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Monoamine oxidase activity in brain microvessels determined using natural and artificial substrates: relevance to the
Abstract:
The possible contribution of cerebrovascular monoamine oxidase (MAO) to the blood-brain barrier to catecholamines was studied in isolated porcine and rat microvessels by determining its activity with various substrates. Michaelis-Menten kinetic constants, Km and Vmax, were determined using noradrenaline (NA) as substrate in a Tris medium. Km values were 0.25 +/- 0.05 mM in control and 0.16 +/- 0.09 mM in ultrasonically disintegrated (USD) preparations (difference not significant); Vmax in USD preparations (1.83 +/- 0.20 n.atoms O2 min-1 mg protein-1) was slightly higher (p less than 0.05) than in control preparations (1.35 +/- 0.11 n.atoms O2 min-1 mg protein-1), suggesting a certain restriction by the plasma membrane of substrate access to the enzyme. This phenomenon was confirmed in a more physiological, ionic medium; the activity was then approximately doubled for 1 mM NA, whereas that for 1 mM beta-phenylethylamine (beta-PEA), a lipid-soluble substrate, tended to decrease with USD treatment. These results show that this highly active form of MAO is unlikely to be saturated by physiological concentrations of catecholamine. It can be estimated that, for a plasma concentration of NA of 1 microM, a facilitated diffusion accelerating the entry of the catecholamine into the cells by at least 15-fold would be necessary in order to exceed the catabolic capacity of MAO. It is concluded that circulating catecholamines are not likely to cross the endothelial barrier of cerebral microvessels intact, and that the small quantities of radioactivity detected in the parenchyma in measurements of the brain uptake index essentially represent metabolites due to MAO activity.
Insights
Cerebrovascular monoamine oxidase (MAO) significantly metabolizes catecholamines in cerebral microvessels. Circulating catecholamines likely do not cross the blood-brain barrier intact, with detected radioactivity representing MAO metabolites.
Area of Science:
- Neuroscience
- Biochemistry
- Pharmacology
Background:
- The blood-brain barrier (BBB) tightly regulates the passage of substances into the brain.
- Monoamine oxidase (MAO) is a key enzyme in catecholamine metabolism.
- The role of cerebrovascular MAO in catecholamine transport across the BBB is not fully understood.
Purpose of the Study:
- To investigate the contribution of cerebrovascular monoamine oxidase (MAO) to the blood-brain barrier's handling of catecholamines.
- To determine the kinetic properties of MAO in isolated porcine and rat cerebral microvessels.
- To assess the likelihood of intact catecholamine passage across the cerebral endothelial barrier.
Main Methods:
- Isolated porcine and rat cerebral microvessels were used.
- Monoamine oxidase (MAO) activity was measured using various substrates, including noradrenaline (NA).
- Michaelis-Menten kinetic constants (Km and Vmax) were determined in different media (Tris and ionic).
Main Results:
- Ultrasonic disintegration (USD) of microvessels slightly increased Vmax, suggesting plasma membrane restriction of substrate access.
- MAO activity was higher in a physiological ionic medium compared to a Tris medium.
- The enzyme's high activity indicates it is unlikely to be saturated by physiological catecholamine concentrations.
Conclusions:
- Circulating catecholamines are unlikely to cross the cerebral microvessel endothelial barrier intact.
- The observed radioactivity in brain parenchyma measurements likely represents metabolites formed by MAO activity.
- Cerebrovascular MAO plays a significant role in metabolizing catecholamines before they can enter the brain parenchyma.