**8.1 Alcohol dehydrogenase (ADH)**

The major pathway of oxidative metabolism of ethanol in the liver involves ADH present in the cytosol. During oxidation an electron carrier nicotinamide adenine

dinucleotide (NAD) is reduced to NADH providing reduced cytosolic environment in the liver cells. Because of byproduct like highly reactive and toxic free radicals and acetaldehyde, the liver cells become more vulnerable to damage [17]. Control of ADH activity is complex and involves: (a) dissociation of the product NADH which is a rate limiting step and (b) product inhibition by NADH and acetaldehyde [14].

ADH comprises of a complex family. In humans on the basis of kinetic and structural properties, enzyme ADH has been categorized into five classes. Most of them are found in liver, stomach and lungs except ADH5 found in most tissues [17]. If alcohol concentration is high, it is eliminated at a higher rate because of high activity of enzymes viz. class II ADH encoded by *ADH4* gene and β3-ADH encoded by *ADH1B\*3* gene [43].

Aldehyde dehydrogenase (ALDH2) rapidly metabolizes acetaldehyde (produced by alcohol oxidation) to acetate and NADH. NADH is then oxidized through electron transport chain, or respiratory chain enzymes [17]. Among various isozymes of ALDH, only the cytosolic ALDH1 and the mitochondrial ALDH2 can metabolize acetaldehyde [44, 45].
