There are three main superfamilies of ADH that arose independently throughout evolution. Although they are all dimeric metalloproteins, Type I ADH is a “medium-chain” family of dehydrogenases and is the most common in vertebrates; Type II is a “short-chain” family of dehydrogenases most common in insects (11), and Type III is more commonly known in microorganisms (20). In vertebrates, most alcohol is oxidized by Type I ADHs, and, as will be discussed in upcoming sections, 5 classes of Type I ADH isoenzymes are found in humans and non-human primates, each of which has different catalytic properties and patterns of tissue-specific expression (Table 1). Therefore, these different ADH families may have responded differently to environmental selective pressures.
