Oxidation of Ethanol by NAD

The reverse reaction also occurs in living systems NADH reduces acetaldehyde to ethanol in the presence of alcohol dehydrogenase. In this process, NADH serves as a hydride donor and is oxidized to NAD while acetaldehyde is reduced. 

The NAD -NADH coenzyme system is involved in a large number of biological oxidation-reductions. Another reaction similar to the ethanol-acetaldehyde conversion is the oxidation of lactic acid to pyruvic acid by NAD and the enzyme lactic acid dehydrogenase  

We shall encounter other biological processes in which the NAD version plays a prominent role in biological oxidation-reduction. 

A reaction characteristic of vicinal diols is their oxidative cleavage on treatment with periodic acid (HIO4). The carbon-carbon bond of the vicinal diol unit is broken and two carbonyl groups result. Periodic acid is reduced to iodic acid (HIO3). 

Vicinal Periodic Aldehyde Aldehyde Iodic Water 

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Describe how one could determine the rate of the alcohol dehydrogenase catalyzed oxidation of ethanol by NAD (Section 20.11). 

Dimethyl Sulfoxide Oxidation of an Alcohol (Swern Oxidation) 666 Oxidation of Ethanol by NAD 669... 

Yeast alcohol dehydrogenase catalyzes the oxidation of ethanol by NAD according to the reaction... 

The liver alcohol dehydrogenase mentioned in the preceding section has the same pro-R stereospecificity for NAD and ethanol as yeast alcohol dehydrogenase. Furthermore, the oxidation of ethanol by a microsomal oxidizing system, or by catalase and H2O2, likewise proceeds with pro-R stereospecificity for the ethanol77>. The catalase-H2C>2 system is so very different, however, from the pyridine nucleotide dehydrogenase, that one wonders whether the similarity in stereospecificity for ethanol is fortuitous. 

Later, an enzymatic method based on oxidation of ethanol by alcohol dehydrogenase (ADH) and nicotinamide adenine dinucleotide (NAD) followed by spectrophotometric analysis was reported. Current methods for detecting ethanol in body fluids are predominantly based on physicochemical techniques. A gas-liquid chromatographic (GLC) method is the most widespread because it is easy, rapid, and has high specificity and accuracy. Analytical methods used to determine alcohol in body fluids are siunma-rized in Table 1. 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

Both ADH and ALDH use NAD+ as cofactor in the oxidation of ethanol to acetaldehyde. The rate of alcohol metabolism is determined not only by the amount of ADH and ALDH2 enzyme in tissue and by their functional characteristics, but also by the concentrations of the cofactors NAD+ and NADH and of ethanol and acetaldehyde in the cellular compartments (i.e., cytosol and mitochondria). Environmental influences on elimination rate can occur through changes in the redox ratio of NAD+/NADH and through changes in hepatic blood flow. The equilib-... 

The NAD /NADH concentration ratio in the cytosol of the liver is maintained at a value of about 1000 but oxidation of ethanol can lower this ratio by at least tenfold. Many dehydrogenase reactions are close to equilibrium so that, for those that react with NAD /NADH, the concentrations of all the other substrates and products will be affected by a change in the NAD /NADH concentration ratio. Hence, a decrease in the NAD /NADH concentration ratio will lower the concentration of the oxidised reactant and raise that of the reduced reactant of aU the dehydrogenation reactions in the cytosol ... 

NAD is one of Nature s most important oxidizing agents it can be considered as a biological equivalent of the chromium(VI) ion. NAD is shorthand for nicotinamide adenine dinucleotide it is a co-enzyme, which together with an enzyme is essential for several life-sustaining processes (Box 2.2). On reduction it forms the corresponding 1,4-dihydropyridine, NADH, The oxidation of ethanol to acetaldehyde (ethanal) is effected by the enzyme alcohol dehydrogenase and mediated by NAD (Scheme 2.31). 

Metabolism of ethanol by alcohol dehydrogenase and the microsomal ethanol-oxidizing system (MEOS). Alcohol dehydrogenase and aldehyde dehydrogenase are inhibited by fomepizole and disulfiram, respectively. NAD +, nicotinamide adenine dinucleotide NADPH, nicotinamide adenine dinucleotide phosphate. 

Alcohol dehydrogenase is present in many organisms that metabolize ethanol, including humans. In human liver it catalyzes the oxidation of ethanol, either ingested or produced by intestinal microorganisms, with the concomitant reduction of NAD+ to NADH. 

We shall start the discussion with a classical experiment related to the stereochemistry of oxidation of ethanol and reduction of acetaldehyde mediated by the enzyme yeast alcohol dehydrogenase in the presence of the oxidized (NAD+) and reduced (NADH) forms, respectively, of the coenzyme nicotinamide adenine dinucleotide (Fig. 54). The stereochemically interesting feature of this reaction stems from the fact that the methylene hydrogens in CH3CH2OH and the faces of the carbonyl in CH3CH = 0 are enantiotopic. The question thus arises which of the CH2-hydrogens... 

Fig. 55. Oxidation of ethanol and reduction of acetaldehyde by NAD+/NADH in the presence of yeast alcohol dehydrogenase (YADH)...

Answer The first step in the synthesis of glucose from lactate in the liver is oxidation of the lactate to pyruvate like the oxidation of ethanol to acetaldehyde, this requires NAD+. Consumption of alcohol forces a competition for NAD+ between ethanol metabolism and gluconeogenesis, reducing the conversion of lactate to glucose and resulting in hypoglycemia. The problem is compounded by strenuous exercise and lack of food because at these times the level of blood glucose is already low. 

The 3-carbamidopyridinium ring is the chemically active portion of the enzymatic cofactors, NAD and NADP (nicotinamide adenine dinucleotide and its phosphate). A typical reaction involving NAD is the stereospecific (with respect to both cofactor and substrate) oxidation of ethanol to acetaldehyde catalyzed by the enzyme, alcohol dehydrogenase (Eq. 33). 

Figure 31. The biocatalyzed conversion of pyruvic acid (21) to lactic acid (22) by an LDH AlcDH system and the use of the NAD+/NADH cofactor. The transformation proceeds with the oxidation of ethanol, and is reversed in a high concentration of lactic acid.

The photolysis of organic materials has also been accomplished by the oxidative quenching of photosensitizers. For example, oxidative quenching of Ru(bpy)3 by MV + in the presence of NADH leads to MV+ and NAD+ through the sequence of reactions outlined in Eqs. 15-17. The reduced photoproduct, MV, mediates H2 evolution in the presence of Pt colloid (Eq. 18), while NAD mediates the two-electron oxidation of ethanol (Eq. 19) or lactic acid (Eq. 20) biocatalyzed by the respective enzyme [225]. 

Conventional laboratory oxidation of the monodeuteriated ethanol 24 gives ethanal (acetaldehyde), which contains 50% of the deuterium present in 24 (Scheme 8.1). However, enzymes are chiral, and enzymic oxidation (by yeast alcohol dehydrogenase and NAD+) removes exclusively in the oxidation of ethanol to ethanal HR is labelled as D in Scheme 8.2. Clearly, the molecule of ethanol is presented to the chiral enzyme so as to form a unique diastereoisomeric complex in which only the proton can be removed in the oxidative elimination. 

The belief that alcoholics are more susceptible to the toxicity of 2,4-DNP during occupational exposure (Perkins 1919) may indicate an interaction with ethanol (and possibly other alcohols) or it may simply be a function of the compromised physiological state of alcoholics. 2,4-DNP appears to markedly increase the rate of ethanol metabolism in rat liver slices by 100-160% (Videla and Israel 1970) and in rats in vivo by 20-30% (Israel et al. 1970). Because 2,4-DNP uncouples mitochondrial electron transport from oxidative phosphorylation, the oxidation of NADH to NAD is accelerated in the mitochondria. Reoxidation of NADH rather than the activity of alcohol dehydrogenase is the rate-limiting step in the metabolism of ethanol, and, therefore, the metabolic effect of 2,4-DNP enhances the clearance of ethanol (Eriksson et al. 1974). Because 2,4-DNP is known to augment the rate of respiration and perspiration, 2.7-8.2% of the initial dose of ethanol was also eliminated by expiration and cutaneous evaporation in the rat (Israel et al. 1970). 

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