In humans, sulfite is generated endogenously by the metabolism of sulfur containing amino acids such as methionine and cysteine. Sulfite is also formed from exposure to sulfur dioxide, one of the major environmental pollutants. Sulfite is used as an antioxidant and preservative in dried fruits, vegetables, and beverages such as wine. Sulfite is also used as a stabilizer in many drugs. Sulfite toxicity has been associated with allergic reactions characterized by sulfite sensitivity, asthma, and anaphylactic shock. Sulfite is also toxic to neurons and cardiovascular cells. Recent studies suggest that the cytotoxicity of sulfite is mediated by free radicals; however, molecular mechanisms involved in sulfite toxicity are not fully understood. Cytochrome c (cyt c) is known to participate in mitochondrial respiration and has antioxidant and peroxidase activities. Studies were performed to understand the related mechanism of oxidation of sulfite and radical generation by ferric cytochrome c (Fe3+ cyt c) in the absence and presence of H2O2. Electron paramagnetic resonance (EPR) spin trapping studies using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were performed with sulfite, Fe3+ cyt c, and H2O2. An EPR spectrum corresponding to the sulfite radical adducts of DMPO (DMPO-SO3-) was obtained. The amount of DMPO-SO3- formed from the oxidation of sulfite by the Fe3+ cyt c increased with sulfite concentration. In addition, the amount of DMPO-SO3- formed by the peroxidase activity of Fe3+ cyt c also increased with sulfite and H2O2 concentration. From these results, we propose a mechanism in which the Fe3+ cyt c and its peroxidase activity oxidizes sulfite to sulfite radical. Our results suggest that Fe3+ cyt c could have a novel role in the deleterious effects of sulfite in biological systems due to increased production of sulfite radical. It also shows that the increased production of sulfite radical may be responsible for neurotoxicity and some of the injuries which occur to humans born with molybdenum cofactor and sulfite oxidase deficiencies (1)

Sulfur dioxide (SO2), which is used as food preservative in apricot sulfurization and several fabricated foods, is a common air pollutant. The aim of this study was to reveal the possible genotoxic effects of SO2 using in vitro human lymphocytes. The different endpoints of genotoxicity: sister chromatid exchange (SCE), micronuclei (MN) tests and cell growth kinetics such as mitotic index (MI) and replication index (RI) were studied. The cells were treated with 0.1, 0.5 and 1.0 ppm concentrations of SO2. It was shown that SO2 caused significant increases in the frequency of SCE and MN in the middle and high dosage groups and also induced mitotic delays and decreased MI and RI. In conclusion, the results have confirmed that SO2 has potent mutagenicity and it can cause genetic damage leading to a malignancy (2).

Due to the contraindications of sulfur dioxide, solutions have been sought that can replace it (3).

Other studies

Vally H, Misso NL. Adverse reactions to the sulphite additives. Gastroenterol Hepatol Bed Bench. 2012 Winter;5(1):16-23.

Abstract. Sulphites are widely used as preservative and antioxidant additives in the food and pharmaceutical industries. Exposure to sulphites has been reported to induce a range of adverse clinical effects in sensitive individuals, ranging from dermatitis, urticaria, flushing, hypotension, abdominal pain and diarrhoea to life-threatening anaphylactic and asthmatic reactions. Exposure to the sulphites arises mainly from the consumption of foods and drinks that contain these additives; however exposure may also occur through the use of pharmaceutical products, as well as in occupational settings. Most studies report a prevalence of sulphite sensitivity of 3 to 10% among asthmatic subjects who ingest these additives. However, the severity of these reactions varies, and steroid-dependent asthmatics, those with marked airway hyperresponsiveness, and children with chronic asthma, appear to be at greater risk. Although a number of potential mechanisms have been proposed, the precise mechanisms underlying sulphite sensitivity remain unclear.

Sulfur Dioxide Activates Cl-/HCO3 - Exchanger via Sulphenylating AE2 to Reduce Intracellular pH in Vascular Smooth Muscle Cells.
Wang Y, Wang X, Chen S, Tian X, Zhang L, Huang Y, Tang C, Du J, Jin H.
Front Pharmacol. 2019 Mar 27;10:313. doi: 10.3389/fphar.2019.00313. eCollection 2019

Abstract. Sulfur dioxide (SO2) is a colorless and irritating gas. Recent studies indicate that SO2 acts as the gas signal molecule and inhibits vascular smooth muscle cell (VSMC) proliferation. Cell proliferation depends on intracellular pH (pHi). Transmembrane cystein mutation of Na+- independent Cl-/HCO3 - exchanger (anion exchanger, AE) affects pHi. However, whether SO2 inhibits VSMC proliferation by reducing pHi is still unknown. Here, we investigated whether SO2 reduced pHi to inhibit the proliferation of VSMCs and explore its molecular mechanisms. Within a range of 50-200 μM, SO2 was found to lower the pHi in VSMCs. Concurrently, NH4Cl pre-perfusion showed that SO2 significantly activated AE, whereas the AE inhibitor 4,4'-diisothiocyanatostilbene- 2,20-disulfonic acid (DIDS) significantly attenuated the effect of SO2 on pHi in VSMCs. While 200 μM SO2 sulphenylated AE2, while dithiothreitol (DTT) blocked the sulphenylation of AE2 and subsequent AE activation by SO2, thereby restoring the pHi in VSMCs. Furthermore, DIDS pretreatment eliminated SO2-induced inhibition of PDGF-BB-stimulated VSMC proliferation. We report for the first time that SO2 inhibits VSMC proliferation in part by direct activation of the AE via posttranslational sulphenylation and induction of intracellular acidification.

Post-synthetic modulation of the charge distribution in a metal-organic framework for optimal binding of carbon dioxide and sulfur dioxide.
Li L, da Silva I, Kolokolov DI, Han X, Li J, Smith G, Cheng Y, Daemen LL, Morris CG, Godfrey HGW, Jacques NM, Zhang X, Manuel P, Frogley MD, Murray CA, Ramirez-Cuesta AJ, Cinque G, Tang CC, Stepanov AG, Yang S, Schroder M.
Chem Sci. 2018 Oct 31;10(5):1472-1482. doi: 10.1039/c8sc01959b. eCollection 2019 Feb 7

Abstract. Modulation of pore environment is an effective strategy to optimize guest binding in porous materials. We report the post-synthetic modification of the charge distribution in a charged metal-organic framework, MFM-305-CH3, [Al(OH)(L)]Cl, [(H2L)Cl = 3,5-dicarboxy-1-methylpyridinium chloride] and its effect on guest binding. MFM-305-CH3 shows a distribution of cationic (methylpyridinium) and anionic (chloride) centers and can be modified to release free pyridyl N-centres by thermal demethylation of the 1-methylpyridinium moiety to give the neutral isostructural MFM-305. This leads simultaneously to enhanced adsorption capacities and selectivities (two parameters that often change in opposite directions) for CO2 and SO2 in MFM-305. The host-guest binding has been comprehensively investigated by in situ synchrotron X-ray and neutron powder diffraction, inelastic neutron scattering, synchrotron infrared and 2H NMR spectroscopy and theoretical modelling to reveal the binding domains of CO2 and SO2 in these materials. CO2 and SO2 binding in MFM-305-CH3 is shown to occur via hydrogen bonding to the methyl and aromatic-CH groups, with a long range interaction to chloride for CO2. In MFM-305 the hydroxyl, pyridyl and aromatic C-H groups bind CO2 and SO2 more effectively via hydrogen bonds and dipole interactions. Post-synthetic modification via dealkylation of the as-synthesised metal-organic framework is a powerful route to the synthesis of materials incorporating active polar groups that cannot be prepared directly.

Comparison and optimization of three commercial methods with an LC-MS/MS method for the determination of sulfites in food and beverages.
Carlos KS, Treblin M, de Jager LS.
Food Chem. 2019 Jul 15;286:537-540. doi: 10.1016/j.foodchem.2019.02.042. 

Abstract. Sulfites are food additives found in a large variety of food products to help reduce oxidation and browning. A small subset of the population has "allergic-like" symptoms upon consumption of sulfite containing foods. There are several sulfite detection methods available in the literature that vary in analysis time and required instrumentation. There are also rapid screening tests available that have not been tested extensively for accuracy and precision. In this study, four different methods (an LC-MS/MS method, a spectrophotometric method, a test strip method and a spot test method) were used to determine the sulfite concentration of 10 different commercially sulfited products. Of these methods, the LC-MS/MS and the spectrophotometric method had the most comparable results. The test strips were only accurate at concentrations greater than 50 mg/kg (ppm) SO2. The spot test method was found to be inaccurate with both standard solutions and samples.

Effect of Sulfites on Antioxidant Activity, Total Polyphenols, and Flavonoid Measurements in White Wine.
Nardini M, Garaguso I.
Foods. 2018 Mar 9;7(3). pii: E35. doi: 10.3390/foods7030035.

Abstract. Polyphenols content and antioxidant activity are directly related to the quality of wine. Wine also contains sulfites, which are added during the winemaking process. The present study aimed to evaluate the effects of sulfites on the assays commonly used to measure the antioxidant activity and polyphenols and flavonoids content of white wines. The effects of sulfites were explored both in the standard assays and in white wine. The addition of sulfites (at 1-10 μg) in the standard assays resulted in a significant, positive interference in the Folin-Ciocalteu's assay used for polyphenols measurements and in both the Ferric Reducing Antioxidant Power and 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt radical cation decolorization assays, which were used for antioxidant activity evaluation. A negative interference of sulfites (at 1-20 μg) was observed for the colorimetric aluminium-chloride flavonoids assay. The addition of sulfites to organic white wines (at 25-200 mg/L wine) clearly resulted in a significant overestimation of antioxidant activity and polyphenols content, and in an underestimation of flavonoids concentration. To overcome sulfite interferences, white wines were treated with cross-linked polyvinylpyrrolidone. The total polyphenols content and antioxidant activity measurements obtained after polyvinylpyrrolidone treatment were significantly lower than those obtained in the untreated wines. Flavonoids were expected to be higher after polyvinylpyrrolidone treatment, but were instead found to be lower than for untreated wines, suggesting that in addition to sulfites, other non-phenolic reducing compounds were present in white wine and interfered with the flavonoid assay. In view of our results, we advise that a purification procedure should be applied in order to evaluate the quality of white wine.

Sulfites inhibit the growth of four species of beneficial gut bacteria at concentrations regarded as safe for food.
Irwin SV, Fisher P, Graham E, Malek A, Robidoux A.
PLoS One. 2017 Oct 18;12(10):e0186629. doi: 10.1371/journal.pone.0186629. 

Abstract. Sulfites and other preservatives are considered food additives to limit bacterial contamination, and are generally regarded as safe for consumption by governmental regulatory agencies at concentrations up to 5000 parts per million (ppm). Consumption of bactericidal and bacteriostatic drugs have been shown to damage beneficial bacteria in the human gut and this damage has been associated with several diseases. In the present study, bactericidal and bacteriostatic effects of two common food preservatives, sodium bisulfite and sodium sulfite, were tested on four known beneficial bacterial species common as probiotics and members of the human gut microbiota. Lactobacillus species casei, plantarum and rhamnosus, and Streptococcus thermophilus were grown under optimal environmental conditions to achieve early log phase at start of experiments. Bacterial cultures were challenged with sulfite concentrations ranging between 10 and 3780 ppm for six hours. To establish a control, a culture of each species was inoculated into media containing no sulfite preservative. By two hours of exposure, a substantial decrease (or no increase) of cell numbers (based on OD600 readings) were observed for all bacteria types, in concentrations of sulfites between 250-500 ppm, compared to cells in sulfite free media. Further testing using serial dilution and drop plates identified bactericidal effects in concentrations ranging between 1000-3780 ppm on all the Lactobacillus species by 4 hours of exposure and bactericidal effects on S. thermophilus in 2000ppm NaHSO3 after 6 hours of exposure.

References_________________________________________________________________________

(1) Sulfite Oxidase Activity of Cytochrome c: Role of Hydrogen Peroxide.

Velayutham M, Hemann CF, Cardounel AJ, Zweier JL.  -  Biochem Biophys Rep. 2016 Mar  Biochem Biophys Rep. 2016 Mar 1;5:96-104.

(2) Genotoxic effects of sulfur dioxide in human lymphocytes.

Uren N, Yuksel S, Onal Y.  - Toxicol Ind Health. 2014 May;30(4):311-5. doi: 10.1177/0748233712457441

(3) Reducing SO₂ in fresh pork burgers by adding chitosan.

Serrano R, Bañón S.  - Meat Sci. 2012 Dec;92(4):651-8. doi: 10.1016/j.meatsci.2012.06.014.