Tissue variant effects of heme inhibitors on the mouse cytochrome c oxidase gene expression and catalytic activity of the enzyme complex

The in vivo effects of heme biosynthesis inhibitors, succinylacetone and Co Cl2 on the cytochrome c oxidase (COX) gene expression and enzyme activity i n different mouse tissues were investigated. Succinylacetone and CoCl2 show ed tissue-specific differences in their ability to modulate heme aa(3) cont ent. A single dose of succinylacetone treatment for 8 h reduced the heme aa (3) content of kidney mitochondria with no effect on the liver. CoCl2 treat ment for 8 h, however, selectively affected the heme aa(3) level in the liv er. Reduced mitochondrial heme aa(3) with both treatments was accompanied b y approximate to 50% reduced, mitochondrial genome-encoded COX I and II mRN As and nuclear genome-encoded COX Vb mRNAs, but no change in COX TV mRNA le vel. Use of isolated mouse liver and brain mitochondrial systems showed a 5 0-80% reduction in mitochondrial transcription and translation rates in hem e-depleted tissues. Blue native gel electrophoresis followed by immunoblot analysis showed that the complex from heme-depleted tissues contained a 30- 50% reduction in levels of subunits I, IV, Vb and near normal levels of sub unit VIc, indicating altered subunit content. Treatment of submitochondrial particles with protein kinase A and ATP resulted in partial dissociation o f COX, suggesting a mechanistic basis for the reduced subunit content of th e complex from heme-depleted tissues. Surprisingly, the enzyme from heme-de pleted tissues showed twofold to fourfold higher turnover rates for cytochr ome c oxidation, suggesting alterations in the kinetic characteristics of t he enzyme following heme reduction. This is probably the first evidence tha t the tissue heme level regulates not only the mammalian COX gene expressio n, but also the catalytic activity of the enzyme, probably by affecting its stability.