Aldehyde dehydrogenase 1A1 (ALDH1A1) is a member of the
aldehyde dehydrogenase superfamily that oxidizes
aldehydes to their corresponding
acids, reactions that are coupled to the reduction of NAD+ to
NADH. We report here that ALDH1A1 can also use
glutathione (GSH) and
dihydrolipoic acid (DHLA) as electron donors to reduce NAD+ to
NADH. The GSH/DHLA-dependent
NAD+-reduction activity of ALDH1A1 is not affected by the
aldehyde dehydrogenase inhibitor or by mutation of the residues in its
aldehyde-binding pocket. It is thus a distinct biochemical reaction from the classic
aldehyde-dehydrogenase activity catalyzed by ALDH1A1. We also found that the ectopic expression of ALDH1A1 decreased the intracellular
NAD+/
NADH ratio, while knockout of ALDH1A1 increased the
NAD+/
NADH ratio. Simultaneous knockout of ALDH1A1 and its
isozyme ALDH3A1 in
lung cancer cell line NCI-H460 inhibited
tumor growth in a xenograft model. Moreover, the ALDH1A1 mutants that retained their GSH/DHLA-dependent NAD+ reduction activity but lost their
aldehyde-dehydrogenase activity were able to decrease the
NAD+/
NADH ratio and to rescue the impaired growth of ALDH1A1/3A1 double knockout
tumor cells. Collectively, these results suggest that this newly characterized GSH/DHLA-dependent
NAD+-reduction activity of ALDH1A1 can decrease cellular
NAD+/
NADH ratio and promote
tumor growth.