Novel NADP-linked alcohol–aldehyde/ketone oxidoreductase in thermophilic ethanologenic bacteria

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Abstract
An NADP-specific alcohol--aldehyde/ketone oxidoreductase was detected in cell extracts of Thermoanaerobium brockii and Clostridium thermohydrosulfuricum, but not in Thermobacteroides acetoethylicus or Clostridium thermocellum. The enzyme was purified from Ta. brockii by differential procedures that included heat treatment and an affinity-chromatography step on Blue Dextran--Sepharose. The 44-fold-purified enzyme displayed one band (mol.wt. approx. 40000) after sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The enzyme had a broad substrate specificity that included linear and branched primary alcohols, linear and cyclic secondary alcohols, linear and cyclic ketones, and acetaldehyde. The NADP-specific alcohol--aldehyde/ketone oxidoreductase was considerably more active towards secondary alcohols than towards other substrates. The enzyme had remarkable stability to heating at 86 degrees C for 70 min, but was rapidly denatured on boiling. Secondary-alcohol dehydrogenase activity displayed a noticeable inflexion point at 50 degrees C in Arrhenius plots and a high Q10 value (greater than 2.0). The enzyme was inactivated by the thiol-blocking reagent p-chloromercuribenzoate, but was not significantly inhibited by common metal-ion-binding agents. The NADP-linked alcohol--aldehyde/ketone oxidoreductase of Ta. brockii appears to have properties distinct from those of previously described primary- and secondary-alcohol dehydrogenases.