METHYLGLYOXAL AS SUBSTRATE AND INHIBITOR OF HUMAN ALDEHYDE DEHYDROGENASE - COMPARISON OF KINETIC-PROPERTIES AMONG THE 3 ISOZYMES

Methylglyoxal was demonstrated to be a substrate for the isozymes El, E2 and E3 of human aldehyde dehydrogenase. Pyruvate was the product fr om the oxidation of methylglyoxal by the three isozymes. At pH 7.4 and 25 degrees C, the major and minor components of the E3 isozyme cataly zed the reaction with V-max of 1.1 and 0.8 mu mol NADH min(-1) mg(-1) protein, respectively, compared to 0.067 and 0.060 mu mol NADH min(-1) mg(-1) protein for the El and E2 isozymes, respectively. The E2 isozy me had a K-m for methylglyoxal of 8.6 mu M, the lowest compared to 46 mu M: for El and 586 and 552 mu M for the major and minor components o f the E3 isozyme, respectively. Both components of the E3 isozyme show ed substrate inhibition by methylglyoxal, with K-i values of 2.0 mM fo r the major component and 12 mM for the minor component at pH 9.0. Sub strate inhibition by methylglyoxal was not observed with the El and E2 isozymes. Methylglyoxal strongly inhibited the glycolaldehyde activit y of the El and E2 isozymes. Mixed-type models of inhibition were empl oyed as an approach to calculate the inhibition constants, 44 and 10.6 mu M for El and E2 isozymes, respectively. (C) 1998 Elsevier Science Inc. All rights reserved.