Human Liver Enzymes Responsible for Metabolic Elimination of Tyramine, a Vasopressor Agent from Daily Food

Dietary tyramine is associated with hypertensive crises because of its ability to induce the release of catecholamines. The roles of monoamine oxidase (MAO), flavin-containing monooxygenase (FMO), and cytochrome P450 2D6 (CYP2D6) were studied in terms of the enzymatic elimination of tyramine in vitro at a substrate concentration of 1.0 μM, which is relevant to in vivo serum concentrations. Tyramine elimination by human liver supernatant fractions was decreased by ∼70% in the absence of NADPH. Pargyline, an MAO inhibitor, decreased tyramine elimination rates by ∼30%. Among recombinant P450 and FMO enzymes, CYP2D6 had a high activity in terms of tyramine elimination. Tyramine elimination rates were inhibited by quinidine and significantly correlated with bufuralol 1'-hydroxylation activities (a CYP2D6 marker). Liver microsomes genotyped for CYP2D6*10/*10 and CYP2D6*4/*4 showed low and undetectable activities, respectively, compared with the wild-type CYP2D6*1/*1. The present results suggest that tyramine is eliminated mainly by polymorphic CYP2D6. Tyramine toxicity resulting from differences in individual metabolic elimination is thus genetically determined.