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Basic toxicokinetics

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Endpoint:
basic toxicokinetics in vitro / ex vivo
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
key study
Study period:
no data available
Justification for type of information:
Read-across
The basis for the read-across concept for this project is the equilibrium between sulfites, hydrogensulfites, and metabisulfites in aqueous solutions depending on pH value which is clearly described in published literature and summarised in the following equations:[1],[2]
SO2 + H2O <->`H2SO3´ H2SO3<->H+ + HSO3- <-> 2H+ +SO32- 2HSO3- <->H2O +S2O52 –
As the nature of the cation should make no significant difference in this case concerning toxicity and solubility (all compounds are very soluble in water), only the chemical and biological properties of the anion are considered relevant. Based on the described equilibrium correlations, we propose unrestricted read-across between the groups of sulfites, hydrogensulfites and metabisulfites. Additionally, it is known that sodium dithionite disproportionates in water to form sodium hydrogen sulfite and sodium thiosulfate (equation II) so that this substance can also be added to the read-across concept.[2],[1]
It is expected for this case that the substance is not stable enough under physiological conditions to fulfil the requirements of study guidelines and so the products of decomposition have to be considered.
2 S2O42-+ H2O→2HSO3-+ S2O32 -

All sulfite, hydrogensulfite and metabisulfite substances are highly soluble in water, establishing upon dissolution an equilibrium that depends on solution pH as follows: ,

1. SO2 + H2O <-> H2SO3
2. H2SO3 <-> H+ + HSO3- <-> 2H+ + SO32-
3. 2 HSO3- <-> H2O + S2O52-

Under oxidising conditions, e.g., in surface waters, sulfite is oxidized to sulfate catalytically by (air) oxygen or by microbial action. A half-life of 77 hour was measured in deionized water, already suggesting substantial abiotic degradation. However, the presence of metal cations in the environment, such as copper, iron and manganese, accelerates the oxidation rate. In soils, HSO3- and SO32- ions are unstable and quickly oxidise. Further, because of the instability of SO32-, metal sulfites are generally too soluble to persist in soils. Thus, the most stable and predominant sulfur form in freshwater and in all but highly reduced environments is sulfate (SO42-). In highly reduced soils and sediments, sulfites may be reduced to sulfides (Lindsay, 1979; OECD SIDS, 2012).

Only the properties of the sulfite anion are considered relevant determinants of environmental toxicity since respective cations, i.e. ammonium, calcium, magnesium, sodium and potassium, are not assumed to contribute substantially to differences therein. Sulfite, although naturally present in the environment and also a metabolite and intermediate of sulfur-containing amino acids in organisms, may have an impact on the environment at elevated levels. Sulfites do not bioaccumulate.

In sum, unrestricted read-across between the sulfites, hydrogensulfites and metabisulfites is justified.
Cross-referenceopen allclose all
Reason / purpose for cross-reference:
reference to same study
Reference
Endpoint:
basic toxicokinetics in vitro / ex vivo
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
key study
Study period:
no data available
Justification for type of information:
Read-across
The basis for the read-across concept for this project is the equilibrium between sulfites, hydrogensulfites, and metabisulfites in aqueous solutions depending on pH value which is clearly described in published literature and summarised in the following equations:[1],[2]
SO2 + H2O <->`H2SO3´ H2SO3<->H+ + HSO3- <-> 2H+ +SO32- 2HSO3- <->H2O +S2O52 –
As the nature of the cation should make no significant difference in this case concerning toxicity and solubility (all compounds are very soluble in water), only the chemical and biological properties of the anion are considered relevant. Based on the described equilibrium correlations, we propose unrestricted read-across between the groups of sulfites, hydrogensulfites and metabisulfites. Additionally, it is known that sodium dithionite disproportionates in water to form sodium hydrogen sulfite and sodium thiosulfate (equation II) so that this substance can also be added to the read-across concept.[2],[1]
It is expected for this case that the substance is not stable enough under physiological conditions to fulfil the requirements of study guidelines and so the products of decomposition have to be considered.
2 S2O42-+ H2O→2HSO3-+ S2O32 -

All sulfite, hydrogensulfite and metabisulfite substances are highly soluble in water, establishing upon dissolution an equilibrium that depends on solution pH as follows: ,

1. SO2 + H2O <-> H2SO3
2. H2SO3 <-> H+ + HSO3- <-> 2H+ + SO32-
3. 2 HSO3- <-> H2O + S2O52-

Under oxidising conditions, e.g., in surface waters, sulfite is oxidized to sulfate catalytically by (air) oxygen or by microbial action. A half-life of 77 hour was measured in deionized water, already suggesting substantial abiotic degradation. However, the presence of metal cations in the environment, such as copper, iron and manganese, accelerates the oxidation rate. In soils, HSO3- and SO32- ions are unstable and quickly oxidise. Further, because of the instability of SO32-, metal sulfites are generally too soluble to persist in soils. Thus, the most stable and predominant sulfur form in freshwater and in all but highly reduced environments is sulfate (SO42-). In highly reduced soils and sediments, sulfites may be reduced to sulfides (Lindsay, 1979; OECD SIDS, 2012).

Only the properties of the sulfite anion are considered relevant determinants of environmental toxicity since respective cations, i.e. ammonium, calcium, magnesium, sodium and potassium, are not assumed to contribute substantially to differences therein. Sulfite, although naturally present in the environment and also a metabolite and intermediate of sulfur-containing amino acids in organisms, may have an impact on the environment at elevated levels. Sulfites do not bioaccumulate.

In sum, unrestricted read-across between the sulfites, hydrogensulfites and metabisulfites is justified.
Reason / purpose for cross-reference:
reference to same study
Reason / purpose for cross-reference:
reference to same study
Objective of study:
other: disappearance from serum or plasma
Qualifier:
no guideline followed
Principles of method if other than guideline:
The hydrate of sulfur dioxide in mammalian plasma and serum was investigated. The longevity of sulfite in contact with mammalian plasma and known components of blood was investigated. The reactivity of sulfite added to serum or plasma in vitro was followed by monitoring the concentration of the sulfite in the reaction mixture.

GLP compliance:
no
Radiolabelling:
no
Species:
rabbit
Strain:
New Zealand White
Sex:
not specified
Details on test animals or test system and environmental conditions:
No details are given.
Route of administration:
other: not applicable
Vehicle:
water
Details on exposure:
Plasma and serum samples were collected from the marginal ear vein of adult rabbits with the aid of a suction device.
Edetate disodium at 2 mg/mL was used as an anticoagulant in the plasma samples.
Duration and frequency of treatment / exposure:
continuously until measurement (after 20 minutes)
Dose / conc.:
1.7 other: nanomols of sulfite
Dose / conc.:
3.6 other: nanomols of sulfite
Dose / conc.:
5.2 other: nanomols of sulfite
No. of animals per sex per dose / concentration:
not applicable
Control animals:
no
Positive control reference chemical:
No positive control substance was tested
Details on study design:
no data
Details on dosing and sampling:
Analysis of sulfite in presence of plasma or serum: To 200 µL of solution containing 40 µL of serum or plasma, approximately 6 microequivalents of hydrochloric acid, and quantities (up to 5.2 nM) of sulfite, 20 µL of PRA and 20 µL of formaldehyde reagent were added. A serum or plasma reagent blank was prepared. The optical density was measured against this blank after 20 minutes at 560 mµ.

in vitro reaction of sulfite with plasma or serum: Serum or plasma was incubated with a known amount of sulfite in a nitrogen atmosphere at 37°C, pH 7.4, using a ratio of 1 volume of serum (or plasma) to 1 volume of buffer and sulfite. These conditions are referred to as standard pH 7.4 incubation conditions. After incubation for the desired period of time, the amount of sulfite remaining in the mixture was analysed according the the procedure described above. This determination indirectly gives the amount of sulfite that "disappeared" from the incubation mixture and which was assumed to have reacted with the serum (or plasma).

Each analysis was replicated six times.
Statistics:
no data
Preliminary studies:
no data
Type:
other: disappearance from serum or plasma
Results:
Sulfite which enters the bloodstream during exposure of a mammal to atmospheric SO2 forms S-sulfonate groups with constituents of the plasma, probably exclusively by sulfitolysis of disulfide groups.
Details on absorption:
no data
Details on distribution in tissues:
no data
Details on excretion:
no data
Metabolites identified:
no
Details on metabolites:
No details are given.
Conclusions:
The results give evidence that sulfite which enters the bloodstream during exposure of a mammal to atmospheric SO2 forms S-sulfonate groups with constituents of the plasma, probably exclusively by sulfitolysis of disulfide groups.
This interaction probably affords protection to many tissues of the body from the insult of high concentrations of sulfite.
Reason / purpose for cross-reference:
reference to same study
Reference
Endpoint:
basic toxicokinetics in vivo
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
key study
Study period:
no data available
Justification for type of information:
Read-across
The basis for the read-across concept for this project is the equilibrium between sulfites, hydrogensulfites, and metabisulfites in aqueous solutions depending on pH value which is clearly described in published literature and summarised in the following equations:[1],[2]
SO2 + H2O <->`H2SO3´ H2SO3<->H+ + HSO3- <-> 2H+ +SO32- 2HSO3- <->H2O +S2O52 –
As the nature of the cation should make no significant difference in this case concerning toxicity and solubility (all compounds are very soluble in water), only the chemical and biological properties of the anion are considered relevant. Based on the described equilibrium correlations, we propose unrestricted read-across between the groups of sulfites, hydrogensulfites and metabisulfites. Additionally, it is known that sodium dithionite disproportionates in water to form sodium hydrogen sulfite and sodium thiosulfate (equation II) so that this substance can also be added to the read-across concept.[2],[1]
It is expected for this case that the substance is not stable enough under physiological conditions to fulfil the requirements of study guidelines and so the products of decomposition have to be considered.
2 S2O42-+ H2O→2HSO3-+ S2O32 -

All sulfite, hydrogensulfite and metabisulfite substances are highly soluble in water, establishing upon dissolution an equilibrium that depends on solution pH as follows: ,

1. SO2 + H2O <-> H2SO3
2. H2SO3 <-> H+ + HSO3- <-> 2H+ + SO32-
3. 2 HSO3- <-> H2O + S2O52-

Under oxidising conditions, e.g., in surface waters, sulfite is oxidized to sulfate catalytically by (air) oxygen or by microbial action. A half-life of 77 hour was measured in deionized water, already suggesting substantial abiotic degradation. However, the presence of metal cations in the environment, such as copper, iron and manganese, accelerates the oxidation rate. In soils, HSO3- and SO32- ions are unstable and quickly oxidise. Further, because of the instability of SO32-, metal sulfites are generally too soluble to persist in soils. Thus, the most stable and predominant sulfur form in freshwater and in all but highly reduced environments is sulfate (SO42-). In highly reduced soils and sediments, sulfites may be reduced to sulfides (Lindsay, 1979; OECD SIDS, 2012).

Only the properties of the sulfite anion are considered relevant determinants of environmental toxicity since respective cations, i.e. ammonium, calcium, magnesium, sodium and potassium, are not assumed to contribute substantially to differences therein. Sulfite, although naturally present in the environment and also a metabolite and intermediate of sulfur-containing amino acids in organisms, may have an impact on the environment at elevated levels. Sulfites do not bioaccumulate.

In sum, unrestricted read-across between the sulfites, hydrogensulfites and metabisulfites is justified.
Reason / purpose for cross-reference:
reference to same study
Reason / purpose for cross-reference:
reference to same study
Objective of study:
other: disappearance from plasma
Qualifier:
no guideline followed
Principles of method if other than guideline:
The hydrate of sulfur dioxide in mammalian plasma was investigated. The longevity of sulfite in contact with mammalian plasma and known components of blood was investigated.
Rabbits were exposed to SO2 by inhalation for 14 or 62 hours and any free sulfite present in the plasma was determined as cyanolytic sulfite.
GLP compliance:
no
Radiolabelling:
no
Species:
rabbit
Strain:
New Zealand White
Sex:
not specified
Details on test animals or test system and environmental conditions:
TEST ANIMALS
- Age at study initiation: 4 to 22 months
- Weight at study initiation: 3 to 4 kg
- Diet: ad libitum
- Water: ad libitum

No more details are given.
Route of administration:
inhalation
Vehicle:
other: air
Details on exposure:
TYPE OF INHALATION EXPOSURE: whole body

GENERATION OF TEST ATMOSPHERE / CHAMPER DESCRIPTION
A plasic glove box with approximately 8 cu ft of space was used as an SO2 exposure chamber for rabbits. Sulfur dioxide was metered into a 12-inch polyethylene tube where it was mixed with ambient air. The mixture was pulled through the chamber at a rate of 2.4 cu ft/minute (1 air change every 3.4 minutes) by a rotary, vane type pump. The concentration of SO2 in the chamber was determined by the West and Gaeke method. The distribution of pollutant within the chamber proved satisfactory, however, there was considerable fluctuations in So2 concentration over an extended period of time due to imperfect regulation of SO2 input. The average SO2 concentration was 23.5 +/- 5 ppm.
One rabbit was exposed at a time.

TEST ATMOSPHERE (if not tabulated)
na data
Duration and frequency of treatment / exposure:
Rabbits 1 and 2: 14 hours + >200 hours post exposure
Rabbits 3 and 4: 62 hours + >200 hours post exposure
Dose / conc.:
23.5 ppm
No. of animals per sex per dose / concentration:
4 rabbits
Control animals:
yes
Positive control reference chemical:
No positive control substance was tested.
Details on study design:
no data
Details on dosing and sampling:
Two or three 1-mL aliquots from each plasma and serum sample were incubated according standard alkaline cyanide conditions. Liberated sulfite was recovered by repeated dialysis for 2.75 hours.
In this analytical procedure, any free sulfite present in the plasma would be determined as cyanolytic sulfite.
Statistics:
no data
Preliminary studies:
no data
Results:
A sharp increase in cyanolytic sulfite was detected immediately after SO2 exposure in all four rabbits.
Type:
other: disappearance from plasma
Results:
Sulfite which enters the bloodstream during exposure of a mammal to atmospheric SO2 forms S-sulfonate groups with constituents of the plasma, probably exclusively by sulfitolysis of disulfide groups.
Details on absorption:
no data
Details on distribution in tissues:
no data
Details on excretion:
no data
Metabolites identified:
no
Conclusions:
The results give evidence that sulfite which enters the bloodstream during exposure of a mammal to atmospheric SO2 forms S-sulfonate groups with constituents of the plasma, probably exclusively by sulfitolysis of disulfide groups.
This interaction probably affords protection to many tissues of the body from the insult of high concentrations of sulfite.

Data source

Reference
Reference Type:
publication
Title:
Sulfur dioxide: Sulfite
Author:
Gunnison, A. F. & Benton, A.W.
Year:
1971
Bibliographic source:
Arch. Environ. Health, Vol. 22: 381 - 388.

Materials and methods

Objective of study:
other: disappearance from serum or plasma
Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
The hydrate of sulfur dioxide in mammalian plasma and serum was investigated. The longevity of sulfite in contact with mammalian plasma and known components of blood was investigated.
The reactivity of sulfite added to serum or plasma in vitro was followed by monitoring the concentration of the sulfite in the reaction mixture.
GLP compliance:
no

Test material

Constituent 1
Reference substance name:
sulfite
IUPAC Name:
sulfite
Details on test material:
- Name of test material (as cited in study report): Sulfite
- Analytical purity: reagent grade
Radiolabelling:
no

Test animals

Species:
human
Sex:
not specified
Details on test animals or test system and environmental conditions:
not applicable

Administration / exposure

Route of administration:
other: not applicable
Vehicle:
water
Details on exposure:
Plasma and serum samples were collected from an arm vein of adult humans with evacuated test tubes.
Edetate disodium at 2 mg/mL was used as an anticoagulant in the plasma samples.
Duration and frequency of treatment / exposure:
continuously until measurement (after 20 minutes)
Doses / concentrationsopen allclose all
Dose / conc.:
1.7 other: nanomols of sulfite
Dose / conc.:
3.6 other: nanomols of sulfite
Dose / conc.:
5.2 other: nanomols of sulfite
No. of animals per sex per dose / concentration:
not applicable
Control animals:
no
Positive control reference chemical:
No positive control substance was tested
Details on study design:
no data
Details on dosing and sampling:
Analysis of sulfite in presence of plasma or serum: To 200 µL of solution containing 40 µL of serum or plasma, approximately 6 microequivalents of hydrochloric acid, and quantities (up to 5.2 nM) of sulfite, 20 µL of PRA and 20 µL of formaldehyde reagent were added. A serum or plasma reagent blank was prepared. The optical density was measured against this blank after 20 minutes at 560 mµ.

in vitro reaction of sulfite with plasma or serum: Serum or plasma was incubated with a known amount of sulfite in a nitrogen atmosphere at 37°C, pH 7.4, using a ratio of 1 volume of serum (or plasma) to 1 volume of buffer and sulfite. These conditions are referred to as standard pH 7.4 incubation conditions. After incubation for the desired period of time, the amount of sulfite remaining in the mixture was analysed according the the procedure described above. This determination indirectly gives the amount of sulfite that "disappeared" from the incubation mixture and which was assumed to have reacted with the serum (or plasma).

Each analysis was replicated six times.
Statistics:
no data

Results and discussion

Preliminary studies:
no data
Main ADME results
Type:
other: disappearance from serum or plasma
Results:
Sulfite which enters the bloodstream during exposure of a mammal to atmospheric SO2 forms S-sulfonate groups with constituents of the plasma, probably exclusively by sulfitolysis of disulfide groups.

Toxicokinetic / pharmacokinetic studies

Details on absorption:
no data
Details on distribution in tissues:
no data
Details on excretion:
no data

Metabolite characterisation studies

Metabolites identified:
no
Details on metabolites:
No details are given.

Applicant's summary and conclusion

Conclusions:
The results give evidence that sulfite which enters the bloodstream during exposure of a mammal to atmospheric SO2 forms S-sulfonate groups with constituents of the plasma, probably exclusively by sulfitolysis of disulfide groups.
This interaction probably affords protection to many tissues of the body from the insult of high concentrations of sulfite.