irradiated DNA Int J Radiat Biol 64 261-263 Box HC, Freund HG, Budzinski EE, Wallace JC, Maccubbin AE (1995) Free radical-induced double base lesions. Radiat Res 141 91-94... 

Reduction of nucleotides to deoxynucleotides takes place by the mechanism illustrated in Figure 14. TPNH acts as a hydrogen donor in the enzymatic reductitm of a small molecular-weight protein, containing a disulphide bridge (oxidized thioredoxin). Reduced thioredoxin so formed, now acts as a hydrogen donor for the reduction of the ribonucleotide diphosphate. The enzyme (ribonucleoside diphos-... 

The electrons undergo the equivalent of a partial oxidation process ia a dark reaction to a positive potential of +0.4 V, and Photosystem I then raises the potential of the electrons to as high as —0.7 V. Under normal photosynthesis conditions, these electrons reduce tryphosphopyridine-nucleotide (TPN) to TPNH, which reduces carbon dioxide to organic plant material. In the biophotolysis of water, these electrons are diverted from carbon dioxide to a microbial hydrogenase for reduction of protons to hydrogen ... 

Xanthine oxidase, mol wt ca 275,000, present in milk, Hver, and intestinal mucosa (131), is required in the cataboHsm of nucleotides. The free bases guanine and hypoxanthine from the nucleotides are converted to uric acid and xanthine in the intermediate. Xanthine oxidase cataly2es oxidation of hypoxanthine to xanthine and xanthine to uric acid. In these processes and in the oxidations cataly2ed by aldehyde oxidase, molecular oxygen is reduced to H2O2 (133). Xanthine oxidase is also involved in iron metaboHsm. Release of iron from ferritin requires reduction of Fe " to Fe " and reduced xanthine oxidase participates in this conversion (133). 

The o-nitrobenzyl and p-nitrobenzyl ethers can b prepared and cleaved by many of the methods described for benzyl ethers. The p-nitrobenzyl ether is also prepared from an alcohol and p-nitrobenzyl alcohol (trifluoroacetic anhydride, 2,6-lutidine, CH2CI2, 67% yield). In addition, the o-nitrobenzyl ether can be cleaved by irradiation (320 nm, 10 min, quant, yield of carbohydrate " 280 nm, 95% yield of nucleotide ). The p-nitrobenzyl ether has been cleaved by electrolytic reduction (—1.1 V, DMF, R4N X, 60% yield) and by reduction with Na2S204 (pH 8-9, 80-95% yield). These ethers can also be cleaved oxidatively (DDQ or electrolysis) after reduction to the aniline derivative. ... 

Under low oxygen conditions, C5 -sugar radicals can react with the base residue on the same nucleotide. In purine nucleotides, the carbon-centered radical 91 can add to the C8-position of the nucleobase (Scheme 8.31). Oxidation of the intermediate nucleobase radical 92 yields the 8,5 -cyclo-2 -deoxypurine lesion 93197,224,225,230-233 Similarly, in pyrimidine nucleotides, the C5 -radical can add to the C6-position of nucleobase. Reduction of the resulting radical intermediate yields the 5, 6-cyclo-5,6-dihydro-2 -deoxypyrimidine lesion 94,234-236... 

The reduced form of diphosphopyridine nucleotide (DPNH), which is used in the reduction of di-O-phosphoglyceronic acid to O-phosphoglycerose, could be regenerated with high efficiency from the oxidized form (DPN ) by the reduced form of 6-thioctic acid, thus supplying the necessary re-... 

Mammalian thioredoxin reductases are a family of selenium-containing pyridine nucleotide-disulfide oxidoreductases. These enzymes catalyze NADPH-dependent reduction of the redox protein thioredoxin (Trx), which contains a redox-active disulfide and dithiol group and by itself may function as an efficient cytosolic antioxidant [77]. One of the functions of Trx/ thioredoxin reductase system is the NADPH-catalyzed reduction of protein disulfide [78] ... 

Alcohol dehydrogenases catalyze oxidation of alcohols in a reaction dependent on the pyridine nucleotide NAD+ [Eq. (5)]. Since the reaction is reversible, alcohol dehydrogenases also catalyze the reduction of aldehydes by... 

The carbonyl reductases catalyze reduction of aldehydes and ketones by reduced pyridine nucleotides (NADH and/or NADPH). As mentioned earlier, alcohol dehydrogenase can perform this function in the presence of a high ratio of NADH to NAD+. Other enzymes capable of carbonyl reduction include the aldehyde and ketone reductases. The aldehyde and ketone reductases have a ubiquitous species distribution, with the enzymes present in organisms ranging from bacteria to vertebrates. The mammalian carbonyl reductases have been extensively reviewed (101). 

Indicine IV-oxide (169) (Scheme 36) is a clinically important pyrrolizidine alkaloid being used in the treatment of neoplasms. The compound is an attractive drug candidate because it does not have the acute toxicity observed in other pyrrolizidine alkaloids. Indicine IV-oxide apparently demonstrates increased biological activity and toxicity after reduction to the tertiary amine. Duffel and Gillespie (90) demonstrated that horseradish peroxidase catalyzes the reduction of indicine IV-oxide to indicine in an anaerobic reaction requiring a reduced pyridine nucleotide (either NADH or NADPH) and a flavin coenzyme (FMN or FAD). Rat liver microsomes and the 100,000 x g supernatant fraction also catalyze the reduction of the IV-oxide, and cofactor requirements and inhibition characteristics with these enzyme systems are similar to those exhibited by horseradish peroxidase. Sodium azide inhibited the TV-oxide reduction reaction, while aminotriazole did not. With rat liver microsomes, IV-octylamine decreased... 

Figure 4 summarizes the direct oxidative pathway with its relations to (a) the gycolytic route of G-6-P utilization, (b) the reduction of pyridine nucleotides, and (c) the influences on some reactions depending on reduced pyridine nucleotides. 

The reactions in Eqs. (10) and (11) indicate the links with glucose metabolism, the former to the direct oxidative pathway, the latter to the glycolytic route. The essential link in the reduction of MHb is the generation of reduced pyridine nucleotides (B14, R12, S10). The... 

The Fe-protein has the protein fold and nucleotide-binding domain of the G-protein family of nucleotide-dependent switch proteins, which are able to change their conformation dependent on whether a nucleoside diphosphate (such as GDP or ADP) is bound instead of the corresponding triphosphate (GTP or ATP). However, nucleotide analogues, which induce the conformational switch of the Fe-protein, do not allow substrate reduction by the MoFe-protein, nor does reduction of the MoFe-protein by other electron-transfer reagents (whether small proteins or redox dyes) drive substrate reduction. Only the Fe-protein can reduce the MoFe-protein to a level that allows it to reduce substrates such as... 

This is another example of the generalization that enzyme reactions of the same type have the same stereospecificity for the pyridine nucleotide, no matter what their cellular origin. There is another mevaldic reductase in the cytosol of liver, which catalyzes the reduction of mevaldic acid to mevalonic acid by NAD or NADP, and this enzyme also has A (or pro R) stereospecificity for the pyridine nucleotide but the hydrogen transfer occurs to the pro R position on C—5. This latter enzyme can use either 3R or 3S mevaldic acid that is, it is indifferent to... 

It should be noted, in this connection, that there are pyridine nucleotide dehydrogenases which catalyze redox reactions which must occur in two steps. Hydroxymethylglutaryl CoA reductase (discussed on p. 51) is one example. Another is uridine diphosphate-glucose dehydrogenase, which catalyzes the oxidation of the C—6 of the glucose (i.e., a primary alcohol) to a carboxyl group. In both cases, there are two molecules of pyridine nucleotide required, and the overall reactions are essentially irreversible. The former enzyme, with A stereospecificity for the pyridine nucleotide, catalyzes the reduction of an acyl-CoA group... 

Dithiothreitol can be used as the source of electrons for the reaction in vitro, although it is widely held that the reduction of the GR in vivo is linked to the pool of reduced nucleotides (NADH or NADPH). A more detailed description of the characterization of each component of the GR is given below. 

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