β-メチル-L-チロシン

使用A tyrosine hydroxylase inhibitor. An antihypertensive in pheochromocytoma

使用Metyrosine is the α-methyl derivative of tyrosine. It competitively inhibits tyrosine hydroxylase action, thus reducing the formation of epinephrine and norepinephrine. It is used for treating patients with pheochromocytoma, in cases where a rise in the level of catecholamines is observed.

一般的な説明Metyrosine (α-Methyl-L-tyrosine, Demser). Althoughinhibition of any of the three enzymes involved in CA biosynthesisshould decrease CAs, inhibitors of the first andthe rate-limiting enzyme TH would be the most effective.As such, metyrosine is a much more effective competitiveinhibitor of E and NE production than agents that inhibitany of the other enzymes involved in CA biosynthesis. Itis often possible to “fool” the enzymes into accepting astructurally similar and unnatural substrate such as metyrosine.Metyrosine differs structurally from tyrosine onlyin the presence of an α-methyl group . It is oneexample of a CA-biosynthesis inhibitor in clinical use.Although metyrosine is used as a racemic mixture, it is the (-)isomer that possesses the inhibitory activity.Metyrosine, which is given orally in dosages ranging from 1 to 4 g/day, is used principally for the preoperative managementof pheochromocytoma, chromaffin cell tumorsthat produce large amounts of NE and E. Although theseadrenal medullary tumors are often benign, patients frequentlysuffer hypertensive episodes. Metyrosine reducesthe frequency and severity of these episodes by significantlylowering CA production (35%–80%). The drug ispolar (log P＝0.73) and excreted mainly unchanged in theurine. Because of its limited solubility in water caused byintramolecular bonding of the zwitterions, crystalluria is apotential serious side effect. It can be minimized by maintaininga daily urine volume of more than 2 L. Inhibitors ofCA synthesis have limited clinical utility because suchagents nonspecifically inhibit the formation of all CAs andresult in many side effects. Sedation is the most commonside effect of this drug.A similar example is the use of α-methyl-m-tyrosine inthe treatment of shock. It differs structurally from metyrosineonly in the presence of m-OH instead of p-OH inmetyrosine. This unnatural amino acid is accepted by the enzymesof the biosynthetic pathway and converted tometaraminol (an α-agonist).