Acetaldehyde to acetic acid

This enzyme, sometimes also called the Schardinger enzyme, occurs in milk. It is capable of " oxidising" acetaldehyde to acetic acid, and also the purine bases xanthine and hypoxanthine to uric acid. The former reaction is not a simple direct oxidation and is assumed to take place as follows. The enzyme activates the hydrated form of the aldehyde so that it readily parts w ith two hydrogen atoms in the presence of a suitable hydrogen acceptor such as methylene-blue the latter being reduced to the colourless leuco-compound. The oxidation of certain substrates will not take place in the absence of such a hydrogen acceptor. 

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

It is possible to carry out such oxidation processes as the conversion of acetaldehyde to acetic acid, or methyl alcohol to formaldehyde in aluminum plants, thus avoiding boiling anhydrous acids. The metal is especially valuable for handling delicate chemicals, which must not acquire metallic taste or color. For these reasons, aluminum has found extensive use in the food, dairy, brewing and fishing industries. 

Ethanol is oxidized by alcohol dehydrogenase (in the presence of nicotinamide adenine dinucleotide [NAD]) or the microsomal ethanol oxidizing system (MEOS) (in the presence of reduced nicotinamide adenine dinucleotide phosphate [NADPH]). Acetaldehyde, the first product in ethanol oxidation, is metabolized to acetic acid by aldehyde dehydrogenase in the presence of NAD. Acetic acid is broken down through the citric acid cycle to carbon dioxide (CO2) and water (H2O). Impairment of the metabolism of acetaldehyde to acetic acid is the major mechanism of action of disulfiram for the treatment of alcoholism. 

FIGURE 4.39 AO-catalyzed oxidation of acetaldehyde to acetic acid. 

The oxidation of acetaldehyde to acetic acid has been studied with NAD-linked ALDH purified from human, rat and Syrian hamster liver (Klyosov et al., 1996). The mitochondrial enzymes from these species have very similar kinetic properties, whereas human cytosolic ALDHl has a value of about 180 pM, compared with 15 pM and 12 pM for rats and hamsters, respectively. Apparently, in human liver, only mitochondrial ALDH oxidizes acetaldehyde at physiological concentrations, whereas both mitochondrial and cytosolic ALDHs of rodents can participate in acetaldehyde metabolism. The rodent cytosolic ALDHs are at least 10 times more sensitive that the human enzyme to inhibition by disulfiram. 

A closely related enzyme is aldehyde oxidase, it also contains two (Mo/2Fe2S-,/FAD) units with a molecular weight of about 300,000. It converts acetaldehyde to acetic acid via electron flow ... 

A simple but effective means of preparing supported metal ion catalysts is to employ ion exchange resins. For example, a cobalt-exchanged H-type resin (Dowex 50) was shown43 to be an effective solid catalyst for the autoxidation of acetaldehyde to acetic acid at 20°C. No leaching of cobalt ions from the resin was observed and the catalyst was used repeatedly (5x) without any significant loss of activity. More recently the use of weak acid resins exchanged with cobalt ions as catalysts for the autoxidation of cyclohexane... 

Liquid-phase processes such as oxidation, hydrogenation, sulfonation, nitration, halogenation, hydrohalogenation, alkylation, sulfonation, polycondensation, polymerization, etc. Examples oxidation of acetaldehyde to acetic acid... 

A subsequent oxidation, catalyzed by aldehyde dehydrogenase (ALDH), converts acetaldehyde to acetic acid, a normal metabolite. 

The drug disulflram (Antabuse) is used in the treatment of chronic alcoholism, producing an acute sensitivity to alcohol. It inhibits the oxidation of acetaldehyde to acetic acid. Metronidazole produces a similar effect, which is why it should never be taken with alcohol. 

Ethanol production is essentially redox neutral however metabolism associated with biomass production generates nett NADH, which is oxidised largely by glycerol production. Other important NADH oxidising reactions with flavour implications are the production of 2,3-butanediol, L-malic acid and succinic add. When glycerol production is stimulated by non-growth associated reactions (i.e. osmotic stress) NAD+ reduction occurs by other reactions including the oxidation of acetaldehyde to acetic acid... 

Certain drugs inhibit non-microsomal metabolic pathways. Metronidazole, like disulfiram, inhibits aldehyde dehydrogenase, the enzyme that normally oxidizes acetaldehyde to acetic acid in the metabolic pathway for ethanol. Allopurinol inhibits xanthine oxidase, the enzyme that catalyses the oxidation of hypoxanthine to xanthine and xanthine to uric acid. Because azathioprine and 6-mercaptopurine are metabolized by xanthine oxidase, the dosage of these drugs (synthetic xanthine analogues), when used concomitantly with... 

Oxidation of acetaldehyde to acetic acid Cascade BC Shah et al [132], Onken [109]... 

Fixed-bed reactors consist of one or more tubes packed with catalyst particles and operated in a. vertical position. The catalyst particles may be a variety of sizes and shapes granular, pelleted, cylinders, spheres, etc. In some instances, particularly with metallic catalysts such as platinum, instead of using single particles, wires of the metal are made into screens. Multiple layers of these screens constitute the catalyst bed. Such screen or gauze catalysts are used in commercial processes for the oxidation of ammonia and the oxidation of acetaldehyde to acetic acid. 

The explosive limits for atmospheric and for reactor conditions. For example, for the oxidation of acetaldehyde to acetic acid, explosive conditions occur outside the operating window. However, for some processes during startup, the reactor passes through the explosive conditions to reach the operating window. 

Antabuse inhibits aldehyde dehydrogenase, the enzyme responsible for oxidizing acetaldehyde to acetic acid, resulting in a buildup of acetaldehyde. Acetaldehyde causes the unpleasant physiological effects of intoxication intense flushing, nausea, dizziness, sweating, throbbing headaches, decreased blood pres-... 

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