MASS-SPECTROMETRIC QUANTIFICATION OF THE MU-OPIOID RECEPTOR AGONIST TYR-D-ARG-PHE-LYS-NH2 (DALDA) IN HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY-PURIFIED OVINE PLASMA

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 sh eep. The presence of DALDA in ovine plasma was determined by reversed- phase highperformance liquid chromatography (RP-HPLC) and mass spectro metry (MS) in plasma samples that were obtained at different times dur ing and following that infusion. A stable isotope-incorporated interna l standard, deuterated DALDA (d(5)-DALDA), was used for the MS quantif ication of DALDA via the protonated molecule ion, (M+H)(+), of DALDA a nd of d(5)-DALDA. Time-course data (mu g 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.