title: Characterization of an NADH-dependent haem-degrading system in ox heart mitochondria.
abstract: We report the identification of an NADH-dependent haem-degrading system in ox heart mitochondria. The activity was localized to the mitochondrial inner membrane, specifically associated with complex I (NADH:ubiquinone oxidoreductase). The mitochondrial NADH-dependent haem-degradation activity was highly effective and displayed a rate nearly 60% higher than that of the microsomal activity. The following observations suggested the enzymic nature of the activity: (i) haem degradation by complex I did not proceed upon exposure to elevated temperature and extremes of pH; (ii) it displayed substrate specificity; (iii) it was inhibited by a substrate analogue; and (iv) it showed a cofactor requirement. Moreover, the activity was distinctly different from the ascorbate-mediated haem-degradation activity. Also, complex I differed from the microsomal NADPH:cytochrome c (P-450) reductase inasmuch as the formation of an effective interaction with the microsomal haem oxygenase could not be detected. Addition of purified haem oxygenase to complex I neither influenced the rate of haem degradation nor resulted in the formation of biliverdin IX alpha. In contrast, addition of haem oxygenase to NADPH:cytochrome c (P-450) reductase enhanced the rate of haem degradation by nearly 8-fold, and more than 60% of the degraded haem could be accounted for as biliverdin IX alpha. The haem-degrading activity of complex I appeared to involve the activity of H2O2, as the reaction was inhibited by nearly 90% by catalase, and propentdyopents were detected as reaction products. Intact haemoproteins such as cytochrome c and myoglobin were not effective substrates. However, the haem undecapeptide of cytochrome c was degraded at a rate equal to that observed for haem. Haematohaem was degraded at a rate 50% lower than that observed for haem. It is suggested that the NADH-dependent haem-degradation system may have a biological role in the regulation of the concentration of respiratory haemoproteins and the disposition of the aberrant forms of the mitochondrial haemoproteins.
mesh_terms: D000818:Animals; D002417:Cattle; D006418:Heme / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D046911:Macromolecular Substances; D008929:Mitochondria, Heart / Q000187:drug effects / Q000378:metabolism*; D009243:NAD / Q000494:pharmacology*; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D013347:Subcellular Fractions / Q000378:metabolism; D013367:Submitochondrial Particles / Q000378:metabolism
chemical_list: D046911:Macromolecular Substances; D009243:NAD; D006418:Heme; D009251:NADPH-Ferrihemoprotein Reductase