Functional and structural polymorphism of rabbit microsomal cytochrome P-450 form 3b

Area of Science:

• Biochemistry
• Pharmacology
• Genetics

Background:

• Microsomal cytochrome P-450 enzymes are crucial for drug and xenobiotic metabolism.
• Cytochrome P-450 form 3b (CYP3B) exhibits diverse hydroxylation activities, including progesterone metabolism.
• Inbred rabbit strains can display distinct enzyme profiles compared to outbred populations.

Purpose of the Study:

• To characterize cytochrome P-450 form 3b from the incipient inbred IIIVO/J rabbit strain.
• To compare its catalytic activities with form 3b isolated from New Zealand White (NZW) rabbits.
• To investigate the basis for observed differences in progesterone metabolism.

Main Methods:

• Isolation and purification of cytochrome P-450 form 3b from rabbit liver microsomes.
• Enzymatic assays for progesterone hydroxylation (6 beta- and 16 alpha-) and other substrates.
• Kinetic analysis (Km determination) of enzyme activity.
• Peptide mapping to assess protein structure.

Main Results:

• IIIVO/J rabbit P-450 3b displayed significantly reduced 6 beta-hydroxylase activity towards progesterone compared to NZW rabbit P-450 3b.
• This reduction was attributed to a higher Km for progesterone and the apparent absence of a distinct enzymatic subform.
• Activities for biphenyl 4-hydroxylation and 2-acetylaminofluorene 7-hydroxylation were similar between the two rabbit sources.
• Peptide mapping revealed closely related yet distinguishable structures for the two P-450 3b variants.

Conclusions:

• The selective loss of 6 beta-hydroxylase activity in IIIVO/J rabbit P-450 3b suggests a genetically determined absence of a specific subform.
• This finding highlights the impact of genetic background on cytochrome P-450 expression and function.
• Rabbit strain differences are critical considerations in P-450 research and xenobiotic metabolism studies.