Mass spectrometric quantification of the mu opioid receptor agonist Tyr- d-Arg-Phe-Lys-NH 2 (DALDA) in high-performance liquid chromatography-purified ovine plasma

Abstract

The mu opioid receptor agonist Tyr- d-Arg-Phe-Lys-Amide ( d - Arg 2- Lys 4- Dermorphin 1-4 amide=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 (d 5-DALDA), was used for the MS quantification of DALDA via the protonated molecule ion, (M+H) +, of DALDA and of d 5-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.