The mechanism of the enzymatic oxidation of aldehydes.

There are at least two distinct mechanisms by which one may envision the enzymatic oxidation of aldehydes. The first of these involves the prior hydration of the aldehyde to a gem diol followed by the removal of a pair of hydrogen atoms or of a hydride ion plus a proton. The second mechanism proposes the removal of a hydride ion or its equivalent from the aldehyde and its ultimate replacement by a hydroxide ion from the medium. It has generally been assumed (1) that the first of these mechanisms is the prevalent one although hydride removal from the aldehyde was postulated by Deitrich, Hellerman, and Wein (2). In a recent report Pocker and Meany (3) showed that the hydration of acetaldehyde was conveniently slow under certain conditions and that the rate of hydration was general acid-base catalyzed. They also found that carbonic anhydrase was capable of catalyzing the hydration of acetaldehyde and that the equilibrium constant for the hydration reaction at 00 was 2.3. This suggested a means of unequivocally deciding whether the hydration of aldehydes is a prerequisite to their enzymatic oxidation to carboxylic acids. Thus, under conditions of substrate limitation, adding an anhydrous solution of the aldehyde as the last component of an aqueous reaction mixture would confer an initial rate advantage if the aldehyde, per se, were the true substrate. This rate advantage would vanish within a few halflives of the hydration reaction. On the other hand, if the hydrate were the true substrate, the rate would be accelerated by allowing the aldehyde to equilibrate with the aqueous reaction mixture prior to initiating the reaction. The test system selected for study was the oxidation of acetaldehyde by milk xanthine oxidase and the results indicate that the hydration of acetaldehyde does not precede its oxidation in this system.