POSSIBLE ROLE OF LIVER CYTOSOLIC AND MITOCHONDRIAL ALDEHYDE DEHYDROGENASES IN ACETALDEHYDE METABOLISM

TO provide a molecular basis for understanding the possible mechanism of action of antidipsotropic agents in laboratory animals, aldehyde de hydrogenase (ALDH) isozymes were purified and characterized from the l ivers of hamsters and rats and compared with those from humans. The mi tochondrial ALDHs from these species exhibit virtually identical kinet ic properties in the oxidation and hydrolysis reactions. However, the cytosolic ALDH of human origin differs significantly from those of the rodents. Thus, for human ALDH-1, the K-m value for acetaldehyde is 18 0 +/- 10 mu M, whereas those for hamster ALDH-1 and rat ALDH-1 are 12 +/- 3 and 15 +/- 3 mu M, respectively. K-m values determined at pH 9.5 are virtually identical to those measured at pH 7.5. In vitro human A LDH-1 is 10 times less sensitive to disulfiram inhibition than are the hamster and rat cytosolic ALDHs. Competition between acetaldehyde and aromatic aldehydes or naphthaldehydes for the binding and catalytic s ites of ALDHs shows their topography to be complex with more than one binding site. This also follows from data on substrate inhibition and activation, effects of NAD(+) on ALDH-catalyzed hydrolysis of p-nitrop henyl esters, substrate specificity toward aldehydes and p-nitrophenyl esters, and inhibition by disulfiram in relation to oxidation and hyd rolysis catalyzed by the ALDHs. The data further suggest that acetalde hyde cannot be considered as a ''standard'' ALDH substrate for studies aimed at aromatic ALDH substrates, e.g. biogenic aldehydes. Apparentl y, in human liver, only mitochondrial ALDH oxidizes acetaldehyde at ph ysiological concentrations, whereas in hamster or rat liver, both the mitochondrial and cytosolic isozymes will do so.