Simultaneous determination of oxidized and reduced coenzyme Q and alpha-tocopherol in biological samples by high performance liquid chromatography with platinum catalyst reduction and electrochemical detection

Rapid and sensitive analysis of reduced and oxidized coenzyme Q10 in human plasma by ultra performance liquid chromatography-tandem mass spectrometry and application to studies in healthy human subjects

BACKGROUND

Coenzyme Q10 is an endogenous antioxidant as well as a popular dietary supplement. In blood circulation, coenzyme Q10 exists predominantly as its reduced ubiquinol-10 form, which readily oxidizes to ubiquinone-10 ex vivo. Plasma concentrations of coenzyme Q10 reflect net overall metabolic demand, and the ratio of ubiquinol-10:ubiquinone-10 has been established as an important biomarker for oxidative stress. However, the lability of ubiquinol-10 makes accurate determination of both forms of coenzyme Q10 difficult. Ex vivo oxidation of ubiquinol-10 to ubiquinone-10 during sample collection, processing and analysis may obfuscate the in vivo ratio.

METHODS

We developed a rapid and sensitive method for the determination of ubiquinol-10 and ubiquinone-10 in human plasma, using coenzyme Q9 analogues as internal standards. Single-step protein precipitation in 1-propanol, a lipophilic and water-soluble alcohol, allowed for rapid extraction.

RESULTS

Analysis by ultra performance liquid chromatography-tandem mass spectrometry provided rapid run-time and high sensitivity, with lower limits of quantitation for ubiquinol-10 and ubiquinone-10 of 5 μg/L and 10 μg/L, respectively.

CONCLUSIONS

This method is suitable for clinical studies with coenzyme Q10 supplementation in various disease states where this lipid-antioxidant may be beneficial. We have applied this method to >300 plasma samples from coenzyme Q10 research studies in chronic haemodialysis patients and postsurgical patients.

Analysis of circulating lipid-phase micronutrients in humans by HPLC: review and overview of new developments

Retinol, tocopherols, coenzyme Q10, carotenoids, and vitamin D are lipophilic compounds shown to function as important health-protective agents by mitigating the damaging effects of oxidative and other injury. Scientific interest in evaluating these compounds has resurfaced in recent years, particularly in the nutritional, clinical and epidemiologic fields, and has precipitated the development of a multitude of new analytical techniques. This review considers recent developments in HPLC-based assays since 2007 for the simultaneous determination of these lipid-phase compounds utilizing exclusively serum or plasma as these matrices are mostly used in clinical and epidemiological investigations. We also provide an overview of blood measurements for selected carotenoids, tocopherols, coenzyme Q10 and retinol from the last 15years of healthy umbilical cord blood, children, and adults.

Analytical problems with the determination of coenzyme Q10 in biological samples

The article discusses analytical problems related to the determination of coenzyme Q10 in biological samples. The assaying of coenzyme Q10 in complex samples, such as plasma, tissues, or food items requires meticulous sample preparation prior to final quantification. The process typically consists of the following steps: deproteinization, extraction, and ultimately reduction of extract volumes. At times drying under a gentle stream of neutral gas is applied. In the case of solid samples, a careful homogenization is also required. Each step of the sample preparation process can be a source of analytical errors that may lead to inaccurate results. The main aim of this work is to point to sources of analytical errors in the preparation process and their relation to physicochemical properties of coenzyme Q10. The article also discusses ways of avoiding and reducing the errors.

Measurement of oxidized and reduced coenzyme Q in biological fluids, cells, and tissues: an HPLC-EC method

Direct measure of coenzyme Q (CoQ) in biological specimens may provide important advantages. Precise and selective high-performance liquid chromatography (HPLC) methods with electrochemical (EC) detection have been developed for the measurement of reduced (ubiquinol) and oxidized (ubiquinone) CoQ in biological fluids, cells, and tissues. EC detection is preferred for measurement of CoQ because of its high sensitivity. Reduced and oxidized CoQ are first extracted from biological specimens using 1-propanol. After centrifugation, the 1-propanol supernatant is directly injected into HPLC and monitored at a dual-electrode. The EC reactions occur at the electrode surface. The first electrode transforms ubiquinone into ubiquinol, and the second electrode measures the current produced by the oxidation of the hydroquinone group of ubiquinol. The methods described provide rapid, precise, and simple procedures for determination of reduced and oxidized CoQ in biological fluids, cells, and tissues. The methods have been successfully adapted to meet regulatory requirements for clinical laboratories, and have been proven reliable for analysis of clinical and research samples for clinical trials and animal studies involving large numbers of specimens.

Analysis of coenzyme Q in human blood and tissues

The major coenzyme Q species in humans is the decaprenyl quinoid derivative coenzyme Q10 (CoQ10), and its measurement is somewhat challenging owing to its hydrophobicity and tendency to be oxidized. There are three major methods which are suited for analysis of CoQ10: HPLC-coupled UV or electrochemical detection, and tandem mass spectrometry. The techniques are discussed, and results of these applications to determine CoQ10 concentrations in various human fluids and tissues are summarized.

Biological validation of coenzyme Q redox state by HPLC-EC measurement: relationship between coenzyme Q redox state and coenzyme Q content in rat tissues

The properties of coenzymes Q (CoQ9 and CoQ10) are closely linked to their redox state (CoQox/total CoQ) x 100. In this work, CoQ redox state was biologically validated by high performance liquid chromatography-electrochemical measurement after modulation of mitochondrial electron flow of cultured cells by molecules increasing (rotenone, carbonyl cyanide chlorophenylhydrazone) or decreasing (antimycin) CoQ oxidation. The tissue specificity of CoQ redox state and content were investigated in control and hypoxic rats. In control rats, there was a strong negative linear regression between tissular CoQ redox state and CoQ content. Hypoxia increased CoQ9 redox state and decreased CoQ9 content in a negative linear relationship in the different tissues, except the heart and lung. This result demonstrates that, under conditions of mitochondrial impairment, CoQ redox control is tissue-specific.

Simultaneous determination of vitamin K analogs in human serum by sensitive and selective high-performance liquid chromatography with electrochemical detection

OBJECTIVES

We report a sensitive and selective method for the simultaneous determination of vitamin K1 and K2 analogs (VKs) with high-performance liquid chromatography in which a platinum catalyst reduction column and an electrochemical detector operated in the oxidation mode are incorporated into the detection system. We also applied this trace analysis method to the simultaneous determination of VKs in human serum to investigate the physiologic and pathophysiologic roles of VKs in the bone metabolism.

METHODS

Our high-performance liquid chromatographic method with postcolumn catalyst reduction and electrochemical detection was applied for the simultaneous determination of VKs in human serum samples. After separation of VKs on a reversed-phase separation column by using a mixture of ethanol and methanol (1:1, v/v), containing 0.025 M of sodium perchlorate as the mobile phase, the VKs were reduced once in a platinum catalyst reduction column connected online and then monitored quantitatively by an electrochemical detector with a glassy carbon working electrode operated in the oxidation mode (+0.6 V versus Ag/AgCl).

RESULTS

VKs were clearly separated from each other within 80 min. The detection limits at a signal-to-noise ratio of 3 were 2 to 10 pg for VKs. Quantitative recovery from serum was obtained in the range of 86% to 91% for VKs. The average coefficients of variation of within-day and between-day assays were 1.6% to 2.1% and 2.4% to 3.6%, respectively, for all VKs.

CONCLUSIONS

This method was sensitive and selective for detection of VKs and was satisfactory in the simultaneous determination of VKs in small volumes of human serum.

Analysis of vitamin E and its oxidation products by HPLC with electrochemical detection

A sensitive HPLC procedure with postcolumn reduction and electrochemical detection was developed for the analysis of vitamin E and its oxidation products, alpha-tocopherylquinone, epoxy-alpha-tocopherylquinones, and 8a-(lipid-dioxy)-alpha-tocopherones. After the separation on a reversed-phase column, on-line zinc-catalyzed reduction allowed the detection of alpha-tocopherylquinone and epoxy-alpha-tocopheryl-quinones, whereas platinum-catalyzed reduction allowed the detection of 8a-(lipid-dioxy)-alpha-tocopherones. The lowest detectable level of each compound was about 0.2 pmol at the signal-to-noise ratio of 3. This method was applied to the detection of alpha-tocopherol products in peroxidized human plasma. When the plasma was peroxidized by the addition of a free radical initiator, peaks corresponding to alpha-tocopherylquinone, epoxy-alpha-tocopherylquinones, and the addition products of alpha-tocopherol with peroxyl radicals derived from cholesteryl ester hydroperoxides and PC hydroperoxides were observed. The amount of these oxidation products in the plasma increased with the depletion of endogenous alpha-tocopherol. The results indicate that the method is useful to detect the oxidation products formed by the peroxyl radical-trapping reactions of alpha-tocopherol in biological systems.

Chromatographic analysis of alpha-tocopherol and related compounds in various matrices

Tocopherols and tocotrienols (Vitamin E) are part of a group of "minor components" of main interest, present in the unsaponifiable fraction of many samples. Their importance in biological, metabolical and nutritional studies makes determination of tocopherols and related compounds of major interest. Present work critically reviews the different ways to perform sample pre-treatment and analysis of these compounds, related to the matrices, other analytes to be measured, sensitivity, and simplicity. The review includes well referenced tables that provide in-depth summaries of methodology for the chromatographic analysis of alpha-tocopherol and related compounds in foods, pharmaceuticals, plants, animal tissues and other matrices.

HPLC Analysis of Reduced and Oxidized Coenzyme Q10 in Human Plasma

Background: The percentage of reduced coenzyme Q10 (CoQ10H2) in total coenzyme Q10 (TQ10) is decreased in plasma of patients with prematurity, hyperlipidemia, and liver disease. CoQ10H2 is, however, easily oxidized and difficult to measure, and therefore reliable quantification of plasma CoQ10H2 is of clinical importance.

Methods: Venous blood was collected into evacuated tubes containing heparin, which were immediately placed on ice and promptly centrifuged at 4 °C. The plasma was harvested and stored in screw-top polypropylene tubes at −80 °C until analysis. After extraction with 1-propanol and centrifugation, the supernatant was injected directly into an HPLC system with coulometric detection.

Results: The in-line reduction procedure permitted transformation of CoQ10 into CoQ10H2 and avoided artifactual oxidation of CoQ10H2. The electrochemical reduction yielded 99% CoQ10H2. Only 100 μL of plasma was required to simultaneously measure CoQ10H2 and CoQ10 over an analytical range of 10 μg/L to 4 mg/L. Intra- and interassay CVs for CoQ10 in human plasma were 1.2–4.9% across this range. Analytical recoveries were 95.8–101.0%. The percentage of CoQ10H2 in TQ10 was ∼96% in apparently healthy individuals. The method allowed analysis of up to 40 samples within an 8-h period.

Conclusions: This optimized method for CoQ10H2 analysis provides rapid and precise results with the potential for high throughput. This method is specific and sufficiently sensitive for use in both clinical and research laboratories.

Comparative study on dynamics of antioxidative action of alpha-tocopheryl hydroquinone, ubiquinol, and alpha-tocopherol against lipid peroxidation

Alpha-tocopheryl quinone is a metabolite of alpha-tocopherol (TOH) in vivo. The antioxidant action of its reduced form, alpha-tocopheryl hydroquinone (TQH2), has received much attention recently. In the present study, the antioxidative activity of TQH2 was studied in various systems in vitro and compared with that of ubiquinol-10 (UQH2) or TOH to obtain the basic information on the dynamics of the antioxidant action of TQH2. First, their hydrogen-donating abilities were investigated in the reaction with galvinoxyl, a stable phenoxyl radical, and TQH2 was found to possess greater second-order rate constant (1.0 x 10(4) M(-1) s(-1)) than UQH2 (6.0 x 10(3) M(-1) s(-1)) and TOH (2.4 x 10(3) M(-1) s(-1)) at 25 degrees C in ethanol. The stoichiometric numbers were obtained as 1.9, 2.0, and 1.0 for TQH2, UQH2, and TOH, respectively, in reducing galvinoxyl. Second, their relative reactivities toward peroxyl radicals were assessed in competition with N,N'-diphenyl-p-phenylenediamine (DPPD) and found to be 6.0 (TQH2), 1.9 (UQH2), and 1.0 (TOH). Third, their antioxidant efficacies were evaluated in the oxidation of methyl linoleate in organic solvents and in aqueous dispersions. The antioxidant potency decreased in the order TOH > UQH2 > TQH2, as assessed by either the extent of the reduction in the rate of oxidation or the duration of inhibition period. The reverse order of their reactivities toward radicals and their antioxidant efficacies was interpreted by the rapid autoxidation of TQH2 and UQH2, carried out by hydroperoxyl radicals. Although neither TQH2 nor UQH2 acted as a potent antioxidant by itself, they acted as potent antioxidants in combination with TOH. TQH2 and UQH2 reduced alpha-tocopheroxyl radical to spare TOH, whereas TOH suppressed the autoxidation of TQH2 and UQH2. In the micelle oxidation, the antioxidant activities of TQH2, UQH2, and TOH were similar, whereas 2,2,5,7,8-pentamethyl-6-chromanol exerted much more potent efficacy than TQH2, UQH2, or TOH. These results clearly show that the antioxidant potencies against lipid peroxidation are determined not only by their chemical reactivities toward radicals, but also by the fate of an antioxidant-derived radical and the mobility of the antioxidant at the microenvironment.

Automated high-performance liquid chromatographic method with precolumn reduction for the determination of ubiquinol and ubiquinone in human plasma

We developed a gradient HPLC method with automated precolumn reduction for direct electrochemical detection of ubiquinol-10 (CoQ10H2) and total coenzyme Q10 (TQ10) in human plasma. The concentration of ubiquinone-10 (CoQ10) was calculated by subtraction of CoQ10H2 from TQ10. Preparation of reducing agent and precolumn reduction was performed by a programmable auto-injector. The two mobile phases used were: A, 100% of methanol containing 50 mM sodium perchlorate and 10 mM perchloric acid; and B, a mixture of ethanol and tert.-butanol (80:20, v/v). Sample preparation was simply a deproteinisation process with 10-fold ethanol. A good linear relationship was obtained for CoQ10H2 concentration from 0.1 to 3 micromol/l. The detection limit was 2.5 nmol/l with an injection volume of 20 microl. The analytical recovery and reproduciblity were generally >90%. To validate the method, 18 freshly collected plasma samples of normal healthy subjects were analysed. The mean ratio of CoQ10H2/CoQ10 was 93:7. The proposed method is sensitive, reliable and can be used for clinical investigation.

Synthesis of [11C] coenzyme Q-related compounds for in vivo estimation of mitochondrial electron transduction and redox state in brain

We have studied the synthesis of [11C]2,3-dimethoxy-5-methyl-6-(10-hydroxy)-decyl-1,4-benzoquinone (idebenone) and [11C]2,3-dimethoxy-5-methyl-1,4-benzoquinone (CoQo) by methylation of their respective desmethyl precursors using [11C]CH3I for in vivo measurement of mitochondrial electron transfer and redox state. The [11C]idebenone was more lipophilic than [11C]CoQo; the latter became hydrophilic by reduction. Clearance of [11C]idebenone from mouse brain was more rapid than that of [11C]CoQo. The results indicated that modification of the isoprenoid side chain in [11C]CoQ is necessary to develop more suitable radiopharmaceuticals.

High-performance liquid chromatography-coulometric electrochemical detection of ubiquinol 10, ubiquinone 10, carotenoids, and tocopherols in neonatal plasma

A micromethod for the rapid simultaneous determination of several lipophilic antioxidants in plasma from newborn infants is presented. Because only 5 microliters of plasma is required, the procedure lends itself for repetitive use in very immature infants at risk for developing so-called "oxygen radical diseases of the premature." The method allows continuous monitoring of antioxidants in such patients and can easily be combined with monitoring other parameters of interest in this context. Reuse of blood samples taken routinely for the determination of hematocrit and bilirubin concentration is possible, reducing the blood volume required to be taken for the oxygen radical-related studies to virtually zero.

Application of derivative spectrophotometry for determination of coenzyme Q10 in pharmaceuticals and plasma

The use of derivative spectrophotometry is proposed in this work for determination of coenzyme Q10 in formulations and in human plasma. The spectrophotometric procedure is simpler and less expensive than chromatographic techniques commonly used for the analysis of coenzyme. The active compound can be determined in the range 0.25-10 ppm for standard solutions and pharmaceuticals and 0.05-1.5 ppm in plasma. The proposed method was applied for coenzyme determination in real samples. The results agree well with declared value and with these obtained by HPLC.

Simultaneous detection of ubiquinol and ubiquinone in human plasma as a marker of oxidative stress

A method is described for the simultaneous detection of ubiquinol-10 and ubiquinone-10 in human plasma. In this procedure, heparinized human plasma was mixed with 5 vol of methanol and 10 vol of hexane. After vigorous shaking and centrifugation, an aliquot of the hexane phase (5 microl) was injected immediately and directly onto a reversed-phase HPLC to minimize the oxidation of ubiquinol to ubiquinone. A post-separation, on-line reduction column converts ubiquinone to ubiquinol which is quantified by electrochemical detection. The detection limit of plasma ubiquinol-10 and ubiquinone-10 is about 4 nM with excellent reproducibilities. Tocopherols, lycopene, and beta-carotene are also detectable in this method. In addition, free cholesterol, and cholesteryl esters can be quantified by their absorption at 210 nm. Using this method we have determined the ratio of ubiquinol to ubiquinone is about 95/5 in human plasma from healthy donors. We suggest that this method will be useful since the ratio of ubiquinol to ubiquinone has been suggested as a good marker of oxidative stress.