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Effect of N-ethylmaleimide on beef and rat liver vitamin K1 epoxide reductase
1Department of Chemistry, Northwestern University, Evanston, Illinois 60208.
Abstract:
There is little difference in the extent of inactivation of beef liver microsomal vitamin K1 epoxide reductase by N-ethylmaleimide (NEM) whether or not the microsomes are pre-treated with dithiothreitol (DTT). The rat liver microsomal enzyme, however, is inactivated by NEM to a much greater extent if the microsomes are pre-treated with DTT. The beef liver enzyme activity is protected from NEM inactivation by the substrate, vitamin K1 epoxide. Ping-pong kinetics are exhibited by the beef liver enzyme. These results support a mechanism for vitamin K1 epoxide reductase in which the function of the required dithiol is to reduce an active site disulfide bond; however, the geometry of the active sites of the enzyme from rat and beef may be different.
Insights
Vitamin K1 epoxide reductase inactivation by N-ethylmaleimide (NEM) differs between beef and rat liver enzymes. Pre-treatment with dithiothreitol (DTT) increases NEM inactivation in rat liver microsomes but not beef liver microsomes.
Area of Science:
- Biochemistry
- Enzymology
- Molecular Biology
Background:
- Microsomal vitamin K1 epoxide reductase is crucial for vitamin K metabolism.
- Understanding enzyme mechanisms aids in drug development and disease research.
Purpose of the Study:
- To investigate the differential inactivation of beef and rat liver vitamin K1 epoxide reductase by N-ethylmaleimide (NEM).
- To elucidate the role of dithiothreitol (DTT) and the substrate, vitamin K1 epoxide, in enzyme inactivation.
- To explore potential differences in the active site geometry between rat and beef liver enzymes.
Main Methods:
- Enzyme assays measuring vitamin K1 epoxide reductase activity.
- Pre-treatment of liver microsomes with dithiothreitol (DTT).
- Inactivation studies using N-ethylmaleimide (NEM) with and without substrate protection.
Main Results:
- Beef liver enzyme showed minimal inactivation by NEM, regardless of DTT pre-treatment.
- Rat liver enzyme inactivation by NEM was significantly enhanced after DTT pre-treatment.
- Vitamin K1 epoxide protected the beef liver enzyme from NEM inactivation.
- Beef liver enzyme exhibited ping-pong kinetics.
Conclusions:
- The findings suggest a mechanism involving a dithiol in the enzyme's active site, likely for reducing disulfide bonds.
- Differences in NEM inactivation between rat and beef enzymes indicate distinct active site geometries.
- Further research is needed to fully characterize the structural and functional differences.