Simultaneous evaluation of HMG-CoA reductase and cholesterol 7alpha-hydroxylase activities by electrospray tandem MS

Optimization, validation and application of an assay for the activity of HMG-CoA reductase in vitro by LC-MS/MS

A stable HMG-CoA reductase (HMGR) reaction in vitro was developed by a sensitive, selective and precise liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. The optimized enzyme reaction condition contained 1.5 μg of HMGR, 20 nM of NADPH with 50 min of reaction time. The method was validated by several intra- and inter-day assays. The production transitions of m/z 147.0/59.1 and m/z 154.0/59.1 were used to detect and quantify mevalonolactone (MVAL) and MVAL-D₇, respectively. The accuracy and precision of the method were evaluated over the concentration range of 0.005-1.000 μg/mL for MVAL and 0.010-0.500 μg/mL for lovastatin acid in three validation batch runs. The lower limit of quantitation was found to be 0.005 μg/mL for MVAL and 0.010 μg/mL for lovastatin acid. Intra-day and inter-day precision ranged from 0.95% to 2.39% and 2.26% to 3.38% for MVAL, 1.46% to 3.89% and 0.57% to 5.10% for lovastatin acid, respectively. The results showed that the active ingredients in Xuezhikang capsules were 12.2 and 14.5 mg/g, respectively. This assay method could be successfully applied to the quality control study of Xuezhikang capsule for the first time.

Measurement of HMG CoA reductase activity in different human cell lines by ultra-performance liquid chromatography tandem mass spectrometry

Hydroxymethylglutaryl coenzyme A reductase (HMGCR) catalyzes the rate limiting step in cholesterol biosynthesis converting HMG-CoA into mevalonic acid (MVA), which equilibrates with mevalonic acid lactone (MVL) under neutral pH conditions. We developed a fast, sensitive, and efficient method to determine HMGCR activity in human cell lines measuring MVL levels by ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). Convenient prepared samples containing MVL-D7 as an internal standard were injected, separated, and eluted from an ACQUITY HSS PFP column. Measurement of MVL was performed by electrospray ionization mass spectrometry with multiple reaction monitoring. Calibration curves were linear and reproducible in the range of 0.15-165 μg/l (r>0.99). Lower limit of quantification was 0.12 μg/l. Intra- and interassay imprecision were <1.3% and <2.9%, respectively. HMGCR enzymatic activity measurements of cells cultivated under different cell culture conditions (with 10% FCS, with 10% lipoprotein-deficient serum and under serum starvation) revealed the applicability of this test system for various experimental settings. This efficient UPLC-MS/MS assay permits rapid and high sensitive determination of HMGCR enzyme activity, tracing potential alterations in cholesterol biosynthesis.

Rapid reverse phase-HPLC assay of HMG-CoA reductase activity

Radioisotope-based and mass spectrometry coupled to chromatographic techniques are the conventional methods for monitoring HMG-CoA reductase (HMGR) activity. Irrespective of offering adequate sensitivity, these methods are often cumbersome and time-consuming, requiring the handling of radiolabeled chemicals or elaborate ad-hoc derivatizing procedures. We propose a rapid and versatile reverse phase-HPLC method for assaying HMGR activity capable of monitoring the levels of both substrates (HMG-CoA and NADPH) and products (CoA, mevalonate, and NADP(+)) in a single 20 min run with no pretreatment required. The linear dynamic range was 10-26 pmol for HMG-CoA, 7-27 nmol for NADPH, 0.5-40 pmol for CoA and mevalonate, and 2-27 nmol for NADP(+), and limit of detection values were 2.67 pmol, 2.77 nmol, 0.27 pmol, and 1.3 nmol, respectively.

Determination of key intermediates in cholesterol and bile acid biosynthesis by stable isotope dilution mass spectrometry

For more than a decade, we have developed stable isotope dilution mass spectrometry methods to quantify key intermediates in cholesterol and bile acid biosynthesis, mevalonate and oxysterols, respectively. The methods are more sensitive and reproducible than conventional radioisotope (RI), gas-chromatography (GC) or high-performance liquid chromatography (HPLC) methods, so that they are applicable not only to samples from experimental animals but also to small amounts of human specimens. In this paper, we review the development of stable isotope dilution mass spectrometry for quantifying mevalonate and oxysterols in biological materials, and demonstrate the usefulness of this technique.

Highly sensitive assay of HMG-CoA reductase activity by LC-ESI-MS/MS

We have developed a new sensitive and specific nonradioisotope assay method to measure the activity of HMG-CoA reductase, the rate-controlling enzyme in the cholesterol biosynthetic pathway. This method was based upon a stable isotope dilution technique by liquid chromatography-tandem mass spectrometry using electrospray ionization in positive mode. Mevalonic acid, the product of HMG-CoA reductase, was converted to mevalonolactone (MVL) in an incubation mixture, extracted by a salting-out procedure, derivatized into the mevalonyl-(2-pyrrolidin-1-yl-ethyl)-amide, and then purified using a disposable silica cartridge. The resulting mevalonylamide was quantified by selected reaction monitoring using the positive electrospray ionization mode. The detection limit of this mevalonylamide was found to be 240 amol (signal-to-noise ratio=3), approximately 833 times more sensitive than that of MVL measured by a conventional radioisotope (RI) method (200 fmol). The variances between sample preparations and between measurements by this method were analyzed by one-way layout and calculated to be 3.2% and 1.8%, respectively. The recovery experiments were performed using incubation mixtures spiked with 0.77-2.31 nmol MVL/mg protein and were validated by a polynomial equation. These results showed that the estimated concentration within a 95% confidence limit was 0.47+/-0.07 nmol/mg protein, which coincided completely with the observed X0 nmol/mg protein with a mean recovery of 94.6%. This method made it possible to measure HMG-CoA reductase activity with a high degree of reproducibility and reliability, and especially with sensitivity superior to that of the conventional RI method.