Measurement of plasma renin activity with use of HPLC-electrospray-tandem mass spectrometry

Background:;The measurement of renin activity is complicated by difficultie s in the quantification of angiotensin 1 (Ang1), the product of the renin-c atalyzed reaction. We report an HPLC-electrospray-tandem mass spectrometry (HPLC-ESI-MS/MS) method for the quantification of Ang1 as a measure of plas ma renin activity (PRA). Methods: After incubation (37 degrees C for 3 or 18 h), samples were prepar ed using C-18 solid-phase extraction. [Val](5)Ang1 was used as the internal standard (IS). Chromatography was performed on a C-18 column, using 200 mL /L ammonium acetate buffer-800 mL/L methanol as the mobile phase. The flow rate was 150 mu L/min, with a chromatographic run time of 5 min/sample. Mas s spectrometric detection was in the positive ionization mode with selected reaction monitoring (Ang1 mit 649.0-->784.0; IS mit 641.9-->770.4). Results: The assay was linear over the range 2.5-500 ng Angl/mL, which corr esponded to a limit of detection (signal-to-noise ratio of 3:1) of PRA of 0 .14 ng Ang1 mL(-1) h(-1). The imprecision (CV) of the assay at PRA values o f 26.1, 13.5, 3.2 and 0.78 ng Ang1 mL(-1) h(-1) was 7.0%, 7.0%, 15%, and 11 %, respectively. Absolute recoveries were 92.3% (Ang1) and 87.4% (IS). Incu bation times of 3 h vs 18 h in the PRA assay gave good agreement at PRA <2 ng Ang1 mL(-1) h(-1), but samples with a PRA of 2-5 ng Ang1 m(L-)1 h(-1) ga ve lower PRA results after incubation for 18 h than after 3 h. We compared the HPLC-ESI-MS/MS assay and an RIA for the determination of PRA, with PRA incubation times of 3 h and 1.5 h, respectively. The mean PRA based on RIA of Ang1 was higher than that obtained using HPLC-ESI-MS/MS. Conclusion: The HPLC-ESI-MS/MS method allows sensitive and specific measure ment of PRA. The higher activities measured with the RIA method highlight i ts potential for overestimation of PRA. (C) 1999 American Association for C linical Chemistry.