Mass spectrometric quantification of the mu opioid receptor agonist Tyr-

The mu opioid receptor agonist Tyr-

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-Arg-Phe-Lys-Amide (

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-Arg²-Lys⁴-Dermorphin_1-4amide=DALDA) was infused continuously for 2 h into sheep. The presence of DALDA in ovine plasma was determined by reversed-phase high-performance liquid chromatography (RP-HPLC) and mass spectrometry (MS) in plasma samples that were obtained at different times during and following that infusion. A stable isotope-incorporated internal standard, deuterated DALDA (d₅-DALDA), was used for the MS quantification of DALDA via the protonated molecule ion, (M+H)⁺, of DALDA and of d₅-DALDA. Time-course data (μg DALDA ml⁻¹ plasma vs. time) were obtained. Tandem MS (MS–MS) provided the product-ion spectrum of the (M+H)⁺ ion of DALDA in one of the samples to confirm the amino acid sequence of DALDA.