Tyramine.

Tyramine is formed when the carboxyl group is removed from tyrosine by aromatic amino acid decarboxylase in tissue or by bacterial tyrosine decarboxylase in fermenting food. That relationship is settled biochemistry and is why tyramine accumulates in protein-rich foods as they age. It also means high free tyrosine in a fermenting substrate is what sets the ceiling on tyramine formation.

Aromatic amino acid decarboxylase and the bacterial tyrosine decarboxylases are both PLP-dependent enzymes. Without the active B6 cofactor the decarboxylation step does not run. This is textbook enzymology and applies equally to the human enzyme and the microbial one.

Both MAO-A and MAO-B carry a covalently bound FAD derived from riboflavin. Intestinal and hepatic MAO-A is what degrades most dietary tyramine before it reaches the general circulation. The cofactor requirement is established, though a clinically meaningful effect of riboflavin status on tyramine handling has not been demonstrated in people.

Semicarbazide-sensitive amine oxidase and diamine oxidase are copper-dependent enzymes that oxidise primary amines including tyramine. They act alongside MAO rather than instead of it. The cofactor dependence is established biochemistry, not a reason to take copper for this purpose.

Tyramine in aged cheese, cured meat and fermented soy is produced by bacterial decarboxylation, and strains within the lactobacilli and enterococci differ sharply in whether they carry the gene. This is why starter culture selection is a real lever on tyramine content in fermented food. It cuts both ways, since a decarboxylase-negative culture keeps tyramine low while a positive one raises it.

Tyramine is an indirect sympathomimetic. It is taken up into noradrenergic nerve terminals and displaces stored noradrenaline into the synapse, which raises blood pressure and heart rate. Caffeine pushes in the same direction through a different route. The combination is worth flagging for anyone with elevated blood pressure rather than assumed to be trivial.

If intestinal MAO activity is reduced, dietary tyramine passes into circulation instead of being degraded in the gut wall, which is the mechanism behind the classic pressor reaction seen with MAO-inhibiting drugs. St John's wort shows weak MAO inhibition in laboratory assays but is generally not considered to produce a clinically relevant tyramine reaction at usual intakes. The row is here as a mechanistic caution, not a demonstrated interaction.

Rhodiola extracts inhibit MAO-A and MAO-B in vitro. Whether that translates into reduced intestinal degradation of dietary tyramine in people has not been shown. Read this as mechanistic rather than clinical.

Colonic bacteria decarboxylate residual tyrosine, so the resident community is one determinant of luminal tyramine load independent of what was eaten. Strains differ in whether they carry the decarboxylase, which means a probiotic can shift this in either direction. The practical size of that shift in people has not been established.