Expression, Activities, and Kinetic Mechanism of Human Stomach Alcohol Dehydrogenase

Abstract

Alcohol dehydrogenase (ADH) is a dimeric molecule containing two zinc atoms per subunit, one for catalytic and the other for structural functions (Eklund and Branden, 1987; Hurley et al., 1994). Human ADH constitutes a complex family. Seven genes have thus far been identified. ADH ₁ through ADH ₅ and ADH ₇ encode α, β, γ, π, χ and μ (or denoted σ) polypeptides, respectively (Smith, 1986; Yoshida et al., 1991; Jornvall and Hoog, 1995). The ADH ₆-encoding subunit has not yet been designated a Greek letter (Yasunami et al., 1991). Both ADH ₂ and ADH ₃ exhibit polymorphism. The allelic variants, ADH ₂ ¹, ADH ₂ ², ADH ₂ ³, and ADH ₃ ¹, ADH ₃ ² code for β₁, β₂, β₃, and γ₁, γ₂, respectively. These polymorphic genes display different allelic distribution among racial populations (Agarwal and Goedde, 1992; Bosron et al., 1993; Yin, 1994). Based on the kinetic and structural features, human ADH can be grouped into five classes (Vallee and Bazzone, 1983; Jornvall and Hoog, 1995). Class I ADH comprises homo- and heterodimers with α, β or γ subunits. Class II, III, IV enzymes are ππ, χχ, and μμ, respectively. The intraclass and interclass sequence homologies at the amino acid/nucleotide level are approximately 90% and 60%, respectively (Yoshida et al., 1991; Jornvall and Hoog, 1995). Kinetic properties of these five class ADHs vary strikingly (Vallee and Bazzone, 1983; Bosron et al., 1993; Yin, 1994). K _m values for ethanol for class I isoenzymes are generally lower than 5 mM, suggesting that they are active after low alcohol consumption. Class II, IV and V enzymes exhibit K _m about 30 mM, hence active at moderate to high ethanol concentrations. Class III enzyme does not seem to significantly contribute to ethanol metabolism due to its extremely high K _m, > 3 M.