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Reaction of the microsomal heme oxygenase with cobaltic protoporphyrin IX, and extremely poor substrate
Abstract:
A reconstituted heme oxygenase system which was composed of a purified heme oxygenase from pig spleen microsomes and a partially purified NADPH-cytochrome c reductase from pig liver microsomes could not catalyze the conversion of cobaltic protoporphyrin IX (Co-heme) to biliverdin, although Co-heme could bind with the heme oxygenase protein to form a complex. The heme oxygenase system in the microsomes from pig spleen, rat spleen, and rat kidney also failed to oxidize Co-heme to biliverdin. Properties of the complex of Co-heme and heme oxygenase closely resembled those of cobalt myoglobin and cobalt hemoglobin; the Co-heme bound to the heme oxygenase protein did not react with cyanide and azide, the Co-heme moiety was reduced but only slowly with sodium dithionite, and the reduced form of the Co-heme did not appear to bind carbon monoxide. The co-heme bound to heme oxygenase was not reduced with the NADPH-cytochrome c reductase system in air. These findings further support the views that heme oxygenase may have a heme-binding crevice similar to those of myoglobin and hemoglobin and that reduction of heme is the prerequisite for the oxidative degradation of heme in the heme oxygenase reaction.
Insights
Heme oxygenase cannot convert cobaltic protoporphyrin IX (Co-heme) to biliverdin. This suggests heme reduction is necessary for heme oxygenase
Area of Science:
- Biochemistry
- Enzymology
- Heme Metabolism
Background:
- Heme oxygenase (HO) is a key enzyme in heme catabolism.
- The mechanism of HO-mediated heme degradation is not fully understood.
- Cobalt-protoporphyrin IX (Co-heme) is a heme analog used to study HO activity.
Purpose of the Study:
- To investigate the ability of a reconstituted heme oxygenase system to catalyze the conversion of Co-heme to biliverdin.
- To characterize the interaction between Co-heme and heme oxygenase protein.
- To elucidate the role of heme reduction in the heme oxygenase reaction.
Main Methods:
- Reconstitution of the heme oxygenase system using purified pig spleen HO and partially purified pig liver NADPH-cytochrome c reductase.
- Incubation of Co-heme with the reconstituted system and microsomal HO systems from various tissues.
- Characterization of the Co-heme/HO complex properties, including binding, reduction, and reactivity with ligands.
Main Results:
- The reconstituted HO system and microsomal HO systems failed to convert Co-heme to biliverdin.
- Co-heme bound to HO, forming a complex with properties similar to cobalt myoglobin and hemoglobin.
- The Co-heme/HO complex showed limited reactivity with cyanide, azide, and carbon monoxide, and slow reduction by dithionite.
Conclusions:
- Heme oxygenase likely possesses a heme-binding site analogous to myoglobin and hemoglobin.
- Reduction of the heme iron is a prerequisite for the oxidative degradation of heme by heme oxygenase.
- Co-heme is not a suitable substrate for heme oxygenase, highlighting the importance of iron in the catalytic mechanism.