There are 6 closely related ADHs whose structure and enzymology have been studied; a seventh (ADH6) has not been found as a protein in vivo (Table 1) (Bosron et al., 1993, Hurley et al., 2002, Edenberg and Bosron, 2018). Their pattern of expression in tissues differs (Figure 2). ADH1A, ADH1B, and ADH1C are called class I ADHs; they are more than 90% identical in amino acid sequence, and can hetero-dimerize with each other. These three ADHs have Km for ethanol in the range of 0.013 to 27 mM (Chi et al., 2018, Hurley et al., 2002, Hurley and Edenberg, 2012) (Table 1), and carry out most of the ethanol oxidation in liver. The other ADH enzymes function as homodimers. When ethanol levels are high (e.g., intoxicating), ADH4 could contribute substantially, perhaps 1/3 of the overall metabolism (Lee et al., 2004), although a recent model shows a smaller contribution (Chi et al., 2018). ADH7 is the only ADH enzyme not expressed in liver; it contributes to ethanol oxidation and local generation of acetaldehyde primarily in the stomach and esophagus. ADH5 is
