MNSOD PROTEIN-CONTENT CHANGES IN HYPOXIC HYPOPERFUSED LUNG-TISSUE/

Previous studies using an in vivo rabbit model in which lung tissue hy poxia/hypoperfusion was created by unilateral lung collapse for 7 days demonstrated a decrease in MnSOD activity in previously hypoxic/hypop erfused lungs. In the present study, we determined whether tissue hypo xia/hypoperfusion decreased MnSOD protein concentration or mRNA expres sion in the lung as well, changes that would suggest pretranslational regulation of enzyme activity. Expression of MnSOD may be critical in determining the degree of tissue injury during re-oxygenation because the mitochondrial electron transport system produces reactive oxygen s pecies (ROS) both during hypoxia and re-oxygenation. We purified MnSOD protein from rabbit livers to a specific activity of similar to 3,500 U/mg protein and found the amino terminal sequence nearly identical t o those of the rat and human MnSOD proteins. Lung MnSOD protein conten t was quantitated by immunoassay, and MnSOD mRNA content was determine d by slot blotting. Results from five control and six experimental rab bits, the right lungs of which had been hypoxic/hypoperfused because o f collapse for 7 days, demonstrated a 32% decrease (P < 0.03) in MnSOD protein content (42 +/- 8 mu g/mg DNA in hypoxic lungs compared with 61 +/- 3 mu g/mg DNA in contralateral lungs) that was not due to decre ased numbers of mitochondria. Lung succinate dehydrogenase activity, a mitochondrial marker, did not change in hypoxic/hypoperfused lungs. T he mRNA for MnSOD did not change relative to B-actin mRNA in lungs tha t had been hypoxic and hypoperfused for 7 days. Lung tissue hypoxia/hy poperfusion, resulting from unilateral lung collapse, was associated w ith parallel decreases in both MnSOD activity and protein content. MnS OD activity in hypoxic lung tissue appears related to changes in enzym e protein content rather than mRNA, suggesting that in this model of l ung tissue hypoxia MnSOD is regulated at the post-transcriptional leve l.