ALLOSTERIC INHIBITION OF HUMAN LIVER ALDEHYDE DEHYDROGENASE BY THE ISOFLAVONE PRUNETIN

Isoflavonoid derivatives including prunetin (4',5-dihydroxy-7-methoxyi soflavone) were shown to be potent inhibitors of human aldehyde dehydr ogenases (Keung W-M and Vallee BL, Proc Natl Acad Sci USA 90: 1247-125 1, 1993). The inhibition reaction was reinvestigated using recombinant ly expressed human aldehyde dehydrogenases. The kinetic analyses showe d that prunetin inhibits competitively against both NAD and propionald ehyde with mitochondrial and cytoplasmic enzymes. The K-i value for th e mitochondrial enzyme was much lower than for the cytoplasmic enzyme. A mixed pattern of inhibition was obtained with the mitochondrial enz yme in the presence of Mg2+. Only one mole of prunetin binds per mole of tetrameric mitochondrial enzyme, which remains unaltered in the pre sence of Mg2+. Prunetin did not displace NADH from the enzyme-NADH com plex. Propionaldehyde did not reverse the loss of fluorescence obtaine d due to enzyme-prunetin complex formation, indicating that prunetin m ay not be interacting at the substrate site. The esterase activity of the mitochondrial enzyme was also inhibited by prunetin in a competiti ve manner. The replacement of lysine 192 by glutamine resulted in a mu tant with a 20% k(cat) and a 100-fold increase in the K-m for NAD comp ared with the native enzyme. However, the K-i value of prunetin agains t NAD was similar to that observed with the native enzyme. Prunetin, e ven at a very high concentration, was not an inhibitor of alcohol and malate dehydrogenase. It was concluded that prunetin may act as an all osteric inhibitor of aldehyde dehydrogenase. (C) 1997 Elsevier Science Inc.