Advances in analytical methods to study cholesterol metabolism: the determination of serum noncholesterol sterols

UPLC-Orbitrap-HRMS application for analysis of plasma sterols

Correct identification and quantification of different sterol biomarkers can be used as a first-line diagnostic approach for inherited metabolic disorders (IMD). The main drawbacks of current methodologies are related to lack of selectivity and sensitivity for some of these compounds. To address this, we developed and validated two sensitive and selective assays for quantification of six cholesterol biosynthesis pathway intermediates (total amount (free and esterified form) of 7-dehydrocholesterol (7-DHC), 8-dehydrocholesterol (8-DHC), desmosterol, lathosterol, lanosterol and cholestanol), two phytosterols (total amount (free and esterified form) of campesterol and sitosterol) and free form of two oxysterols (7-ketocholesterol (7-KC) and 3β,5α,6β-cholestane-triol (C-triol). For quantification of four cholesterol intermediates we based our analytical approach on sterol derivatization with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD). Quantification of all analytes is performed using UPLC coupled to an Orbitrap high resolution mass spectrometry (HRMS) system, with detection of target ions through full scan acquisition using positive atmospheric pressure chemical ionization (APCI) mode. UPLC and MS parameters were optimized to achieve high sensitivity and selectivity. Analog stable isotope labeled for each compound was used for proper quantification and correction for recovery, matrix effects and process efficiency. Precision (2.4%-12.3% inter-assay variation), lower limit of quantification (0.027 nM-50.5 nM) and linearity (5.5 μM (R2 0.999) - 72.3 μM (R2 0.997)) for phyto- and oxysterols were determined. The diagnostic potential of these two assays in a cohort of patients (n = 31, 50 samples) diagnosed with IMD affecting cholesterol and lysosomal/peroxisomal homeostasis is demonstrated.

Validation of an In-House-Developed GC-MS Method for 5α-Cholestanol According to ISO 15189:2012 Requirements

OBJECTIVE

The aim of this study was to validate a gas chromatography-mass spectrometry (GC-MS) method for the measurement of 5α-cholestanol in the clinical laboratory in agreement with ISO 15189:2012.

MATERIALS AND METHODS

The GC-MS performance was evaluated and proficiency testing data were used to estimate the measurement uncertainty of the method considering the recommendations of international guidelines.

RESULTS

The calibration curves were linear from 6 to 50 μmol/L, with r2 >.99. The limit of detection and limit of quantitation were determined to be 0.36 and 2.58 μmol/L, respectively. The bias ranged from -18.9% to 15.2% for 6.5, 18.3, and 66 μmol/L. The intra- and interassay reproducibility was <20% at the various concentrations studied. The expanded uncertainty was determined to be 50.9%.

CONCLUSION

The GC-MS method for the measurement of 5α-cholestanol has proved to have acceptable analytical performance for use in the clinical laboratory.

Methodological Aspects of Phytosterol Measurements in Biological Samples

Phytosterol measurement has gained a lot of interest during the last two decades after foods and supplements with added 4-desmethyl phytosterols were recognized and used as effective and safe non-pharmacologic hypocholesterolemic agents, and also after the mechanisms of intestinal absorption and hepatic excretion of sterols were unraveled. In addition, the wide use of serum phytosterols as biomarkers of cholesterol absorption has increased the interest in their measurement. In this review, the basic methods are discussed without going into details of the practical operations. The analysis includes first lipid extraction and saponification from various biologic matrices such as serum/plasma, feces, or tissues, after which the individual sterols are separated by adsorption chromatography (gas-liquid or liquid or high performance liquid chromatography) based on the polarity of the various sterols. We also deal with some specific aspects of phytosterol measurements in biological samples such as the need of harmonization of their analysis in biological samples, the discrepancies in the results of sitosterol and campesterol concentrations between different studies, and what is known about their biological day-to-day fluctuation. Phytosterols have a remarkable role in human health, so that their complicated and time consuming measurements call attention to routine ways of standardization between the sterol research laboratories.

Determination of non-cholesterol sterols in serum and HDL fraction by LC/MS-MS: Significance of matrix-related interferences

Background

Non-cholesterol sterols (NCS) are promising biomarkers for estimation of cholesterol homeostasis properties. In addition, determination of NCS in high-density lipoprotein (HDL) fraction (HDL-NCS) could provide information on cholesterol efflux. However, matrix effects interfere in liquid chromatography-mass spectrometry (LC-MS) analysis of NCS, thereby impairing the method sensitivity. The aims of this study were development, optimization and validation of LC-MS method for quantification of NCS in serum and HDL-NCS. Additionally, matrix effect interferences and methods application in individual serum samples were examined.

Methods

HDL precipitating reagent was used for HDL isolation. Matrix effect was examined by comparing different surrogates by simple regression analysis. Validation was conducted according to the FDA-ICH guideline. 20 healthy volunteers were recruited for testing of method application.

Results

The observed matrix effect was 30%, and matrix comparison showed that cholesterol was the dominant contributor to the matrix effect. Cholesterol concentration was adjusted by construction of the calibration curve for serum and HDL fraction (5 mmol/L and 2.5 mmol/L, respectively). The intraand interrun variabilities for NCSs were 4.7-10.3% for serum NCS and 3.6-13.6% for HDLNCS and 4.6-9.5% for serum NCSs and 2.5-9.8% for HDL-NCS, respectively. Recovery studies showed satisfactory results for NCSs: 89.8-113.1% for serum NCS and 85.3-95.8% for HDL-NCS.

Conclusions

The method was successfully developed and optimized. The matrix interference was solved by customising calibration curves for each method and sample type. The measurement of NCS in HDL fraction was proposed for the first time as potentially useful procedure in biomedical researches.

Preanalytical and analytical challenges in gas chromatographic determination of cholesterol synthesis and absorption markers

INTRODUCTION

Cholesterol homeostasis disruption contributes to the development of different pathologies. Non-cholesterol sterols (NCSs) serve as cholesterol synthesis markers (desmosterol and lathosterol), and cholesterol absorption surrogate markers (campesterol, stigmasterol and β-sitosterol). The study aimed to resolve certain new pre-analytical and analytical problems and ensure a reliable and validated method.

MATERIALS AND METHODS

Method optimization, validation and stability studies were executed in human serum and plasma. Freeze-thaw cycles were done with and without antioxidant. Gas chromatography-mass spectrometer (GC-MS) was used for NCSs confirmation and plasticizer identification, while GC-flame ionization detector (GC-FID) was used for NCSs quantitation.

RESULTS

Intra- and inter-assay variabilities for all NCSs were 2.75-9.55% and 5.80-7.75% for plasma and 3.10-5.72% and 3.05-10.92% for serum, respectively. Recovery studies showed satisfactory percentage errors for all NCSs: 93.4-105.7% in plasma and 87.5-106.9 in serum. Derivatized samples were stable up to 7days at -20°C and derivatization yield was affected by presence of plasticizers. Fatty acid amids were identified as interfering plastic leachates. Statistically different NCSs concentrations were observed after the 1st freeze-thaw cycle, in antioxidant-free samples, and after the 4th cycle in antioxidant-enriched samples.

CONCLUSIONS

All of the in-house procedures proved to be useful for minimizing the preanalytical and analytical variations, as proven by the validation results.

Sitosterolemia: A multifaceted metabolic disorder with important clinical consequences

Sitosterolemia is a metabolic disorder characterized by increased intestinal absorption and tissue accumulation of phytosterols. Although sitosterolemia is considered a rare disease, its prevalence may be significantly higher than initially thought. Indeed, accumulating evidence suggests that patients with unexplained hematologic abnormalities or premature cardiovascular disease in the absence of classic risk factors may exhibit disordered phytosterol metabolism. In this review, we present a patient with sitosterolemia, describe the pathophysiology and the clinical picture of this disorder, and discuss the clinical value of phytosterol supplementation in patients with primary dyslipidemias.

A novel chemiluminescence sensor for sensitive detection of cholesterol based on the peroxidase-like activity of copper nanoclusters

A sensitive and selective chemiluminescence (CL) sensor based on the peroxidase-like activity of copper nanoclusters was established for the detection of cholesterol. Copper nanoclusters catalyse the CL reaction between luminol and H₂O₂. Because H₂O₂ is the oxidative product of cholesterol in the presence of cholesterol oxidase, the oxidation of cholesterol can be quantitatively converted to a CL response by combining the two reactions. The proposed method is simple and can be completed in a few minutes with high sensitivity. Under the optimal conditions, the CL intensity was proportional to the concentration of cholesterol over a wide range of 0.05-10 mM, with a detection limit of 1.5 μM. Furthermore, the method was successfully applied to determine cholesterol in milk powder and human serum with satisfactory accuracy and precision. This method expands the applications of nano-mimic enzymes in the field of CL-based sensors.

Cholesterol absorption and synthesis markers in Portuguese hypercholesterolemic adults: A cross-sectional study

OBJECTIVE

The dynamics of cholesterol homeostasis and the development of cardiovascular disease (CVD) are complex and multifactorial, to which adds individual variability in the proportion of cholesterol from exogenous versus endogenous sources. The aim of this study was to undertake the first characterization of cholesterol absorption and synthesis profiles in Portuguese hypercholesterolemic adults through the quantification of surrogate markers, and the analysis of the predictive value of age and sex on the cholesterol homeostasis biomarkers.

METHODS

Serum samples for the measurement of lipid profiles and cholesterol homeostasis markers were obtained for 100 men and 112 women, aged 30-65, with TC ≥ 5.2 mmol/L (~200mg/dL) and/or LDL-C ≥ 2.6 mmol/L (~100mg/dL), none of whom were on any lipid-lowering therapy.

RESULTS

Overall, sex-specific significant differences were observed in the cholesterol homeostasis markers and lipid profiles; women had lower cholesterol synthesis marker concentrations (P<0.01 for lathosterol) and lipid parameters (except for HDL-C concentrations). Age-related significant differences were also found, including higher concentrations of cholesterol absorption markers in association with increasing age.

CONCLUSION

In our study, the predictors of higher levels of cholesterol absorption markers were higher age and female gender.

Biomarkers of cholesterol homeostasis in a clinical laboratory database sample comprising 667,718 patients

BACKGROUND

Circulating noncholesterol sterols/stanols (NCS) are used in clinical lipidology as surrogate measures of cholesterol synthesis and absorption, where they can be valuable tools in assessing cholesterol metabolism and personalizing therapies in patients with dyslipidemia.

OBJECTIVES

To describe the distributions of plasma NCS concentrations and inter-NCS correlations in a large cohort of American patients constituting a clinical laboratory database, and to investigate the relationship between circulating NCS, age, sex, and apolipoprotein E (APOE) genotype.

METHODS

A total of 667,718 patient blood samples submitted for testing to Health Diagnostic Laboratory, Inc. (Richmond, VA) were analyzed for cholesterol absorption markers (sitosterol, campesterol, and cholestanol) and one cholesterol synthesis marker (desmosterol). NCS percentiles were determined, along with intermarker correlations (Pearson's R). Analysis of variance was used to assess the effect of age and sex on NCS level, and to evaluate the relationship between cholesterol synthesis/absorption status and APOE genotype in a subset of 336,866 patients.

RESULTS

Mean NCS concentrations were: sitosterol, 2.45 μg/mL; campesterol, 3.3 μg/mL; cholestanol, 2.92 μg/mL; and desmosterol 0.99 μg/mL. The correlations between each NCS and its ratio to total cholesterol ranged from 0.72 (cholestanol) to 0.94 (desmosterol). NCS levels were significantly affected by age and sex (P < .0001), and prevalence of cholesterol hyperabsorption was higher in APOE ε4 allele carriers compared with the other APOE genotypes.

CONCLUSIONS

We have described sample distributions of NCS biomarkers and characterized their relationship to age, sex, and APOE genotype. These data may facilitate research into altered cholesterol homeostasis and human disease, and help physicians optimize lipid-lowering therapies.