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Tranylcypromine lowers human platelet MAO B activity but not concentration
Abstract:
Monoamine oxidase content in extracts of human blood platelets was determined independently of MAO activity measurements with a recently developed monoclonal antibody against human platelet monoamine oxidase (MAO B) in a competitive radioimmunoassay. Administration of a single oral dose of the irreversible inhibitor tranylcypromine to normal human subjects resulted in a rapid and marked inhibition of platelet MAO catalytic activity within 1 day, followed by recovery of activity to normal levels within 2 weeks. In contrast, there was no change in the net concentration of MAO protein during this time. Since recovery of MAO B activity approximated the reported half-life of blood platelets, these results suggest that recovery of platelet MAO activity after tranylcypromine treatment is due to the replacement of old platelets by new ones which contain catalytically active MAO B.
Insights
Recovery of platelet monoamine oxidase (MAO B) activity after tranylcypromine treatment is not due to enzyme regeneration. Instead, new platelets replace old ones, restoring MAO B activity.
Area of Science:
- Biochemistry
- Pharmacology
- Hematology
Background:
- Monoamine oxidase B (MAO B) is crucial for neurotransmitter metabolism.
- Understanding MAO B regulation in human platelets is vital for psychopharmacology.
- Tranylcypromine is an irreversible inhibitor of MAO B.
Purpose of the Study:
- To investigate the mechanism of monoamine oxidase B (MAO B) activity recovery in human platelets after irreversible inhibition.
- To differentiate between enzyme regeneration and platelet turnover as the cause of MAO B activity restoration.
Main Methods:
- Competitive radioimmunoassay using a monoclonal antibody to quantify MAO B protein.
- Measurement of platelet MAO B catalytic activity.
- Administration of tranylcypromine to human subjects.
Main Results:
- Tranylcypromine rapidly inhibited platelet MAO B activity.
- MAO B protein concentration remained unchanged post-inhibition.
- Platelet MAO B activity recovered within two weeks, correlating with platelet half-life.
Conclusions:
- Platelet MAO B activity recovery after tranylcypromine is attributed to the replacement of inhibited platelets with newly formed ones.
- This suggests that MAO B in platelets is not regenerated but relies on new cell synthesis.