A simple, sensitive and efficient assay for the determination of d- and l-lactic acid enantiomers in human plasma by high-performance liquid chromatography

Semiconducting polymer dots based l-lactate sensor by enzymatic cascade reaction system

BACKGROUND

l-lactate detection is important for not only assessing exercise intensity, optimizing training regimens, and identifying the lactate threshold in athletes, but also for diagnosing conditions like L-lactateosis, monitoring tissue hypoxia, and guiding critical care decisions. Moreover, l-lactate has been utilized as a biomarker to represent the state of human health. However, the sensitivity of the present l-lactate detection technique is inadequate.

RESULTS

Here, we reported a sensitive ratiometric fluorescent probe for l-lactate detection based on platinum octaethylporphyrin (PtOEP) doped semiconducting polymer dots (Pdots-Pt) with enzymatic cascade reaction. With the help of an enzyme cascade reaction, the l-lactate was continuously oxidized to pyruvic and then reduced back to l-lactate for the next cycle. During this process, oxygen and NADH were continuously consumed, which increased the red fluorescence of Pdots-Pt that responded to the changes of oxygen concentration and decreased the blue fluorescence of NADH at the same time. By comparing the fluorescence intensities at these two different wavelengths, the concentration of l-lactate was accurately measured. With the optimal conditions, the probes showed two linear detection ranges from 0.5 nM to 5.0 μM and 5.0 μM-50.0 μM for l-lactate detection. The limit of detection was calculated to be 0.18 nM by 3σ/slope method. Finally, the method shows good detection performance of l-lactate in both bovine serum and artificial serum samples, indicating its potential usage for the selective analysis of l-lactate for health monitoring and disease diagnosis.

SIGNIFICANCE

The successful application of the sensing system in the complex biological sample (bovine serum and artificial serum samples) demonstrated that this method could be used for sensitive l-lactate detection in practical clinical applications. This detection system provided an extremely low detection limit, which was several orders of magnitude lower than methods proposed in other literatures.

Paper-based chiral biosensors using enzyme encapsulation in hydrogel network for point-of-care detection of lactate enantiomers

Chiral analysis is of pivotal importance in a variety of fields due to the different biological activities and functions of enantiomers. Here, we develop a simple paper-based chiral biosensor that can perform sample-to-answer simultaneous analysis of lactate enantiomers in human serum samples. By modification of alginate hydrogel with "egg-box" three-dimensional network structure on a glass microfiber paper, reagents of enantiomer-selective enzymatic reactions are efficiently encapsulated forming the sensing regions for chiral analysis. Dual enzyme catalytic system (lactate dehydrogenase and glutamic pyruvic transaminase) is utilized to enhance the response of the biosensor. A smartphone with color analysis software is used to collect and analyze the fluorescence signal from the product nicotinamide adenine dinucleotide. The results show that the sensor has excellent selectivity toward lactate enantiomers with low limit-of-detection of (30.0 ± 0.7) μM for L-lactate and (3.0 ± 0.2) μM for D-lactate, and wide linear detection range of 0.1-3.0mM and 0.01-0.5 mM for L-lactate and D-lactate respectively. The proposed method is successfully applied to the simultaneous detection of L-/D-lactate concentrations in human serum with satisfactory accuracy. Our study provides a robust approach for developing chiral biosensors, which would have promising application prospect in point-of-care testing (POCT) analysis of various biological and food samples.

In Vitro Animal Model for Estimating the Time since Death with Attention to Early Postmortem Stage

Estimating the postmortem interval (PMI) has remained the subject of investigations in forensic medicine for many years. Every kind of death results in changes in metabolites in body tissues and fluids due to lack of oxygen, altered circulation, enzymatic reactions, cellular degradation, and cessation of anabolic production of metabolites. Metabolic changes may provide markers determining the time since death, which is challenging in current analytical and observation-based methods. The study includes metabolomics analysis of blood with the use of an animal model to determine the biochemical changes following death. LC-MS is used to fingerprint postmortem porcine blood. Metabolites, significantly changing in blood after death, are selected and identified using univariate statistics. Fifty-one significant metabolites are found to help estimate the time since death in the early postmortem stage. Hypoxanthine, lactic acid, histidine, and lysophosphatidic acids are found as the most promising markers in estimating an early postmortem stage. Selected lysophosphatidylcholines are also found as significantly increased in blood with postmortal time, but their practical utility as PMI indicators can be limited due to a relatively low increasing rate. The findings demonstrate the great potential of LC-MS-based metabolomics in determining the PMI due to sudden death and provide an experimental basis for applying this attitude in investigating various mechanisms of death. As we assume, our study is also one of the first in which the porcine animal model is used to establish PMI metabolomics biomarkers.

A photonic crystal fiber-based fluorescence sensor for simultaneous and sensitive detection of lactic acid enantiomers

A photonic crystal fiber (PCF)–based fluorescence sensor is developed for rapid and sensitive detection of lactic acid (LA) enantiomers in serum samples. The sensor is fabricated by chemical binding dual enzymes on the inner surface of the PCF with numerous pore structures and a large specific surface area, which is suitable to be utilized as an enzymatic reaction carrier. To achieve simultaneous detection of l-LA and d-LA, the PCF with an aldehyde-activated surface is cut into two separate pieces, one of which is coated with l-LDH/GPT enzymes and the other with d-LDH/GPT enzymes. By being connected and carefully aligned to each other by a suitable sleeve tube connector, the responses of both l-LA and d-LA sensors are determined by laser-induced flourescence (LIF) detection. With the aid of enzyme-linked catalytic reactions, the proposed PCF sensor can greatly improve the sensitivity and analysis speed for the detection of LA enantiomers. The PCF sensor exhibits a low limit of detection of 9.5 μM and 0.8 μM, and a wide linear range of 25–2000 μM and 2–400 μM for l-LA and d-LA, respectively. The sensor has been successfully applied to accurate determination of LA enantiomers in human serum with satisfactory reproducibility and stability. It is indicated that the present PCF sensors would be used as an attractive analytical platform for quantitative detection of trace-amount LA enantiomers in real biological samples, and thus would play a role in disease diagnosis and clinical monitoring in point-of-care testing.

Progress in pretreatment and analysis of organic Acids: An update since 2010

Organic acids, as an important component of food, have great influence on the flavor, texture, freshness of food. By lowering the pH of food to bacteriostatic acidity, organic acids are also used as additives and preservatives. Because organic acids are crucial to predict and evaluate food maturity, production and quality control, the rapid and sensitive determination methods of organic acids are necessary. This review aims to summarize and update the progress of the determination of organic acids in food samples. Pretreatment methods include simple steps (e.g., "dilute and shoot," protein precipitation, filtration, and centrifugation) and advanced microextraction methods (e.g., hollow fiber liquid phase microextraction, stir bar sorptive extraction and dispersive micro-solid phase extraction). Advances in novel materials (nanomaterial), solvents (ionic liquids and supercritical fluids) and hybrid methods are clearly displayed in detail. Continuous progress which has been made in electrochemical method, two-dimensional chromatography, high resolution mass is thoroughly illustrated.

Screening of dominant strains in red sour soup from Miao nationality and the optimization of inoculating fermentation conditions

Red sour soup is a traditional fermented product in southwest China. Currently, the existing production process mainly adopts the method of natural fermentation, with long fermentation cycles and poor stability between batches. Rapid establishment of dominant strains can accelerate the formation of lactic acid, which can inhibit the growth of miscellaneous bacteria. It is also helpful for the inhibition of nitrite accumulation, shortening of fermentation. In this study, the dominant strain H9, with lactic acid-producing ability, was isolated from the natural fermented red sour soup, and was identified as Lactobacillus buchneri, based on the 16s rRNA sequence analysis and biochemical identification. Then, the optimization of fermentation conditions was performed using L. buchneri H9 strain as external bacteria. The optimized fermentation conditions were temperature of 22°C, starch dosage of 11.24 g/L, and initial inoculation of 3.5 × 108 cfu/L. The concentration of lactic acid reached 8.029 g/L after 8 days of inoculating fermentation, which exceeded 6.221 g/L for 20 days of natural fermentation. Compared with natural fermentation, the peak of nitrite during inoculating fermentation appeared earlier and the peak height was lower. While the nitrite content in inoculating fermentation decreased to safety threshold more quickly. The volatile flavor compounds analysis showed that 41 types of volatile compounds were detected in the inoculating fermentation product, while 45 in the natural fermentation product. Over 88% compounds were overlapped, which means similar flavor between two fermentation products. These results provide a sufficient scientific basis for the industrialized production of inoculating fermentation of red sour soup.

Lactate determination in human vitreous humour by capillary electrophoresis and time of death investigation

Forensic inquests, particularly, in assessing time since death currently recognize the importance of the analysis of vitreous humour (VH) biomarkers. Present research, studies, and validates the determination of lactate (La) in VH by CZE with indirect UV detection. The BGE (pH 8.9) consisted of Tris buffer (37 mM) containing 4-methoxybenzoic acid (4 mM) and alkyl-trimethyl-ammonium bromide (1.2 mM). Each VH specimen was diluted with a butyric acid solution (internal standard 0.057 mM) and La and butyrate were separated within 3-5 min (30 kV). The La LOQ and LOD were 4 and 2 mM, respectively. The calibration curve linearity ranged from 4 to 80 mM; intra- and interruns precisions were less than 10% for standard as well as for VH specimen, respectively. To investigate postmortem interval (PMI) and VH lactate level correlation, human VH specimens were collected during autopsy (n = 40) and stored at -20°C until assay. La levels ranged from 16 to 42 mM; PMI values ranged from 10 to 141 h. La (mM) and PMI (h) correlation was statistically significant (r² = 0.527; p < 0.05). In conclusion, the present CZE analysis is efficacious to determine VH La as a biomarker for PMI investigation.

Lactic Acid Determination in Human Plasma Using Ultrasound-Assisted Emulsification Microextraction Followed by Gas Chromatography

A rapid, sensitive, and accurate analytical method was developed for determination of lactic acid (LA) in human plasma to monitor lactic acidosis. This method was based on an ultrasound-assisted emulsification microextraction (USAEME) method followed by gas chromatography with flame ionization detection (GC–FID). Derivatization of LA was carried out by a low density alcoholic solvent which performs both as an extraction solvent and derivatization agent, simultaneously. In this procedure, 100 μL of binary mixtures of pentan-1-ol with toluene (70 : 30, v/v %) was slowly injected into a 10 mL acidified aqueous sample of LA placed into an ultrasonic water bath. The resulting emulsion was centrifuged and after derivatization, 2 μL of organic phase was analysed by GC–FID. The effective variables were evaluated to optimize the efficiency of USAEME. Under the optimum conditions, good linearity in the range of 0.06–7.77 mmol L–1 was obtained with a correlation coefficient (R2) of 0.991 and a limit of detection (LOD) of 0.04 mmol L–1 for water samples. The inter-day and intra-day repeatability of the proposed method in human plasma were evaluated in terms of the relative standard deviation (RSD %) and were found to be <10 %. The results revealed that the USAEME–GC–FID method can be applied successfully for determination of LA in human plasma samples with satisfactory accuracy and precision.

Development and validation of a novel derivatization method for the determination of lactate in urine and saliva by liquid chromatography with UV and fluorescence detection

We developed a novel and straightforward derivatization method for the determination of lactate by reversed phase high-performance liquid chromatography (RP-HPLC) with fluorescence and UV detection in biological matrices as urine and saliva. The derivatization of lactate was achieved employing 9-chloromethyl anthracene (9-CMA) as fluorescence reagent, which has never been previously used to obtain a lactate derivative. Lactate reacts with 9-CMA with high selectivity in a very short time, without requiring extraction procedures from the aqueous solution, and the reaction reaches 70% completion in 30 min. The ester derivative obtained can be easily determined by RP-HPLC with fluorescence detection at 410 nm (λ ex=365 nm) and UV detection at 365 nm. The method was also optimized in order to allow for the simultaneous determination of lactate and creatinine for the application to urine samples. The lactate calibration curve was linear in the investigated range 2 × 10(-4)-3 × 10(-2)mM and the limit of detection, calculated as three times the standard deviation of the blank divided by the calibration curve slope, was 50 nM for both fluorescence and UV detection. Intra-day and inter-day repeatability were lower than 5% and 6%, respectively. The method proposed was successfully applied to the analysis of urine and saliva samples.

HPLC determination of D-3-hydroxybutyric acid by derivatization with a benzofurazan reagent and fluorescent detection: application in the analysis of human plasma

A simple and sensitive new method for the determination of D-3-hydroxybutyric acid (D-3-HBA) in human plasma after derivatization is described. The proposed method is based on the reaction of (2S)-2-amino-3-methyl-1-[4-(7-nitro-benzo-2,1,3-oxadiazol-4-yl)-piperazin-1-yl]-butan-1-one (NBD-PZ-Val) with D-3-HBA in the presence of O-(7-azobenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU) and N-ethyldiisopropylamine (DIEA) to produce a fluorescent derivative. The formed derivative was monitored fluorimetrically at λ(ex)=489 nm and λ(em)=532 nm. The HPLC analysis was carried out by use of a C18 analytical column (Synergy Hydro 150 mm × 3 mm, i.d., 4 μm) with a binary gradient elution program of 0.1% aqueous trifluoroacetic acid versus methanol. The method showed satisfactory linearity (r(2)=0.9997) in the range from 20 to 500 μmol/L. The limit of detection (LOD) of the method was 7.7 μmol/L, while the limit of quantitation (LOQ) was 25.8 μmol/L. The analytical method was successfully applied to human plasma samples from normal healthy subjects.

Online enzyme discrimination and determination of substrate enantiomers based on electrophoretically mediated microanalysis

We proposed the first application of an electrophoretically mediated microanalysis (EMMA) method for fast online discrimination and determination of substrate enantiomers, which was achieved by just one EMMA assay. Lactate dehydrogenase (LDH)-catalyzed reaction was studied to evaluate the feasibility and performance of the presented method. The L- and D-LDH chiral enzymatic reactions, which are highly stereoselective to the lactate enantiomers, were initiated successively in one capillary, and the corresponding products, nicotinamide adenine dinucleotide (NADH), were online discriminated and detected by UV absorption. Excellent linear dependence of the two NADH peak intensities on the concentration of the corresponding lactate enantiomers was obtained within a wide range of 0.1-10 mM. The limit of detection was 26 μM for D-lactate and 49 μM for L-lactate (S/N = 3). Good repeatability of online chiral discrimination was demonstrated with relative standard deviation (RSD) < 6.3% for NADH peak height and RSD < 1.5% for migration time (n = 5). K(m) values for L- and D-lactate were measured and were consistent with those of the off-line enzyme assays. The presented method was successfully applied to determine the L-/D-lactate in several yogurt and wine samples. Our study shows a new application of the EMMA method utilizing high stereoselectivity of enzymes for fast online chiral analysis.

L(+) and D(-) lactate are increased in plasma and urine samples of type 2 diabetes as measured by a simultaneous quantification of L(+) and D(-) lactate by reversed-phase liquid chromatography tandem mass spectrometry

BACKGROUND

Plasma and urinary levels of D-lactate have been linked to the presence of diabetes. Previously developed techniques have shown several limitations to further evaluate D-lactate as a biomarker for this condition.

METHODS

D- and L-lactate were quantified using ultraperformance liquid chromatography tandem mass spectrometry with labelled internal standard. Samples were derivatized with diacetyl-L-tartaric anhydride and separated on a C(18)-reversed phase column. D- and L-lactate were analysed in plasma and urine of controls, patients with inflammatory bowel disease (IBD), and patients with type 2 diabetes (T2DM).

RESULTS

Quantitative analysis of D- and L-lactate was achieved successfully. Calibration curves were linear (r(2) > 0.99) over the physiological and pathophysiological ranges. Recoveries for urine and plasma were between 96% and 113%. Inter- and intra-assay variations were between 2% and 9%. The limits of detection of D-lactate and L-lactate in plasma were 0.7 μmol/L and 0.2 μmol/L, respectively. The limits of detection of D-lactate and L-lactate in urine were 8.1 nmol/mmol creatinine and 4.4 nmol/mmol creatinine, respectively. Plasma and urinary levels of D- and L-lactate were increased in patients with IBD and T2DM as compared with controls.

CONCLUSION

The presented method proved to be suitable for the quantification of D- and L-lactate and opens the possibility to explore the use of D-lactate as a biomarker.