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Essential sulfhydryl groups of rat liver monoamine oxidase
Abstract:
The inhibition by some thiol reagents of partly purified mitochondrial monoamine oxidase (MAO) (EC 1.4.3.4) from rat liver was studied, and the molar content of sulfhydryl groups in the enzyme determined. Sodium nitroprusside and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) inhibited the enzyme, apparently reversibly, while sodium arsenite was not inhibitory. Concentrations of the respective inhibitors causing 50% inhibition after 15 min of preincubation with the enzyme at pH 7.0 and 37 degrees C are 5.80 times 10(-4) M and 4.35 times 10(-5) M. The thiol compounds cysteine, dithiothreitol, and 2-mercaptoethanol did not inhibit MAO. The average number of sulfhydryl groups per mole of enzyme, determined by reaction with DTNB, increased from 3.6 +/- 0.2 freely reacting sulfhydryl groups (n = 4) to 18.4 to total sulfhydryl groups (n = 2) on denaturation with 8 M urea.
Insights
This study investigated thiol reagents
Area of Science:
- Biochemistry
- Enzymology
- Molecular Biology
Background:
- Mitochondrial monoamine oxidase (MAO) is crucial for neurotransmitter metabolism.
- Understanding MAO inhibition is vital for developing therapeutic agents.
- The role of sulfhydryl groups in MAO activity requires further elucidation.
Purpose of the Study:
- To investigate the inhibitory effects of specific thiol reagents on rat liver mitochondrial monoamine oxidase (MAO).
- To determine the molar content of sulfhydryl groups within the MAO enzyme structure.
Main Methods:
- Partly purified rat liver mitochondrial monoamine oxidase (MAO) was used.
- Enzyme inhibition assays were performed with sodium nitroprusside and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB).
- Sulfhydryl group content was quantified using DTNB reaction and urea denaturation.
Main Results:
- Sodium nitroprusside and DTNB inhibited MAO activity in a dose-dependent manner.
- The 50% inhibitory concentrations (IC50) were determined for sodium nitroprusside (5.80 x 10(-4) M) and DTNB (4.35 x 10(-5) M).
- DTNB assay revealed approximately 3.6 free sulfhydryl groups per mole of MAO, increasing to 18.4 total sulfhydryl groups upon denaturation.
Conclusions:
- Specific thiol reagents like DTNB can inhibit mitochondrial monoamine oxidase (MAO).
- The enzyme contains a significant number of sulfhydryl groups, with a portion accessible for reaction.
- These findings contribute to understanding MAO's structure-function relationship and potential inhibition mechanisms.