Monoamine oxidase
Monoamine oxidase inhibitors (MAOIs) are a class of drugs that inhibit the enzyme monoamine oxidase. This enzyme is responsible for breaking down many compounds; basically, anything with a primary amine moiety is likely to be oxidized by monoamine oxidase. An important substrate of the enzyme MAO is tyramine. MAOIs inhibit the enzyme either reversibly, in which MAO is inhibited only until the drug is cleared from the system, or irreversibly, in which the substrate binds permanently to the enzyme, rendering it inactive and effectively destroying it. Irreversible MAOIs are potentially more dangerous, because the body takes about two weeks to regenerate MAO enzymes to functional levels. Two subtypes of MAO exist: MAO-A and MAO-B; this is relevant to adrenergic storms, as there are significant differences between the two types, such as their differential expression throughout the body, and range of substrates. While both MAO-A and MAO-B metabolize tyramine, only MAO-A is present in the gastrointestinal tract and singularly metabolizes the majority of consumed tyramine. (The small portion normally passing into circulation is mostly degraded in the liver where both MAO types act.) Consequently, MAOIs that irreversibly inhibit MAO-A will permit high levels of circulating tyramine able to cause tyramine-induced hypertensive crisis. Aged cheese, beer, red wine, some mushrooms, and fermented products such as pickles are foods containing high levels of tyramine that passed into circulation can cause such a hypertensive crisis.