['We tested the hypothesis of whether an inhibition of the nitroglycerin (GTN) bioactivating enzyme mitochondrial aldehyde dehydrogenase (ALDH-2) contributes to GTN tolerance in human blood vessels.', 'The hemodynamic effects of GTN are rapidly blunted by the development of tolerance, a phenomenon associated with increased formation of reactive oxygen species (ROS). Recent studies suggest that ROS-induced inhibition of ALDH-2 accounts for tolerance in animal models.', 'Segments of surgically removed arteria mammaria and vena saphena from patients undergoing coronary bypass surgery were used to examine the vascular responsiveness to GTN and the endothelium-dependent vasodilator acetylcholine. The ALDH-2 activity and expression in these segments were assessed by the conversion of a benzaldehyde or its derivative to the benzoic acid metabolite and by Western blotting technique.', 'In contrast to patients not treated with nitrates (n = 36), patients treated with GTN for 48 h (n = 14) before surgery showed tolerance to GTN and endothelial dysfunction in arterial and venous vessels. In vivo GTN tolerance was mimicked in vitro by incubation of nontolerant vessels with the ALDH-2 inhibitor benomyl. In vivo GTN treatment decreased vascular aldehyde dehydrogenase activity compared with nontolerant vessels and decreased the expression of ALDH-2 in arterial tissue. Incubation of control venous vessels with GTN caused a significant attenuation of aldehyde dehydrogenase activity that was reversed by presence of the sulfhydryl group donor dithiothreitol.'] Does oxidative inhibition of the mitochondrial aldehyde dehydrogenase promote nitroglycerin tolerance in human blood vessels?