The identification of the heat-stable microsomal protein required for methoxyflurane metabolism as cytochrome b5

Area of Science:

• Biochemistry
• Pharmacology
• Enzymology

Background:

• Methoxyflurane anesthetic metabolism by cytochrome P-450LM2 is dependent on a heat-stable microsomal protein.
• The precise identity and function of this protein factor were previously uncharacterized.

Purpose of the Study:

• To identify the heat-stable microsomal protein involved in methoxyflurane metabolism.
• To elucidate the role of this protein in the cytochrome P-450LM2-mediated metabolic pathway.

Main Methods:

• Treatment of the heat-stable protein with diethylpyrocarbonate to assess its effect on methoxyflurane metabolism.
• Co-elution studies during purification to identify the protein factor.
• Reconstitution experiments using apocytochrome b5 and heme.
• Computer graphics analysis of the cytochrome b5 molecule.

Main Results:

• Diethylpyrocarbonate treatment caused a dose-dependent loss of the protein's ability to facilitate methoxyflurane metabolism.
• The heat-stable factor was identified as cytochrome b5 based on co-elution during purification.
• Reconstituted cytochrome b5 restored metabolic activity, confirming its identity.
• Maximal methoxyflurane metabolism rates were achieved with a 1:1:1 ratio of cytochrome P-450LM2, reductase, and cytochrome b5.

Conclusions:

• Cytochrome b5 is an obligatory heat-stable protein factor required for methoxyflurane metabolism by cytochrome P-450LM2.
• Cytochrome b5 likely functions as an electron donor to cytochrome P-450LM2 during the O-demethylation of methoxyflurane.