Myocardial ischaemia inhibits mitochondrial metabolism of 4-hydroxy-trans-2-nonenal

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Abstract
Myocardial ischaemia is associated with the generation of lipid peroxidation products such as HNE (4-hydroxy-trans-2-nonenal); however, the processes that predispose the ischaemic heart to toxicity by HNE and related species are not well understood. In the present study, we examined HNE metabolism in isolated aerobic and ischaemic rat hearts. In aerobic hearts, the reagent [3H]HNE was glutathiolated, oxidized to [3H]4-hydroxynonenoic acid, and reduced to [3H]1,4-dihydroxynonene. In ischaemic hearts, [3H]4-hydroxynonenoic acid formation was inhibited and higher levels of [3H]1,4-dihydroxynonene and [3H]GS-HNE (glutathione conjugate of HNE) were generated. Metabolism of [3H]HNE to [3H]4-hydroxynonenoic acid was restored upon reperfusion. Reperfused hearts were more efficient at metabolizing HNE than non-ischaemic hearts. Ischaemia increased the myocardial levels of endogenous HNE and 1,4-dihydroxynonene, but not 4-hydroxynonenoic acid. Isolated cardiac mitochondria metabolized [3H]HNE primarily to [3H]4-hydroxynonenoic acid and minimally to [3H]1,4-dihydroxynonene and [3H]GS-HNE. Moreover, [3H]4-hydroxynonenoic acid was extruded from mitochondria, whereas other [3H]HNE metabolites were retained in the matrix. Mitochondria isolated from ischaemic hearts were found to contain 2-fold higher levels of protein-bound HNE than the cytosol, as well as increased [3H]GS-HNE and [3H]1,4-dihydroxynonene, but not [3H]4-hydroxynonenoic acid. Mitochondrial HNE oxidation was inhibited at an NAD+/NADH ratio of 0.4 (equivalent to the ischaemic heart) and restored at an NAD+/NADH ratio of 8.6 (equivalent to the reperfused heart). These results suggest that HNE metabolism is inhibited during myocardial ischaemia owing to NAD+ depletion. This decrease in mitochondrial metabolism of lipid peroxidation products and the inability of the mitochondria to extrude HNE metabolites could contribute to myocardial ischaemia/reperfusion injury.