Zinc-containing ADHs

Different species of higher plants contain ADH. The peanut enzyme has been purified and shown to be a zinc metalloenzyme (488). From tea seeds a mixture of two isozymes of molecular weight 150,000 have been purified and foimd to be unstable in the absence of thiols (439). Partially purified pea ADH is also unstable (490). Other species in which the enzyme is present, apart from those previously reviewed (1), include oats (491), barley (493), and wheat (493). The occurrence of isozymes has been noticed in wheat (494) ... 

There are two main routes for ethanol metabolism in the liver. The first and most important of these begins with the zinc-containing, qdoplasmic, alcohol dehydrogenase (ADH) group of enzymes (especially alcohol dehydrogenase IB). These enzymes are not particularly specific as they can also metabolise longer-chain alcohols produced in the co-oxidation pathway for fatty acids. ADH converts alcohol to acetaldehyde with a Km of 1.6 mmol/L ... 

Alcohol dehydrogenases (ADH EC 1.1.1.1), for which several X-ray structures are available ", catalyze the biological oxidation of primary and secondary alcohols via the formal transfer of a hydride anion to the oxidized form of nicotinamide adenine dinucleotide (NAD ), coupled with the release of a proton. Liver alcohol dehydrogenase (LADH) consists of two similar subunits, each of which contains two zinc sites, but only one site within each subunit is catalytically active. The catalytic zinc is coordinated in a distorted tetrahedral manner to a histidine residue, two cysteine residues and a water molecule. The remaining zinc is coordinated tetrahedrally to four cysteine residues and plays only a structural role . 

The active site of LADH contains an Asp-His-Zn triad (see Figure 11). This pattern is quite common in zinc-enzymes. The aspartate affects the structure, electronic properties and energetics of the active site and thus the catalytic activity. Indeed, Asp49 is conserved in all mammalian ADHs . 

The discard fractions during purification contain zinc. The presence of substantial concentrations of zinc in the absence of activity (Table X, fractions 3, 5) indicates that zinc extraneous to [(ADH)ZnJ is removed. The activity and zinc of fraction 8 probably reflect incomplete removal of crystals from this fraction by centrifugation. 

It has been reported (Leiner and Leiner, 1944) and confirmed (Bowness et al., 1952) that retina contains high concentrations of zinc. Horse liver ADH oxidizes vitamin Ai and reduces retinene, probably being identical with retinene reductase (Bliss, 1949 Theorell and Bonnichsen, 1951a). 

ADH is active as a dimer, with an active site containing zinc present in each subunit. The human has at least seven genes that encode isozymes of ADH, each with a slightly different range of specificities for the alcohols it oxidizes. 

Yeast and mammalian alcohol dehydrogenases differ in substrate specificity and catalytic activity. The yeast enzyme is more specific for acetaldehyde and ethanol, but mammalian enzymes have a broad substrate specificity, and even with primary alcohols maximum activity is not observed with ethanol. Because of the large amount of alcohol dehydrogenase present in human liver and its role in alcohol metabolism in man, human liver alcohol dehydrogenase is of particular interest. It was first purified by Wartburg et and crystallized by Mourad and Woronick. Human liver alcohol dehydrogenase is a dimer with subunit structures analogous to those of horse liver, and each subunit probably contains two zinc atoms. Several different types of human ADH have been isolated. with minor variations in amino acid... 

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