The liquid chromatography-multiple reaction monitoring-tandem mass spectrometry (LC-MRM-MS/MS) method using (13)C stable isotope-labeled dipeptides was newly developed to simultaneously determine the absorption of three antihypertensive peptides (Val-Tyr, Met-Tyr, and Leu-Tyr) into blood of spontaneously hypertensive rats in one run-in assay. After extracting (13)C-labeled peptides in blood sample with a C(18) cartridge, the extract was applied to a (13)C monoisotopic transition LC-MRM-MS/MS system with D-Val-Tyr included as internal standard. An excellent separation of each dipeptide in LC was achieved at the elution condition of 5-100% methanol in 0.1% formic acid at a flow rate of 0.25 ml/min. The (13)C-labeled peptides ionized by electron spray were detected in the positive ion mode within 15 min. The established method showed high reproducibility with less than 10% coefficient of variation as well as high accuracy of more than 85%. After the administration of a mixture containing the three (13)C-labeled dipeptides to rats at each dose of 30 mg/kg, we could successfully determine the intact absorption of each (13)C-labeled peptide with the maximal absorption amount of 1.1 ng/ml plasma for Val-Tyr by the proposed LC-MRM-MS/MS method.

The candidates responsible for vasorelaxation action of rutin-free tartary buckwheat extract (TBSP) were examined in this study. As a result of reversed-phase high-performance liquid chromatography (HPLC) separations, five prominent peaks in the acidic fraction of TBSP were obtained at 260 nm. Among the five collected peaks, we successfully identified four compounds by nuclear magnetic resonance (NMR) and mass spectrometry (MS) measurements: (+)-osbeckic acid as a dimer ([M - H](-) m/z: 371.2 > 184.9 > 140.9), 5-hydroxymethyl-2-furoic acid, protocatechuic acid, and p-hydroxybenzoic acid. A vascular contractive measurement in 1.0 μM phenylephrine-contracted Sprague-Dawley rat thoracic aorta rings revealed that (+)-osbeckic acid dimer evoked a potent vasorelaxant effect with an EC50 value of 887 μM compared to other isolates (EC50: 5-hydroxymethyl-2-furoic acid, 3610 μM; protocatechuic acid, 2160 μM; p-hydroxybenzoic acid, no inhibition). Dimeric (+)-osbeckic acid was stable in solutions and at high temperatures, while its degraded peak on the HPLC chromatogram was observed when it was dissolved in dimethyl sulfoxide.