Multiple forms and gene loci of human alcohol dehydrogenase (ADH EC: 1.2.1.3) and aldehyde dehydrogenase (ALDH, EC: 1.2.1.3) in the major pathway of alcohol metabolism have been found and characterized in the last two decades. With the coenzyme NAD, these enzymes catalyze the reversible conversion of organic alcohols to ketones or aldehydes, and aldehyde to acetic acid. The ADH genes are mapped to chromosome 4p21-25, but the ALDH genes are localized at different chromosomes. The cytochrome P450 2E1 (CYP2E1) gene, which is mapped to chromosome 10q24.3-qter contributes also the conversion of ethanol to acetaldehyde. Genetic polymorphisms have been reported in these alcohol metabolizing enzymes. The metabolisms of alcohol and acetaldehyde in liver and blood after drinking alcohol are thought to be influenced by the interactive action of these enzymes. Amongst the five major classes of the ADH subunits (alpha, beta, gamma, pi, chi, sigma), beta and gamma subunits show genetic polymorphisms. Recently a new nomenclature for ALDH genes has been recommend based on divergent evolution and chromosomal mapping. Two major isoforms designated as cytosolic ALDH1 and mitochondrial ALDH2 can be distinguished by their electrophoretic and kinetic properties as well as by their subcellular localization. Mitochondrial ALDH2 is a major enzyme in the oxidation of acetaldehyde derived from ethanol metabolism. The catalytic deficiency of ALDH2 isozyme is responsible for flushing and other vasomotor symptoms caused by higher acetaldehyde levels after alcohol intake. So far, frequencies of the two alleles of ALDH2 in Mongoloid have been reported in the different population groups. The catalytic deficiency of ALDH2 is caused by a structural point mutation at amino acid position 487, where a substitution of Glu to Lys resulting from a transition of G (C) to A (T) at 1510 nucleotide from the initiation codon has occurred. Individuals deficient in ALDH2 activity refrain from excessive drinking of alcohol due to the aversive reactions, leading to protection against alcoholism. Prevalence of the ALDH2*1 allele is associated with alcoholism, and subsequent studies have confirmed the allelic association with alcoholism in different ethnic groups. The effects of polymorphisms of ADH2 and CYP2E1 remained controversial, even in the same ethnic group. Investigation of mutations for the transacting cis-element in promoter region of the ALDH2 gene will provide important information with respect to regulation of this gene. Transfection assays using the first 600 bp of the upstream nucleotide sequences indicated that a region from -75 to -120 was necessary for the ALDH2 gene expression, and especially NF-Y/CP1 binding site from -92 to -96 (CCAAT box) is important in the expression of the gene. A novel polymorphism due to the nucleotide replacement at -357 G to A was found in all the population groups. Alcoholism is thought to be a multifactorial disease with complex mode of inheritance in addition to psychological and social factors, and many studies of family, adoption and twins concerning alcoholism have revealed that hereditary factor is an important determinant for developing alcoholism. Genetic association studies have contributed to the identification of a number of genetic risk factors for the chronic diseases influenced by genetic disorders and environmental factors.
