Beschreibung:
<jats:p>The main results obtained during the investigation of the peroxidatic reaction between NAD<jats:sup>+</jats:sup> and H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>, catalyzed by horse liver alcohol dehydrogenase, are here presented.</jats:p><jats:p>
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<jats:list-item><jats:p>As previously reported [Favilla, R. & Cavatorta, P. (1975) <jats:italic>FEBS Lett. 50</jats:italic>, 324–329], the reaction can be followed spectrophotometrically around 300 nm. It consists of a biphasic pattern: the faster step reflects the enzyme‐catalyzed formation of a NAD<jats:sup>+</jats:sup> derivative, here called ‘compound I’ (ɛ<jats:sub>300</jats:sub>= 15000 M<jats:sup>−1</jats:sup> cm<jats:sup>−1</jats:sup>), from the reagents NAD<jats:sup>+</jats:sup> and H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>. The slower step is enzyme independent and consists of at least two distinct processes, each of them leading independently to the disappearance of the intermediate compound I. These two processes are competitive and conditions can be selected such that either one or the other occurs preferentially: one of them leads to the formation of NADX (ɛ<jats:sub>300</jats:sub>= 23000 M<jats:sup>−1</jats:sup> cm<jats:sup>−1</jats:sup>), the final stable product of the overall reaction, which can be recovered quantitatively only if catalase is added soon after the end of the faster enzymatic step. The second process of the slow step is due to further reaction of compound I with excess H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> and the final product Y can be recovered almost quantitatively only if catalase is not added after the fast step.</jats:p></jats:list-item>
<jats:list-item><jats:p>A careful re‐examination of the stoichiometry of the reaction unequivocally indicated that one molecule of NAD<jats:sup>+</jats:sup> is converted to compound I per molecule of H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> reacted at the enzyme active site during the fast step.</jats:p></jats:list-item>
<jats:list-item><jats:p>One proton has been found to be released per mole of NAD<jats:sup>+</jats:sup> converted to compound I during the enzymatic fast step. This very significant observation allowed us to deduce the complete stoichiometric balance of the overall reaction.</jats:p></jats:list-item>
<jats:list-item><jats:p>An initial activation of the peroxidatic activity of horse liver alcohol dehydrogenase, followed by a slow progressive decrease of the same activity, was observed when the enzyme was preincubated with 1 mM EDTA. This behaviour has been interpreted as being due to the essential role played by the active‐site zinc ion in catalysing the peroxidatic reaction.</jats:p></jats:list-item>
<jats:list-item><jats:p>The peroxidatic reaction has been found to be catalyzed also by yeast alcohol dehydrogenase, thus suggesting a possible physiological meaning to this general ability of alcohol dehydrogenases.</jats:p></jats:list-item>
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