An ethanol‐inducible MDR ethanol dehydrogenase/acetaldehyde reductase in <i>Escherichia coli</i>

<jats:p>An ethanol‐active medium‐chain dehydrogenase/reductase (MDR) alcohol dehydrogenase was isolated and characterized from <jats:italic>Escherichia coli</jats:italic>. It is distinct from the fermentative alcohol dehydrogenase and the class III MDR alcohol dehydrogenase, both already known in <jats:italic>E. coli</jats:italic>. Instead, it is reminiscent of the MDR liver enzyme forms found in vertebrates and has a <jats:italic>K</jats:italic><jats:sub>m</jats:sub> for ethanol of 0.7 m<jats:sc>m</jats:sc>, similar to that of the class I enzyme in humans, however, it has a very high <jats:italic>k</jats:italic><jats:sub>cat</jats:sub>, 4050 min<jats:sup>−1</jats:sup>. It is also inhibited by pyrazole (<jats:italic>K</jats:italic><jats:sub>i</jats:sub> = 0.2 µ<jats:sc>m</jats:sc>) and 4‐methylpyrazole (<jats:italic>K</jats:italic><jats:sub>i</jats:sub>= 44 µ<jats:sc>m</jats:sc>), but in a ratio that is the inverse of the inhibition of the human enzyme. The enzyme is even more efficient in the reverse direction of acetaldehyde reduction (<jats:italic>K</jats:italic><jats:sub>m</jats:sub> = 30 µ<jats:sc>m</jats:sc> and <jats:italic>k</jats:italic><jats:sub>cat</jats:sub> = 9800 min<jats:sup>−1</jats:sup>), suggesting a physiological function like that seen for the fermentative non‐MDR alcohol dehydrogenase. Growth parameters in complex media with and without ethanol show no difference. The structure corresponds to one of 12 new alcohol dehydrogenase homologs present as ORFs in the <jats:italic>E. coli</jats:italic> genome. Together with the previously known <jats:italic>E. coli</jats:italic> MDR forms (class III alcohol dehydrogenase, threonine dehydrogenase, ζ‐crystallin, galactitol‐1‐phosphate dehydrogenase, sensor protein rspB) there is now known to be a minimum of 17 MDR enzymes coded for by the <jats:italic>E. coli</jats:italic> genome. The presence of this bacterial MDR ethanol dehydrogenase, with a structure compatible with an origin separate from that of yeast, plant and animal ethanol‐active MDR forms, supports the view of repeated duplicatory origins of alcohol dehydrogenases and of functional convergence to ethanol/acetaldehyde activity. Furthermore, this enzyme is ethanol inducible in at least one <jats:italic>E. coli</jats:italic> strain, K12 TG1, with apparently maximal induction at an enthanol concentration of ≈17 m<jats:sc>m</jats:sc>. Although present in several strains under different conditions, inducibility may constitute an explanation for the fairly late characterization of this <jats:italic>E. coli</jats:italic> gene product.</jats:p>