Metals/bisulfite system involved generation of 24-sulfonic-25-ene ginsenoside Rg1, a potential quality control marker for sulfur-fumigated ginseng.

Ginseng is a most popular health-promoting food with ginsenosides as its main bioactive ingredients. Illegal sulfur-fumigation causes ginsenosides convert to toxic sulfur-containing derivatives, and reduced the efficacy/safety of ginseng. 24-sulfo-25-ene ginsenoside Rg1 (25-ene SRg1), one of the sulfur-containing derivatives, is a potential quality control marker of fumigated ginseng, but with low accessibility owing to its unknown generation mechanism. In this study, metals/bisulfite system involved generation mechanism was investigated and verified. The generation of 25-ene SRg1 in sulfur-fumigated ginseng is that SO2, formed during sulfur-fumigation, reacted with water and ionized into HSO3-. On the one hand, under the metals/bisulfite system, HSO3- generates HSO5- and free radicals which converted ginsenoside Rg1 to 24,25-epoxide Rg1; on the other hand, as a nucleophilic group, HSO3- reacted with 24,25-epoxide Rg1 and further dehydrated to 25-ene SRg1. This study provided a technical support for the promotion of 25-ene SRg1 as the characteristic quality control marker of sulfur-fumigated ginseng.

Sulfites detection by surface-enhanced Raman spectroscopy: A feasibility study.

The exhaustive control required for the correct wine production needs of many chemical analysis throughout the process. The most extended investigations for wine production control are focused on the quantification of total and free SO2. Most methods described in the literature have an adequate detection limit, but they usually lack reproducibility and require a previous sample treatment for the extraction of the SO2 from the wine-matrix. In this context, Surface-Enhanced Raman Spectroscopy (SERS) can be a promising technique for free SO2 determination without the need for any sample pre-processing. This work describes a proof of concept of a new methodology based on SERS and supported by Density Functional Theory (DFT) calculations to identify the active vibrational modes of the key molecules that contribute to the concentration of free SO2 in solution. Theoretical predictions and experimental outcomes are brought together to chemometrics to get a simple and real-time free SO2 monitoring. This general procedure could pave the way towards an implementation of a portable SERS detection module for in-field measurements.

Multi-well plate as headspaces for paper-based colorimetric detection of sulfur dioxide gas: An alternative method of sulfite titration for determination of formaldehyde.

This work describes the analysis of formaldehyde using a 96-well microplate as multiple headspaces for the separation of sulfur dioxide gas generated from the sulfite remaining after its reaction with the formaldehyde in the sample. The quantitation of the gas is by colorimetric detection of an indicator paper placed over the microplate. The samples are aqueous extracts of various foods that are possibly adulterated with formaldehyde. A known excess amount of sulfite is added to the extract solution aliquoted in the well. The remaining sulfite is acidified with hydrochloric acid to generate sulfur dioxide gas which diffuses through the headspace above the solution to be absorbed at the moist strip of the indicator paper placed over the mouth of the wells. Anthocyanins extracted from the butterfly pea flower is used as the pH indicator giving a color change from the increase of hydrogen ions by hydrolysis of the absorbed sulfur dioxide gas. The exposed paper strip is scanned, and the digital images of the colored region analyzed using ImageJ software. The optimized method has a linear range of 200-1000 mg L-1 formaldehyde with limit of detection ((2.57*SD of intercept)/(slope of calibration line)) of the aqueous extract of 40 mg L-1 and coefficient of determination (r2) > 0.9979. Samples of fresh produce, such as seafood, meat, and vegetables, and various processed food were analyzed for their possible formaldehyde content. The results obtained from the headspace paper-based colorimetric detection are not statistically different from the values obtained from the titration method by paired t-tests.

A fully integrated 3D printed platform for sulfite determination in beverages via gas diffusion membrane extraction and digital video treatment.

In this study, we have described a miniaturized, simple, and low-cost device for sulfite determination in beverages by coupling Gas Diffusion Microextraction to paper-based analytical devices. The color change of an acid-base indicator - promoted by the generated gaseous SO2 - impregnated onto the paper surface was monitored in the function of time by video recording using a smartphone. The analytical information was related to the Hue, Saturation, Value (HSV) color space extracted from the video file. The complete analytical platform was built using a 3D printer, allowing the easy fabrication of a low-cost tailored device. Under optimized conditions, a linear relation from 5 to 90 mg L-1 was obtained using 30 µL of the reagent, 1 mL of sample, and 10 min of analysis. The relative standard deviation and the limit of detection were 2.2 % and 1.6 mg L-1, respectively. The method was successfully employed in several beverages, such as juices, soda, and coconut water.

Baseline composition and microbial quality assessment of raw milk from small ruminants and Maghrebi camels in the oasis area of Tunisia.

The milk quality and characteristics of the local Gharbi sheep and autochthonous goat population were studied and compared to those of the local Maghrebi camel. Milk samples from 378 lactating animals raised in the Tunisian oasis region were obtained and processed for various physicochemical compositions (pH, density, acidity, dry matter, fat, protein, lactose, casein, ash, and casein-protein ratio), mineral concentrations (Ca, P, Na, and K), and bacteriological properties (total mesophilic aerobic bacteria (TMAB), total coliform count (TCC), lactic acid bacteria (LAB), sulfite-reducing Clostridium (CSR), yeast and molds (Y/M), Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and Salmonella) using standard methods. Milk from sheep breeds had a higher average of all physical parameters (pH, density, and dornic acidity) than milk from goat species. The sheep population produced milk with a similar pH to the camel population, but with a higher density and acidity content. The pH and acidity were higher in Neggas than in goat species, while density was similar in both. For chemical composition, the results showed significant heterogeneity in milk content across all species. Except for the casein-protein ratio, which favors goat species, the analysis indicates that sheep species were superior to populations of goats and camels in all chemical compositions. The present results showed considerable variation in the mineral content of milk from different species. The levels of calcium and phosphorus are higher in sheep than in goat and camel milk. Compared to small ruminants, milk from camels is the richest in Na and K. Additionally, more calcium is present in the milk of camels than that of goats. Goat milk, the lowest in Ca and Na, contains more P than camel milk and more K than sheep's milk on average. The poorest microbial quality was that of camel milk for all bacterial counts. Based on TMAB, TCC, and E. coli counts, the microbiological quality of goat milk was higher than that of ovine milk, while ovine milk had better quality based on LAB, Y/M, and S. aureus values. For Escherichia coli and Staphylococcus aureus, there were no significant variations between the species studied. Results showed that all milk samples studied were completely free of two dangerous pathogens, Salmonella and sulfite-reducing Clostridium. The bacteriological quality of small ruminant's milk was acceptable and met the regulatory limits set by Tunisian dairy legislation. Regarding camel milk, the microbial analysis revealed poor quality that exceeds standard criteria.

Reagent-less and sub-minute quantification of sulfite in food samples using substrate-integrated hollow waveguide gas sensors coupled to deep-UV LED.

The increasing consumption of processed foods demands the usage of chemical preservatives to ensure freshness and extended shelf life. For this purpose, sodium sulfite and its derivatives have been widely used in a variety of food products to inhibit microbial spoilage and for mitigating oxidative decay. However, the excessive consumption of sulfite may cause health problems, thus requiring rapid and accurate analytical methods for the rapid identification of threshold levels. Conventionally, sulfite is volatilized from food samples by acidification followed by trapping of the gaseous SO2 and determination using a suitable analytical technique. Herein, we propose a yet unprecedented reagent-less approach via direct absorbance measurements of gaseous SO2 at 280 nm after sample acidification. The detection system combines a deep-UV LED and a SiC photodiode with a substrate-integrated hollow waveguide (iHWG) gas cell. Absorbance measurements were performed using a log-ratio amplifier circuitry, resulting in noise levels <0.7 mAU. This innovative concept enabled the determination of sulfite in beverages in the range of 25-1000 mg L-1 with suitable linearity (r2 > 0.99) and an analysis time <30 s. The limit of detection (LOD) was calculated at 14.3 mg L-1 (3σ) with an iHWG providing an optical path length of 75 mm. As a proof of concept, this innovative analytical platform was employed for sulfite quantification in concentrated grape juice, coconut water and beer, with suitable accuracy in terms of recovery (83-117%) and favorable comparison with the official Monier-Williams method. Given the inherent modularity and adaptability of the device concept, we anticipate the application of the proposed analytical platform for the in-situ studies addressing sulfite and other volatilized preservatives in a wide variety of food products with tailorable detectability.

Desulfoluna limicola sp. nov., a sulfate-reducing bacterium isolated from sediment of a brackish lake.

A novel sulfate-reducing bacterium, strain ASN36T, was isolated from sediment of a brackish lake in Japan. Cells of strain ASN36T were not motile and rod-shaped, with length of 2.0-4.9 µm and width of 0.6-0.9 µm. Growth was observed at 5-35 °C with an optimum growth temperature of 25-30 °C. The pH range for growth was 6.6-8.8 with an optimum pH of 7.3. Major fatty acids were C16:1 ω7c and C16:0. Under sulfate-reducing conditions, strain ASN36T utilized lactate, malate, pyruvate, butyrate, ethanol, butanol, glycerol, yeast extract and H2/CO2 as growth substrate. Fermentative growth occurred on malate and pyruvate. The novel isolate used sulfate, sulfite and thiosulfate as electron acceptors. The genome of strain ASN36T is composed of a chromosome with length of 6.3 Mbp and G + C content of 55.1 mol%. Analyses of the 16S rRNA gene indicated that strain ASN36T is related to Desulfoluna species. Overall genome relatedness indices indicated that strain ASN36T does not belong to any existing species. In contrast to the closest relatives, strain ASN36T lacks genes for reductive dehalogenase required for organohalide respiration and does not use halogenated aromatics as electron acceptors. On the basis of its genomic and phenotypic properties, strain ASN36T (= DSM 111985 T = JCM 39257 T) is proposed as the type strain of a new species, Desulfoluna limicola sp. nov.

Desulfofustis limnaeus sp. nov., a freshwater sulfate-reducing bacterium isolated from marsh soil.

A novel sulfate-reducing bacterium, strain PPLLT, was isolated from marsh soil. Cells of strain PPLLT were rod-shaped with length of 1.5 µm and width of 0.7 µm. Growth was observed at 22-37 °C (optimum 35 °C) and pH 6.8-8.4 (optimum 7.3). Lactate, succinate, fumarate, formate and malate were utilized as electron donors for sulfate reduction. Fermentative growth was not observed on tested organic acids. Besides sulfate, sulfite, thiosulfate and elemental sulfur were utilized as electron acceptors. Hydrogen is used only in the presence acetate or yeast extract. The major fatty acid was C16:0. The complete genome of strain PPLLT was composed of a circular chromosome with length of 4.2 Mbp and G + C content of 57.7 mol%. Sequence analysis of the 16S rRNA gene showed that strain PPLLT was affiliated with the genus Desulfofustis in the family Desulfocapsaceae. On the basis of differences in the phylogenetic and phenotypic properties between the strain and the type strain of the genus Desulfofustis, strain PPLLT (DSM 110475T = JCM 39161T) is proposed as the type strain of a new species, with name of Desulfofustis limnaeus sp. nov.

Methyl Salicylate, an Odor-Active Compound in Bordeaux Red Wines Produced without Sulfites Addition.

Red wines produced without the addition of any SO2 are currently the source of a new consumer trend. The first characterization approaches regarding these specific wines were devoted to sensory studies that highlighted differences according to the use of SO2 during winemaking. The goal of this paper is to extend our knowledge of such aromatic specificities. Examining experimental wines produced with and without the addition of SO2, aroma fractionation, gas chromatography coupled with olfactometry, and mass spectrometry were applied to identify compounds at the origin of the specificities of these wines. Thus, we identified methyl salicylate as being impacted by the use of SO2 during the winemaking process. Studying a large number of commercial Bordeaux red wines, methyl salicylate was significantly quantified at a higher content in wines without added SO2. A sensory approach revealed a significant impact of methyl salicylate on wines without the sulfite aroma, particularly concerning fruity aromas and wine freshness.

Amperometric Sensor for Selective On-Site Analysis of Free Sulfite in Wines.

Accurate and rapid on-site analysis of free SO2 content is crucial in the process of winemaking from a producer and consumer perspective. Herein, we present an amperometric sensor based on commercially available screen-printed electrodes coupled with an electrochemical oxygen filter. The developed amperometric method gave a linear response in a concentration range up to 200 mg L-1 with a limit of quantification of 7.5 mg L-1. The applicability of the developed sensor was successfully tested on 27 white and red wine samples and compared to the Ripper method (iodometry) that is a standard procedure for free SO2 determination. The sensor exhibits similar precision and accuracy but shows no interference from oxidizable species such as ascorbic acid, which is a major advantage over iodometric titration. The performance of the sensor was in addition positively evaluated during on-site analysis in a winery.

Simple and sensitive determination of sulfites in Chinese herbal teas by ultrahigh-performance liquid chromatography tandem mass spectrometry.

Sulfites are used widely in food and beverage production to prevent browning or oxidation. However, the overingestion of sulfites is harmful to human health and may cause medical complications. Chinese herbal teas have been widely consumed for centuries. However, sulfite levels in Chinese herbal teas are rarely investigated and reported. Here, we present a simple, sensitive, and quantitative method to determine sulfites in Chinese herbal teas using ultrahigh-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) coupled with dispersive solid phase extraction. The method utilized a SeQuant ZIC-HILIC column for separation, and the optimal gradient eluents consisted of acetonitrile and aqueous solution with 0.1% acetic acid and 10 mM ammonium acetate. Porous chitosan/partially reduced graphene oxide/diatomite (CS/prGO/DM) composites were used as efficient dispersive solid phase extraction adsorbents for sample preparation. Several parameters were investigated during the extraction process, including sample-to-extraction solvent volume ratios, the extraction procedure and dosage of the adsorbent. Under the optimum conditions, the developed method gave a good determination coefficient (r2 > 0.99), low detection limits (0.51-12.1 µg kg-1) and high recoveries in the range of 83.8-102.7% at different spiked levels. The method has the great advantages of being time saving, good reproducibility and much lower detection limits when compared to titration methods. The method was further applied to analyze real herbal tea samples collected from the local market, demonstrating that our developed method is robust and useful for determining sulfites in practical application.

A mitochondria-targeted near-infrared fluorescent probe for detection and imaging of HSO3- in living cells.

Sulfur dioxide, an essential gas signaling molecule mainly produced in mitochondria, plays important roles in many physiological and pathological processes. Herein, a near-infrared fluorescent probe, A1, with good mitochondria targeting ability was developed for colorimetric and fluorescence detection of HSO3-. Probe A1 has a conjugated cyanine structure that can selectively react with HSO3- through the nucleophilic addition. The reaction with HSO3- destroys the conjugated structure of probe A1, resulting in fluorescence quenching, and accompaniedby color change of probe A1 solution from purple-red to colorless. Probe A1 showed high selectivity and good sensitivity to HSO3- in PBS. And the limit of detection was calculated to be 1.28 and 0.037 µM for colorimetry and fluorescence spectrophotometry respectively. In addition, probe A1 mainly entered the mitochondria in living cells, and was successfully used for imaging the exogenous/endogenous HSO3- in cells. These results suggest the potential applications of probe A1 in biological systems.

Disposable Microchamber with a Microfluidic Paper-Based Lid for Generation and Membrane Separation of SO2 Gas Employing an In Situ Electrochemical Gas Sensor for Quantifying Sulfite in Wine.

This work presents a fully disposable microchamber for gas generation of a sample solution. The microchamber consists of a cylindrical well-reactor and a paper-based microfluidic lid (µFluidic lid), which also serves as the reagent loading and dispensing unit. The base of the reactor consists of a hydrophobic membrane covering an in-house graphene electrochemical gas sensor. Fabrication of the gas sensor and the three-layer µFluidic lid is described. The µFluidic lid is designed to provide a steady addition of the acid reagent into the sample solution instead of liquid drops from a disposable syringe. There are three steps in the procedure: (i) acidification of the sample in the reactor to generate SO2 gas by the slow dispensing of the acid reagent from the µFluidic lid, (ii) diffusion of the liberated SO2 gas through the hydrophobic membrane at the base of the reactor, and (iii) in situ detection of SO2 by cathodic reduction at the graphene electrode. The device was demonstrated for quantitation of the sulfite preservative in wine without heating or stirring. The selectivity of the analysis is ensured by the combination of the gas-diffusion membrane and the selectivity of the electrochemical sensor. The linear working range is 2-60 mg L-1 SO2, with a limit of detection (3SD of intercept/slope) of 1.5 mg L-1 SO2. This in situ method has the shortest analysis time (8 min per sample) among all voltammetric methods that detect SO2(g) via membrane gas diffusion.

A quinoline-based probe for the ratiometric fluorescent detection of sulfite in lysosomes of living cells.

A lysosome-targeting ratiometric fluorescent probe was synthesized for detecting sulfite based on sulfite-triggered nucleophilic addition reaction. Due to the specific reaction, the fluorescence intensity ratio (I530/I390) of the probe in an almost aqueous solution (0.5% DMSO) changed significantly after the addition of HSO3-, corresponding to the change in the fluorescence color of the solution from green to blue. The recognition was conducted using high-resolution mass spectrometry, proton nuclear magnetic resonance, and density functional theory calculations. The fluorescent probe could be utilized to quantitatively monitor HSO3- in lysosomes of living C6 glioma cells and real-water samples.

Induced sulfur metabolism by sulfur dioxide maintains postharvest quality of 'Thompson Seedless' grape through increasing sulfite content.

BACKGROUND: The commercial preservation of table grapes largely depends on the application of sulfur dioxide (SO2 ). However, little is known about whether SO2 participates in sulfur metabolism to improve the postharvest quality of table grapes. In this study, the contents of sulfur-containing compounds, activities of enzymes, and expression of genes involved in sulfur metabolism in table grapes (Vitis vinifera cv. Thompson Seedless) were evaluated. RESULTS: The results indicated that SO2 treatment maintained the postharvest quality of table grapes. The sulfite content in rachises and berries, but not the sulfate content, increased in response to SO2 treatment. SO2 caused high activities of sulfite reductase, O-acetylserine (thiol)-lyase, and γ-glutamylcysteine synthetase, thereby increasing the contents of cysteine, hydrogen sulfide, and glutathione in the rachises and berries. The expression of VvSURTL, VvATPS1, VvATPS2, and VvAPR3 decreased in response to SO2 treatment; however, the transcript levels of VvSiR1 and VvOASTL exhibited the opposite tendency. CONCLUSION: These findings indicated that the sulfite converted from SO2 participated in sulfur metabolism and maintained the postharvest quality of table grapes by modulating the contents of metabolites, activities of enzymes, and expression of genes related to sulfur metabolism. © 2021 Society of Chemical Industry.

Solid-state and solution characterizations of [(TMPA)Cu(II)(SO3)] and [(TMPA)Cu(II)(S2O3)] complexes: Application to sulfite and thiosulfate fast detection.

Sulfite (SO32-) and thiosulfate (S2O32-) ions are used as food preservative and antichlor agent respectively. To detect low levels of such anions we used Cu(II) complex of the Tris-Methyl Pyridine Amine (TMPA) ligand, denoted L. Formation of [LCu(SO3)] (1) and [LCu(S2O3)] (2) in solution were monitored using UV-Vis, EPR and cyclic voltammetry, while the solid-state X-ray structures of both complexes were solved. In addition, we also evaluated the pH range in which the complexes are stable, and the anions binding affinity values for the [LCu(solvent)]2+ (3) parent complex. As a matter of illustration, we determined the sulfite content in a commercial crystal sugar.

A Novel Fluorescence Probe for the Reversible Detection of Bisulfite and Hydrogen Peroxide Pair in Vitro and in Vivo.

The detection of changes in the reactive oxygen species (ROS)/reactive sulfur species (RSS) couple is important for studying the cellular redox state. Herein, we developed a 1,8-naphthalimide-based fluorescence probe (NI) for the reversible detection of bisulfite (HSO3 - ) and hydrogen peroxide (H2 O2 ) in vitro and in vivo. NI has been designed with a reactive ethylene unit which specifically reacts with HSO3 - by a Michael addition reaction mechanism, resulting in the quenching of yellow fluorescence at 580 nm and the appearing of green fluorescence at 510 nm upon excitation at 500 nm and 430 nm, respectively. The addition product (NI-HSO3 ) could be specifically oxidized to form the original C=C bond of NI, recovering the fluorescence emission and color. The detection limits of NI for HSO3 - and NI-HSO3 for H2 O2 were calculated to be 2.05 µM and 4.23 µM, respectively. The reversible fluorescence response of NI towards HSO3 - /H2 O2 couple can be repeated for at least five times. NI is reliable at a broad pH range (pH 3.0-11.5) and features outstanding selectivity, which enabled its practical applications in biological and food samples. Monitoring the reversible and dynamic inter-conversion between HSO3 - and H2 O2 in vitro and in vivo has been verified by fluorescence imaging in live HeLa cells, adult zebrafish and nude mice. Moreover, NI has been successfully applied to detect of HSO3 - levels in food samples.

Sulfate transport mutants affect hydrogen sulfide and sulfite production during alcoholic fermentation.

Hydrogen sulfide is a common wine fault, with a rotten-egg odour, which is directly related to yeast metabolism in response to nitrogen and sulfur availability. In grape juice, sulfate is the most abundant inorganic sulfur compound, which is taken up by yeast through two high-affinity sulfate transporters, Sul1p and Sul2p, and a low affinity transporter, Soa1p. Sulfate contributes to H2 S production under nitrogen limitation, by being reduced via the Sulfur Assimilation Pathway (SAP). Therefore, yeast strains with limited H2 S are highly desirable. We report on the use of toxic analogues of sulfate following ethyl methane sulfate treatment, to isolate six wine yeast mutants that produce no or reduced H2 S and SO2 during fermentation in synthetic and natural juice. Four amino acid substitutions (A99V, G380R, N588K and E856K) in Sul1p were found in all strains except D25-1 which had heterozygous alleles. Two changes were also identified in Sul2p (L268S and A470T). The Sul1p (G380R) and Sul2p (A470T) mutations were chosen for further investigation as these residues are conserved amongst SLC26 membrane proteins (including sulfate permeases). The mutations were introduced into EC1118 using Crispr cas9 technology and shown to reduce accumulation of H2 S and do not result in increased SO2 production during fermentation of model medium (chemically defined grape juice) or Riesling juice. The Sul1p (G380R) and Sul2p (A470T) mutations are newly reported as causal mutations. Our findings contribute to knowledge of the genetic basis of H2 S production as well as the potential use of these strains for winemaking and in yeast breeding programmes.

A novel ratiometric fluorescent probe for differential detection of HSO3- and ClO- and application in cell imaging and tumor recognition.

By connecting 1,8-naphthalimide and indole sulfonate, a ratio fluorescent probe capable of differential detection of hydrogen sulfite and hypochlorite was synthesized for the first time. It was able to achieve the qualitative detection of HSO3- and ClO- with high sensitivity and selectivity, respectively. It provides a multi-purpose probe and is based on different emission channels without mutual interference. The probe has the advantages of larger Stokes shift (ClO-: 115 nm, HSO3-: 88 nm), longer λem (ClO-: 515 nm, HSO3-: 548 nm) and better water solubility (DMF/PBS = 1:99, v/v). In addition, the probe is a ratio fluorescence probe, which can detect fluorescence intensity with two different emission waves. It provides internal self-calibration, reduces interference from the background and increases detection accuracy. In vitro cytotoxicity and imaging experiments show that the probe can effectively perform the detection of exogenous HSO3- and ClO- in cells. It can also achieve the detection of HSO3- and ClO- in the plasma environment. Because the probe can detect endogenous ClO-, it also has a good prospect for biological application in identifying tumor cells. Graphical abstract.

High-Specific Fluorescence Probe for SO32- Detection and Bioimaging.

It is well known that sulfite (SO32-) plays an indispensable role in various physiological processes. Abnormal levels of SO32- can trigger a wide variety of diseases involving respiratory, nervous and cardiovascular systems. Hence, it is necessary to find an efficient approach for detection of SO32-. In this study, a pyrene derivative, (E)-4-(3-oxo-3-(pyren-1-yl)prop-1-en-1-yl)phenyl acrylate (PPA), was designed and synthesized for monitoring SO32-. The probe possessed simple synthetic steps, excellent anti-interference ability and specific response to SO32- in the presence of other substances. The reaction between PPA and SO32- was ascribed to Michael addition and the detection mechanism was confirmed by HRMS spectra analysis and FTIR analysis. Additionally, PPA responded linearly to detect SO32- within the rang of 0-100 µM. The limit of detection was calculated as low as 0.17 µM in accordance with the recommendation of IUPAC (CDL =3sb/m). Notably, PPA was further applied in biological imaging in HepG2 cells, which provided a possibility to monitor SO32- in vivo.