Monoamine Oxidase

monoamine oxidase [män·ō′am‚ēn ′äk·sə‚dās]

(biochemistry)

A mitochondrial enzyme which oxidatively deaminates intraneuronal biogenic amines, some of which are important neurotransmitters in the peripheral and central nervous system.

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Monoamine oxidase

Either of two enzymes found in the outer membrane of mitochondria that degrade biogenic amines and are thus responsible for the destruction of transmitter substances at neuronal synapses. Nerve cells release neurotransmitter into the synapse in response to stimulation. The neuron must then dispose of this neurotransmitter to stop the signal or a new signal cannot get through. This is accomplished by one of three mechanisms: diffusion; reuptake into the presynaptic area; and degradation by a number of enzymes, including monoamine oxidase. See Synaptic transmission

Monoamine oxidase inhibitors are drugs that block degradation of amine transmitters within the cell; however, not all of their effects can be attributed directly to monoamine oxidase inhibition, since a number of different neuronal effects have been described. The most prominent consequence of monoamine oxidase inhibition is a rapid increase in the intracellular concentrations of monoamines. In addition, the level of serotonin in the brain is raised to a greater extent than that of norepinephrine and dopamine. After these amine concentrations rise, secondary adaptive consequences occur, including a reduction in amine synthesis via an apparent feedback mechanism, which has been most clearly demonstrated for the noradrenergic system. See Neurobiology, Serotonin

Two types of monoamine oxidase have been identified. These are designated A and B and are distinguished by having different substrate specificity. Type A preferentially deaminates norepinephrine, cortical dopamine, and serotonin, and is selectively inhibited by clorgyline. Type B degrades phenylethylamine, dopamine, and benzylamine, and is sensitive to deprenyl or pargyline inhibition. Commonly used monoamine oxidase inhibitors are nonselective inhibitors that affect types A and B. Seventy-five percent of monoamine oxidase in the human is type B.

Monoamine oxidase inhibitors are used in medicine for controlling hypertension and for treating depression and other disorders. Other psychiatric disorders, such as obsessive-compulsive disorder, bulimia, somatoform pain disorder, panic disorder, and schizophrenia, have been reported to occasionally respond to treatment with monoamine oxidase inhibitors. There is also some evidence that patients with so-called atypical depression preferentially respond to monoamine oxidase inhibitors. See Affective disorders

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Monoamine Oxidase 

an enzyme of the oxidoreductase class that catalyzes the oxidative deamination of monoamines, including biogenic amines. Monoamine oxidase is present in the outer membrane of mitochondria, with which it forms strong lipoprotein complexes. Flavin adenine dinucleotide (FAD), covalently bonded to protein, is a coenzyme (prosthetic group) of monoamine oxidase.

The presence of sulfhydryl (—SH) groups that are not, however, part of the active center of the enzyme is important for the activity of monoamine oxidase. Differences in the properties of enzyme systems of amino deamination (substrate specificity, reactivity to inhibitors such as 8-oxyquinoline, hydrazine derivatives, and certain alkaloids), observed in different species of animals and in different kinds of tissue, led to the concept of monoamine oxidase multiplicity. However, all attempts to isolate specific types of monoamine oxidase were unsuccessful. Monoamine oxidase apparently takes part not only the decomposition but also in the formation of biologically active compounds.

I. S. SEVERINA