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Characterization of the purified microsomal FAD-containing monooxygenase from mouse and pig liver
Abstract:
The FAD-containing monooxygenase (FMO) has been purified from both mouse and pig liver microsomes by similar purification procedures. Characterization of the enzyme from these two sources has revealed significant differences in catalytic and immunological properties. The pH optimum of mouse FMO is slightly higher than that of pig FMO (9.2 vs. 8.7) and, while pig FMO is activated 2-fold by n-octylamine, mouse FMO is activated less than 20%. Compounds, including primary, secondary and tertiary amines, sulfides, sulfoxides, thiols, thioureas and mercaptoimidazoles were tested as substrates for both the mouse and pig liver FMO. Km- and Vmax-values were determined for substrates representative of each of these groups. In general, the mouse FMO had higher Km-values for all of the amines and disulfides tested. Mouse FMO had Km-values similar to those of pig FMO for sulfides, mercaptoimidazoles, thioureas, thiobenzamide and cysteamine. Vmax-values for mouse FMO with most substrates was approximately equal, indicating that as with pig FMO, breakdown of the hydroxyflavin is the rate limiting step in the reaction mechanism. Either NADPH or NADH will serve as an electron donor for FMO, however, NADPH is the preferred donor. Pig and mouse FMOs have similar affinity for NADPH (Km = 0.97 and 1.1 microM, respectively) and for NADH (Km = 48 and 73 microM, respectively). An antibody, prepared by immunizing rabbits with purified pig liver FMO, reacts with purified pig liver FMO but not with mouse liver FMO, indicating structural differences between these two enzymes. This antibody inhibited pig FMO activity up to 60%.
Insights
Flavin-containing monooxygenase (FMO) enzymes from mouse and pig liver exhibit distinct catalytic and immunological properties. These differences highlight species-specific variations in FMO function and structure.
Area of Science:
- Biochemistry
- Enzymology
- Comparative Biology
Background:
- Flavin-containing monooxygenases (FMOs) are crucial enzymes involved in the metabolism of xenobiotics and endogenous compounds.
- Understanding species-specific differences in FMOs is vital for drug development and toxicology.
- Previous studies have indicated potential variations in FMO activity across different species.
Purpose of the Study:
- To purify and characterize flavin-containing monooxygenase (FMO) from mouse and pig liver microsomes.
- To compare the catalytic and immunological properties of mouse and pig FMO.
- To identify species-specific differences in FMO enzyme kinetics and substrate specificity.
Main Methods:
- Purification of FMO from mouse and pig liver microsomes.
- Enzyme characterization including pH optimum determination and substrate activation studies.
- Kinetic analysis (Km and Vmax) for various amine and sulfur-containing substrates.
- Immunological characterization using an antibody generated against pig FMO.
Main Results:
- Mouse and pig FMO displayed different pH optima and activation by n-octylamine.
- Kinetic parameters (Km, Vmax) varied for amine and disulfide substrates, with mouse FMO generally showing higher Km values.
- Both enzymes utilized NADPH and NADH, with NADPH being the preferred electron donor, and exhibited similar affinities for these cofactors.
- An antibody against pig FMO did not cross-react with mouse FMO, confirming structural divergence and inhibiting pig FMO activity.
Conclusions:
- Mouse and pig liver FMOs possess distinct catalytic and immunological profiles.
- Significant structural and functional differences exist between mouse and pig FMO, impacting substrate metabolism.
- These findings underscore the importance of considering species-specific enzyme characteristics in pharmacological and toxicological assessments.