Monoamine oxidases of the human brain and liver

The two molecular forms of monoamine oxidase (MAO), MAO-A and MAO-B, were determined quantitatively in discrete regions of the human brain at autopsy, and in cerebral microvessels, choroid plexus and liver samples from the same subjects. MAO was assessed by specific[3H] pargyline binding, which is stoichiometric and irreversible, and by measuring the rate of oxidation of several MAO substrates. Basal ganglia structures (caudate, putamen, globus pallidus and substantia nigra) and hippocampus had about twice the levels of MAO that were present in the cerebral cortex and cerebellum. Cerebral microvessels, which constitute the blood-brain barrier, had minimal MAO, while the choroid plexus, which constitutes the blood-cerebrospinal fluid barrier, and the liver had higher MAO levels than any brain region. The vast majority of MAO (80-95%) in these tissues was of the B type, except in microvessels, where total MAO activity was low. Specific [3H]pargyline binding correlated well with the oxidation rates for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and benzylamine in all tissues. Both specific [3H]pargyline binding and the rate of oxidation of MAO substrates increased with age.