Allosteric inhibition of human liver aldehyde dehydrogenase by the isoflavone prunetin

Isoflavonoid derivatives including prunetin (4',5-dihydroxy-7-methoxyisoflavone) were shown to be potent inhibitors of human aldehyde dehydrogenases (Keung W-M and Vallee BL, Proc Natl Acad Sci USA 90: 1247-1251, 1993). The inhibition reaction was reinvestigated using recombinantly expressed human aldehyde dehydrogenases. The kinetic analyses showed that prunetin inhibits competitively against both NAD and propionaldehyde with the mitochondrial and cytoplasmic enzymes. The Ki value for the mitochondrial Enzyme was much lower than for the cytoplasmicenzyme. A mixed pattern of inhibition was obtaiend with the mitochondrial Enzyme in the presence of Mg2+. Only one mole of prunetin binds per mole of tetrameric mitochondrial Enzyme, which remains unaltered in the presence of Mg2+. Prunetin did not displace NADH from the enzyme-NADH complex. Propionaldehyde did not reverse the loss of fluorescence obtained due to enzyme-prunetin complex formation, indicating that prunetin may not be interacting at the substrate site. The esterase activity of the mitochondrial Enzyme was also inhibited by prunetin in a competitive manner. The replacement of lysine 192 by glutamine resulted in a mutant with a 20% kcat and a 100-fold increase in the Km for NAI) compared with the native Enzyme. However, the Ki value of prunetin against NAD was similar to that observed with the native Enzyme. Prunetin, even at a very high concentration, was not an inhibitor of alcohol and malate dehydrogenase. It was concluded that prunetin may act as an allosteric inhibitor of aldehyde dehydrogenase.