MAO inhibitors (MAOIs) are substances that interfere with the enzyme's normal activity by binding to it. MAOIs are categorized based on their selectivity for MAO-A or -B and on the reversibility of the inhibition they cause (Holschneider & Shih, 2000).
Reversible MAOIs bind reversibly to MAO: they eventually detach, leaving the enzyme unchanged and active. Irreversible inhibitors (also known as inactivators or suicide inhibitors) form a covalent bond with MAO, leaving it permanently inactive (Banik, 1990). After irreversible inhibition, "recovery of enzymatic activity and restoration of amine metabolism generally requires up to two weeks, presumably because [MAO] must be replaced by synthesis" (Holschneider & Shih, 2000, sec. 6).
MAO inhibitors can be selective (show a preference for either MAO-A or -B at a given concentration) or non-selective (have approximately equal binding affinities for both isoforms). MAO inhibitors are virtually never 100% selective towards either isoform, gradually losing their selectivity at higher concentrations (citation needed).
| MAOIs in medical use | Excluding withdrawn, abandoned, and experimental MAOIs | | | | | Isocarboxazid (Marplan) | Antidepressant. Irreversible, non-selective (Riederer et al., 2004). | | Phenelzine (Nardil) | Antidepressant. Irreversible, non-selective (Riederer et al., 2004). | | Tranylcypromine (Parnate) | Antidepressant. Irreversible, non-selective (Riederer et al., 2004). | | Moclobemide (Aurorix) | Antidepressant. Reversible, MAO-A selective (RIMA) (Riederer et al., 2004). selectivity loss | | Toloxatone (Humoryl) | Antidepressant. Reversible, MAO-A selective (RIMA) (Riederer et al., 2004). | | Selegiline (Eldepryl, Emsam, Zelapar) | Anti-parkinson. Irreversible, MAO-B selective Loses selectivity above 10 mg/day (Holschneider & Shih, sec. 6). bioavailability | | Rasagiline (Azilect) | Anti-parkinson. Irreversible, MAO-B selective (Riederer et al., 2004). | | Linezolid (Zyvox, Zyvoxid) | Antibiotic. Reversible, non-selective (Taylor et al., 2006). | | Furazolidone (Furoxone) | Antibiotic. reversibility, selectivity | | Procarbazine (Matulane) | Antineoplastic. reversibility, selectivity | | Methylene Blue | Redox dye. Reversible, MAO-A selective (RIMA). Related redox dyes (toluidine blue O, thionine, brilliant cresyl blue, toluylene blue) are also RIMAs (Ramsay et al., 2007; Oxenkrug et al., 2007). |
| Recreationally used MAOIs | Including "research chemicals" | | | | | Coffee | Contains significant amounts of norharman (β-carboline) and harman (1-methyl-β-carboline), which are reversible MAO inhibitors (Herraiz & Chaparro, 2006). The hypothesis that coffee consumption lowers MAO activity in humans was not yet demonstrated. | | Tobacco | Smokers have approximately 20-30% lower MAO-A activity and approximately 30-40% lower MAO-B activity than non-smokers (Fowler et al., 1996; Berlin & Anthenelli 2001, Fowler et al 2003). Tobacco smoke contains norharman (β-carboline) and harman (1-methyl-β-carboline), which are partially responsible for MAO inhibition in smokers (Herrais & Chaparro, 2005). Because overnight abstinence (11.3 hours) from smoking does not cause any significant normalization of brain MAO-B activity in smokers (Fowler et al., 2000) and tobacco smoke was found to irreversibly inhibit MAO in vitro (Yu & Boulton, 1987), tobacco should be considered an irreversible MAO inhibitor, although it is a mild one. | | Harmine & related | Reversible, MAO-A selective (RIMA) (Kim et al., 1997, Wouters, 1988). Present in Peganum harmala (Syrian Rue) (Hemmateenejad et al., 2006) and Banisteriopsis caapi (Yagé) (Callaway et al., 2005). other beta-carbolines | | α-Methyltryptamine (AMT) | Reversible, MAO-A selective (RIMA) (Arai et al., 1986; Renyi, 1986). 5-MeO-AMT | | α-Ethyltryptamine (etryptamine, AET) | Reversible, MAO-A selective (RIMA) (Renyi, 1986). | | 4-methylthioamphetamine (4-MTA) | Reversible, MAO-A selective (RIMA) (Hurtado-Guzman et al., 2003). | | 2C-T-series | 2,5-dimethoxy-4-thiophenethylamine derivatives |
| Other MAOIs | Foods, herbals, and other miscellaneous MAOIs | | | | | Nutmeg | Myristicin, the major component of the essential oil of nutmeg, is considered a weak MAOI. MAO inhibition may be significant when large amounts are ingested (citation needed). |
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