Nicotinamide adenine dinucleotides NAD/NADH

Although zinc itself is not redox-active, some class I enzymes containing zinc in their active sites are known. The most prominent are probably alcohol dehydrogenase and copper-zinc superoxide dismutase (Cu,Zn-SOD). AU have in common that the redox-active agent is another transition-metal ion (copper in Cu,Zn-SOD) or a cofactor such as nicotinamide adenine dinucleotide (NAD+/NADH). The Zn(II) ion affects the redox reaction only in an indirect manner, but is nevCTtheless essential and cannot be regarded simply as a structural factor. 

Enzyme biosensors are usually constructed with enzymes that are oxido-reductases. The largest known group of oxidoreductases is dehydrogenases, of which more than 250 depend on the soluble coenzyme nicotinamide adenine dinucleotide, NAD "/NADH couple. Excellent review for electrocatalytic... 

Strolin-Benedetti et al., 2006). Most of these are zinc-containing cytosolic enzymes that use nicotinamide adenine dinucleotide/reduced nicotinamide adenine dinucleotide (NAD+/NADH) as the cofactor. In contrast, aldehyde dehydrogenases utilize NAD-I- and catalyze the irreversible oxidation of aldehydes to carboxylic acids (Testa and Kramer, 2007 Strolin-Benedetti et al., 2006 Marchitti et al., 2008). Some forms of aldehyde dehydrogenases are cytosolic and others are mitochondrial ... 

Today s systems are in most cases based on the bioluminescence with ATP and luciferase from the firefly. As an alternative system it is also possible to use a colour test nicotinamide adenine dinucleotides (NAD/NADH) and nicotinamide adenine dinucleotide phosphates (NADP/NADPH), which are also compounds used for the energy transfer in the metabolism in living cells or compounds found in food debris. 

Scheme 14.12. A representation of a potential pathway for the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) while tetrahydrofolate is oxidized to dihydrofolate using nicotinamide adenine dinucleotide (NAD /NADH). EC numbers and some graphic materials in this scheme have been taken from appropriate links in a URL starting with http //www.chem.qmul.ac.uk/iubmb/enzyme.

Enzyme-catalysed reactions are widely used for analytical purposes, for the determination of substrates (e.g. glucose oxidase for determination of glucose) and of inhibitors (such as pesticides, by their inhibition of cholinesterase) and activators. Although enzymes are very useful as analytical reagents, they are not classified individually in this Dictionary. However, enzymes themselves are extensively assayed by clinical chemists, biochemists, forensic scientists and food chemists, and the substrates used for such assays are carefully chosen to achieve optimum sensitivity, selectivity and reliability. Such substrates are listed in this Dictionary, as are the co-enzymes (co-factors) required by many redox enzymes, for example nicotinamide adenine dinucleotide (NAD /NADH) which is a co-enzyme for many dehydrogenases, e.g. 

Insects poisoned with rotenone exhibit a steady decline ia oxygen consumption and the iasecticide has been shown to have a specific action ia interfering with the electron transport iavolved ia the oxidation of reduced nicotinamide adenine dinucleotide (NADH) to nicotinamide adenine dinucleotide (NAD) by cytochrome b. Poisoning, therefore, inhibits the mitochondrial oxidation of Krebs-cycle iatermediates which is catalysed by NAD. 

Indicators There are certain compounds that are suitable as indicators for sensitive and specific clinical analysis. Nicotinamide adenine dinucleotide (NAD) occurs in oxidized (NAD" ) and reduced (NADH) forms. Nicotinamide adenine dinucleotide phosphate (NADP) also has two states, NADP" and NADPH. NADH has a very high uv—vis absorption at 339 nm, extinction coefficient = 6300 (M cm) , but NAD" does not. Similarly, NADPH absorbs light very strongly whereas NADP" does not. 

Coenzymes such as adenosine diphosphate (ADP), adenosine SGtriphosphate (ATP), nicotinamide adenine dinucleotide (NAD), and nicotinamide adenine dinucleotide, reduced (NADH), are involved in some reactions (4). 

The leading substrate (A) is nicotinamide adenine dinucleotide (NAD ), and NAD and NADH (product Q) compete for a common site on E. A specific example is offered by alcohol dehydrogenase (ADH) ... 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

Figure 12.6 The polyol pathway. NADPH, reduced nicotinamide adenine dinucleotide (NAD) phosphate NADH, reduced NAD.

Rhodium and ruthenium complexes have also been studied as effective catalysts. Rh(diphos)2Cl [diphos = l,2-bis(diphenyl-phosphino)ethane] catalyzed the electroreduction of C02 in acetonitrile solution.146 Formate was produced at current efficiencies of ca. 20-40% in dry acetonitrile at ca. -1.5 V (versus Ag wire). It was suggested that acetonitrile itself was the source of the hydrogen atom and that formation of the hydride HRh(diphos)2 as an active intermediate was involved. Rh(bpy)3Cl3, which had been used as a catalyst for the two-electron reduction of NAD+ (nicotinamide adenine dinucleotide) to NADH by Wienkamp and Steckhan,147 has also acted as a catalyst for C02 reduction in aqueous solutions (0.1 M TEAP) at -1.1 V versus SCE using Hg, Pb, In, graphite, and n-Ti02 electrodes.148 Formate was the main... 

The second type of biological electron transfer involves a variety of small molecules, both organic and inorganic. Examples of these are (a) nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) as two electron carriers and (b) quinones and flavin mononucleotide (FMN), which may transfer one or two electrons. The structure of NAD and its reduced counterpart NADH are shown in Figure 1.12. 

The liberated nicotinamide-adenine-dinucleotide hydrogenase (NADH) has an absorption maxima at 340 nm, whereas lactic acid. NAD+ and pyruvic acid do not absorb at all at this wavelenath. 

In the second stage, the building blocks are degraded by various pathways in tissues to a common metabolic intermediate, acetyl CoA. Most of the energy contained in metabolic fuels is conserved in the chemical bonds (electrons) of acetyl CoA. A smaller portion is conserved in reducing nicotinamide adenine dinucleotide (NAD) to NADH or flavin adenine dinucleotide (FAD) to FADH. Reduction indicates the addition of electrons that may be free, part of a hydrogen atom (H), or a hydride ion (H ). 

Although the structures for molecules having niacin activity are simple, the forms in which they act in human biochemistry are not so simple. Nicotinic acid and nicotinamide are precursors for three complex coenzymes in multiple oxida-tion/reduction (redox) reactions nicotinamide mononucleotide, NMN nicotinamide adenine dinucleotide, NAD+ and nicotinamide adenine dinucleotide phosphate, NADP. I shall use NAD+ as representative of the class. NADH is the corresponding reduced form. ... 

Streptozocin (Zanosar), a water-soluble nitrosourea produced by the fungus Streptomyces achromogenes, acts through methylation of nucleic acids and proteins. In addition, it produces rapid and severe depletion of the pyridine nucleotides nicotinamide adenine dinucleotide (NAD) and its reduced form (NADH) in liver and pancreatic islets. 

There are some methods that are specific to HCHO. For example, the Hantzsch reaction of HCHO, collected with a diffusion scrubber, with ammonium acetate, acetic acid, and acetylacetone to form diacetyldihydrolutidine, which is measured using its fluorescence at 470 nm, has been applied to air measurements (Dasgupta et al., 1988, 1990 Kleindienst et al., 1988a,b Lawson et al., 1990 Khare et al., 1997). Reaction with 1,3-cyclohexanedione and ammonium acetate to form a dihydropyridine derivative that is measured by fluorescence has been used in conjunction with a diffusion scrubber (Fan and Dasgupta, 1994). Enzymatic methods have been used in which formaldehyde dehydrogenase catalyzes the oxidation of HCHO to HCOOH in the presence of -nicotinamide adenine dinucleotide, NAD+, which is reduced to NADH. The latter is measured by fluorescence at 450 nm (Lazrus et al., 1988 Ho and Richards, 1990). 

An enzyme reactor with immobilized 3 -hydroxysteroid dehydrogenase has been successfully used for the analysis of residues of 17 -methyltestosterone in trout by high-performance liquid chromatography (HPLC) (269). Following their separation by reversed-phase chromatography, the major tissue metabolites of 17 -methyltestosterone, namely 5 -androstane-17 -methyl-3, 17 -diol, and 5 -androstane-17 -methyl-3, 17 -diol, were enzymatically modified in the presence of a coreactant, nicotinamide-adenine dinucleotide (NAD), to the corresponding ketone. The position at 3 was enzymatically oxidized, and NADH, the reduced form of NAD, was produced as a coproduct and subjected to fluorescence detection. Reoxidation of NADH to NAD provides the possibility for electrochemical detection. 

FIGURE 13-15 NAD and NADR (a) Nicotinamide adenine dinucleotide, NAD +, and its phosphorylated analog NADP+ undergo reduction to NADH and NADPH, accepting a hydride ion (two electrons and one proton) from an oxidizable substrate. The hydride ion is added to either the front (the A side) or the back (the B side) of the planar nicotinamide ring (seeTable 13-8). (b)The UV absorption spec-... 

Ethanol also may be determined using alcohol dehydrogenase and measuring the change in nicotinamide adenine dinucleotide (NAD+) to the reduced form (NADH) at 340 nm. 

There are two aspects to this process (a) the oxidation and subsequent phosphorylation of glyceraldehyde-3-phosphate (G3P) to 1,3-bisphospho-glycerate (1,3-BGP), and (b) the reduction of nicotinamide adenine dinucleotide (NAD+) to its reduced form designated as NADH. The net reaction can be written as... 

The alcohol dehydrogenases are zinc metalloenzymes which can oxidize a wide variety of alcohols to their corresponding aldehydes or ketones using nicotinamide adenine dinucleotide (NAD+) as coenzyme. These reactions are readily reversible so that carbonyl compounds may be reduced by NADH. 

During conversion of ethanol to acetaldehyde, hydrogen ion is transferred from alcohol to the cofactor nicotinamide adenine dinucleotide (NAD+) to form NADH. As a net result, alcohol oxidation generates an excess of reducing equivalents in the liver, chiefly as NADH. The excess NADH production appears to underlie a number of metabolic disorders that accompany chronic alcoholism. 

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