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Re: reversible chemical distinctions?

Posted by Adam on November 7, 1999, at 22:08:02

In reply to Re: reversible chemical distinctions?, posted by dj on November 6, 1999, at 13:28:29

Irreversible monoamine oxidase inhibitors (sometimes referred to as suicide monoamine oxidase inhibitors) bind to monoamine oxidase
and react with it chemically. This reaction leads to the formation of an unbreakable covalent bond between the enzyme and the drug
metabolite/enzyme adduct. By unbreakable I mean it cannot be dissociated by the enzyme itself or under conditions that would preserve
the enzyme. Because of where this bond is formed, MAO cannot react with its normal substrates. This effectively destroys the enzyme.

Reversible MAOIs do not form unbreakle bonds with monamine oxidase. They can be dissociated from MAO without destroying it under
normal physioligical conditions, leaving the enzyme unaltered by the interaction. Because of the placement, strength, and duration
of this interaction, the reversible MAOI can still interfere with the enzymes ability to catalyse reactions with its normal substrates.

In either case, the result is increased levels of some neurotransmitters.

"Specific" MAOIs have greater affinities for one or the other of the two MAO species, MAO-A or MAO-B. MAO-A preferentially breaks down
(oxidatively deaminates) serotonin and norepinepherne, though it does degrade dopamine to a lesser extent. MAO-B breaks down only
dopamine and phenylethylamine. Both forms of the enzyme can break down tyramine. So, an MAOI that inhibits one but not both of the MAOs
will lessen the danger of the "cheese effect", the adverse reactions one can experience from elavated levels of the amino acid tyramine
in the brain.

Tranylcypromine, phenelzine, and isocarboxazid are examples of irreversible, non-specific MAOIs. They bind both MAO-A and B and destroy them.

Clorgyline and pargyline are examples of irreversible inhibitors of MAO-A.

Moclobemide, befloxatone, and brofaromine are examples of reversible inhibitors of MAO-A (RIMAs) They transiently bind with MAO-A and leave
the enzyme unchanged by the interaction.

Selegiline and rasagiline are expamples of irreversible inhibitors of MAO-B (selegiline has a much higher affinity for MAO-B, but starts to
have significant effects on MAO-A at high doses).

I have seen references to reversible inhibitors of MAO-B (RIMBs?) but I do not know what these drugs are.

> Perhaps, JB, you or someone else might be kind and thoughtful enough to translate the MAOI distinctions you made below into English with an explanation of why they are labeled as irreversible?
> > I'd say the effect was nothing like with either Nardil (an irreversible, non-selective, Hydrazine MAOI) or Parnate (an irreversible, non-selective, NON-hydrazine MAOI)or Marplan (an irrerversile, non-selective, Hydrazine MAOI). Hope this helps at least a little.
> >
> > Good luck, Judy. JohnB




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