Review ArticleBioactivation of Nitroglycerin by the Mitochondrial Aldehyde Dehydrogenase
Section snippets
Mitochondrial Aldehyde Dehydrogenase-Catalyzed GTN Biotransformation. How Was It Discovered?
Several candidate enzymes, including glutathionine-S-transferases (GSTs), cytochrome P450 reductase/cytochrome P450, and xanthine oxidoreductase, have been proposed to catalyze GTN biotransformation. However, their possible roles in generating NO bioactivity from GTN remain controversial (Chen et al. 2002). For example, although GSTs can metabolize GTN, the GST inhibitor sulfobromophthalein has no effect on GTN-induced increases in cyclic guanosine monophosphate (cGMP). Moreover, a homozygous
Characterization of mtALDH-Catalyzed GTN Biotransformation
Mitochondrial aldehyde dehydrogenase purified from bovine liver catalyzes predominantly 1,2-GDN formation (1,2-GDN/1,3-GDN, ∼8:1) from low levels of GTN (1 μM). In the presence of NAD+, the 1,2-GDN/1,3-GDN ratio increases to more than 20:1 and the rate of GTN metabolism increases ∼10-fold. The optimum pH of 9.0 is about the same as that of aldehyde dehydrogenase activity (Chen et al. 2002). In a subsequent analysis of overexpressed human mtALDH, Kollau et al. (2005) reported that NAD+ addition
Inhibition of mtALDH-Mediated GTN Reductase Activity by ALDH Inhibitors
Our finding that mtALDH functions as a GTN reductase provides a new explanation for the inhibitory effect of GTN on aldehyde dehydrogenase activity (alcohol-GTN drug interaction) and also indicates that ALDH substrates and inhibitors should suppress GTN reductase activity. We examined the effects of the substrate acetaldehyde and of different classes of ALDH inhibitors on GTN bioactivation.
1,2-GDN formation by purified bovine liver mtALDH was inhibited by the classic substrate analog ALDH
Mitochondrial Nitrite Reductase Activity
The results discussed above suggest that nitrite generated from GTN by mtALDH within the mitochondria (Eq. (1)) is further reduced or converted to NO bioactivity and that this NO bioactivity is exported from mitochondria to the cytosol, where it activates sGC (sGC is not detected in purified mitochondria by immunoblotting; Chen and Stamler, unpublished observation). To demonstrate directly the generation and export of NO bioactivity by mitochondria, we used a reporter assay to test whether
Biochemistry
As expected, GTN biotransformation to 1,2-GDN is essentially eliminated in the mitochondria of mtALDH−/− mice (>90% decrease compared to wild-type mitochondria), whereas 1,3-GDN production is not affected. Accordingly, mtALDH−/− mitochondria almost completely lose the ability to generate cGMP from 1 μM GTN in the RFL-6 cell reporter assay (Chen et al. 2005).
In intact aorta, biotransformation of GTN to 1,2-GDN was eliminated at low GTN concentrations (<1 μM) and significantly attenuated at 1–10
Clinical Implications
Nitrovasodilator therapy has not provided survival benefits. It is possible that mitochondrial damage and impaired respiration (Needleman and Hunter 1966) that are consequent upon the localization of GTN biotransformation to mitochondria may provide at least a partial explanation for the failure of long-term GTN therapy to improve mortality. Glyceryl trinitrate-related adverse atherogenic effects and increased morbidity also merit further investigation (Nakamura et al., 1999, Gori and Parker,
Summary and Perspective
Recent biochemical, pharmacologic, and genetics studies have demonstrated clearly that mtALDH is the essential enzyme in the biotransformation of clinical levels of GTN, and that it plays a significant role in the development of GTN tolerance. These studies, and in particular analyses in mtALDH knockout mice, also indicate that there is an additional mechanism for GTN biotransformation (high Km pathway), but that this pathway probably plays no role in the development of mechanism-based GTN
Acknowledgments
We thank Dr. Douglas T. Hess for valuable discussions and critical reading and editing of the manuscript.
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