[Properties of intestinal monoamine oxidase in the rat]
Abstract:
Monoamine oxidase (MAO) activity (substrate: tyramine) has been studied in rat intestinal wall mitochondrial fractions identified by monitoring succinate dehydrogenase and cytochrome oxidase activities. The MAO activity, which was not due to contamination with mitochondria, has been also found in nuclear and microsomal (+hyaloplasm) fractions. Deamination of tyramine, serotonin and dopamine by rat intestinal mitochondrial MOA obeyed the Michaelis--Mentern kinetics. The Vmax values were the highest for deamination of tyramine, the lowest--for norepinephrine. The lowest Km value was recorded in the systems with 2-phenylethylamine. Data on the inhibitory effect of low concentrations of deprenyl suggest that 50% of the total tyramine deaminating activity in rat intestinal mitochondria was due to presence of MAO type B. Low concentrations of chlorgyline inhibited the deamination of tyramine in these systems by 20-30% suggesting a possibility of presence in the rat intestinal mitochondria of a tyramine deaminating activity distinct from MAO type A. Pyrazidol or harmine, which are selective inhibitors of the MAO type A, caused only partial (30-40%) inhibition of MAO activity (substrate: tyramine) in rat intestinal mitochondria. Controlled heating experiments indicated higher thermostability of MAO type B (substrate: 2-phenylethylamine) as compared with MAO type A (substrate: serotonin) in rat intestinal mitochondria. The data obtained suggest that rat intestinal mitochondria, contrary to human intestinal mucosa (cf. ref. 2), contain about 50% of MAO type B, which is comparatively thermostable and does not resemble in this respect the MAO type B in many other biological sources.
Insights
Rat intestinal mitochondria contain significant Monoamine oxidase (MAO) type B activity, distinct from human tissues. This MAO type B is notably thermostable, differing from MAO B in other biological sources.
Area of Science:
- Biochemistry
- Enzymology
- Pharmacology
Context:
- Monoamine oxidase (MAO) is crucial for neurotransmitter and xenobiotic metabolism.
- Understanding MAO distribution and properties in the gastrointestinal tract is vital for drug development and understanding physiological processes.
- Previous studies suggest MAO type A predominates in human intestinal mucosa.
Purpose:
- To characterize Monoamine oxidase (MAO) activity in rat intestinal wall mitochondrial fractions.
- To determine the kinetic properties (Michaelis-Menten kinetics) of MAO activity towards various substrates.
- To investigate the presence and relative contribution of MAO type A and MAO type B in rat intestinal mitochondria.
Summary:
- MAO activity was detected in rat intestinal mitochondrial, nuclear, and microsomal fractions.
- Kinetic analysis revealed distinct Vmax and Km values for tyramine, serotonin, dopamine, and norepinephrine deamination.
- Inhibition studies with selective MAO inhibitors (deprenyl, chlorgyline) and thermal stability assays indicated approximately 50% MAO type B activity, which is unusually thermostable compared to MAO B from other sources.
Impact:
- This study reveals a significant presence of a distinct, thermostable MAO type B in rat intestinal mitochondria.
- Findings challenge the prevailing view of MAO distribution in the intestinal tract, particularly in comparison to human tissues.
- The unique properties of rat intestinal MAO B may have implications for understanding drug metabolism and developing targeted therapies.
