Development and validation of a liquid chromatography–tandem mass spectrometric method for the determination of α-methyltyrosine in human plasma

A sensitive and selective LC–MS–MS method for the isolation and quantification of α-methyltyrosine (AMT) from human plasma is described. The method employs a simple protein precipitation using zinc sulfate and sodium hydroxide. This precipitation procedure produced samples with high aqueous content that could be directly injected into a LC–MS–MS system without compromising reverse-phase chromatographic performance. Chromatographic separation was performed on a MetaChem MonoChrom C18 column (2.0 mm × 50 mm; 5 μm) at a flow rate of 1 mL/min. Compounds were eluted using a gradient mixture of water–acetic acid (100:0.1, v/v) and acetonitrile–acetic acid (100:0.1, v/v). The structural analog α-hydroxymethyltyrosine was used as the internal standard. Mass spectrometric detection was carried out with a triple quadrupole mass spectrometer. The method was validated and used to determine human plasma AMT concentrations, and has been implemented to derive pharmacokinetic parameters.