Identification of 4-(N,N-dipropylamino)benzaldehyde as a potent, reversible inhibitor of mouse and human class I aldehyde dehydrogenase.

As the physiologic roles for the different classes of aldehyde dehydrogenase (ALDH) enzymes are elucidated, the identification of specific, reversible inhibitors becomes of great pharmacologic interest. Previous structure-function studies identified dialkylamino substituted benzaldehyde compounds as a novel class of reversible inhibitors of class I ALDH. To examine further structural requirements for inhibition, we tested a series of 4-(N,N-dialkylamino)benzaldehyde analogs as inhibitors of propanal oxidation by mouse liver and human erythrocyte class I ALDH. 4-(N,N-dipropylamino)benzaldehyde (DPAB) was identified as the most potent, reversible inhibitor of propanal oxidation by class I ALDH in spectrophotometric enzyme assays. In kinetic studies, DPAB showed mixed-type inhibition with respect to the aldehyde substrates propanal, phenylacetaldehyde, benzaldehyde, and aldophosphamide. DPAB exhibited uncompetitive inhibition with respect to the cofactor NAD. Inhibition constants (Ki) for DPAB, estimated from Dixon plots, were 10 nM (propanal) and 77 nM (phenylacetaldehyde) for mouse ALDH and 3 nM (propanal) and 70 nM (phenylacetaldehyde) for human ALDH. These Ki values are 100-fold lower than those reported for class I specific inhibitors. At low (< 1 microM) DPAB concentrations, inhibition of propanal and aldophosphamide oxidation was > 75%, whereas inhibition of benzaldehyde (32%) and phenylacetaldehyde (19%) oxidation was reduced markedly. These results indicate that DPAB exhibits potent, reversible inhibition of mouse and human class I ALDH. The degree of inhibition was highly dependent on the structure of the aldehyde substrate.