3-Methoxytyramine is first extracted using an ion exchange matrix followed by an acylation process. The assay procedure uses the basic principle of radioimmunoassay, involving competition between a radioactive and a non-radioactive antigen for a fixed number of antibody binding sites. The amount of 125I-labelled antigen bound to the antibody is inversely proportional to the analyte concentration of the sample. When the system is in equilibrium, the antibody bound radioactivity is precipitated with a second antibody in the presence of polyethylene glycol. The precipitate is counted in a gamma counter. Quantification of unknown samples is achieved by comparing their activity with a reference curve prepared with known standards. Metanephrine, Normetanephrine and 3-Methoxytyramine are the metabolites of the catecholamines Epinephrine, Norepinephrine and Dopamine, respectively. Cells derived from neuroendocrine tumors (e.g. pheochromocytoma) are known to produce catecholamines, which are secreted episodically via vesicles into the blood stream. But beside this, a small portion of the catecholamines is metabolized inside the cells to the corresponding catecholamines metabolites – namely Metanephrine, Normetanephrine and 3-Methoxytyramine – which are secreted at low levels continuously into the blood stream. 125I – Radioimmunoassay for the determination of free 3-Methoxytyramine (3-MT) in plasma. For research use only, not for use in diagnostic procedures.