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Basic toxicokinetics

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Administrative data

Endpoint:
basic toxicokinetics in vitro / ex vivo
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
key study
Justification for type of information:
The basis for the read-across concept for this project is the equilibrium between sulfites, hydrogensulfites, and metabisulfites in aqueous solutions depending on pHvalue 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 -
 
Not completely included in this read-across concept is the substance class of thiosulfates. Although thiosulfates may also disproportionate in aqueous solution to form polythionic acids and SO2(HSO3-), the required conditions are somewhat different (more acidic) and are therefore not strictly comparable with physiological conditions, except for the case of oral application where read-across should be considered unrestricted due to the strongly acidic conditions in the stomach:
       HS2O3-+ H2S2O3→HS3O3- + SO2 + H2O
Nevertheless, read-across for all other routes (dermal, inhalation) should also be considered.
The proposed read-across concept only applies to toxicological and ecotoxicological/environmental fate endpoints.
[1]Hollemann Wiberg, Lehrbuch der Anorganischen Chemie, 101.Auflage
[2]Handbook of Chemistry and Physics, Ed. Lide, DR, 88thedition, CRC Press

Data source

Reference
Reference Type:
publication
Title:
Alternative pathways of sulfite oxidation in human polymorphonuclear leukocytes
Author:
Constantin, D. et al.
Year:
1994
Bibliographic source:
Pharmacology & Toxicology. 74: 136 - 140.

Materials and methods

Objective of study:
metabolism
Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
In the present study, the oxidation of sulfite in human polymorphonuclear leukocytes was investigated and the sulfite-oxidase catalyzed reaction and the non-enzymatic pathway were compared.
GLP compliance:
not specified

Test material

Constituent 1
Chemical structure
Reference substance name:
Sodium sulphite
EC Number:
231-821-4
EC Name:
Sodium sulphite
Cas Number:
7757-83-7
Molecular formula:
NA2SO3
IUPAC Name:
disodium sulfite
Details on test material:
- Name of test material (as cited in study report): Sodium sulfite
Radiolabelling:
no

Test animals

Species:
human
Details on test animals or test system and environmental conditions:
Not applicable - Since this is a in vitro study there is no information on test animals.

Administration / exposure

Route of administration:
other: human polymorphonuclear leukocytes
Duration and frequency of treatment / exposure:
Cell culture used: Human polymorphonuclear leukocytes were isolated from buffy coat (obtained from the Blood Donor center of Sabbatsbergs Hospital, Stockholm, Sweden).
(For further information on the preparation on the human polymorphonuclear leukocytes see "Any other information on materials and method incl. tables" below.)
Doses / concentrations
Remarks:
Doses / Concentrations:
For information on the preparation on the human polymorphonuclear leukocytes see "Any other information on materials and method incl. tables" below.
No. of animals per sex per dose / concentration:
Not appropriate
Statistics:
The results are expressed as means +/- S.E.M. Groups of data were compared for significant differences using the non-paired t-test.

Results and discussion

Main ADME results
Type:
metabolism
Results:
Two different oxidation routes of sulfite to sulfate have been identified in the human polymorphonuclear leukocytes. 1. Via sulfite oxidase and 2. via an one electron oxidation step with an intermediate formation of sulfur trioxide radicals.

Metabolite characterisation studies

Metabolites identified:
yes
Details on metabolites:
Addition of sulfite to polymorphonuclear leukocytes significantly stimulated the uptake of oxygen.
The oxygen consumption varied substantially between cells from different donors and were divided in those with low (0 -200 nmol O2/ml/min.) and high (>200 nmol O2/ml/min.) capacity.
The interindividual difference in oxygen uptake was also reflected in the rates of sulfite disappearance and sulfate formation, the correlation between these two parameters being fairly good. The correlation was not affected by varying the concentration of sulfite added to the leukocytes. It is assumed that the variation in oxygen consumption mainly reflects the cells capacity to oxidize sulfite direct to sulfate, thus the activity of sulfite oxidase.
Only 30 % to 40% of the sulfite added to cells with low sulfite oxidase activity was oxidized to sulfate after 30 min. incubation whereas on average about 60% was oxidized in cells with high activity.
In the presence of sulfite, addition of phorbol myristate acetate to cells with low sulfite oxidase activity increased the O2 consumption substantially (up to 600 nmol/ml/ min.) In cells with high enzyme activity an inhibitory effect of phorbol myristate acetate on oxygen consumption was observed.
The effect of phorbol myristate acetate can also be seen on the oxidation of sulfite to sulfate. In cells with low sulfite oxidase activity the addition of phorbol myriastate acetate increases the rate of sulfate formation whereas in cells with high activity phorbol myriastate acetate has an inhibitory effect.
The EPR spectrum shows signals consistent with the presence of sulfur trioxide radicals formed during autooxidation of sulfite. A similar spectrum is observed after addition of sulfite to non-phorbol myristate acetate stimulated human polymorphonuclear leukocytes.
When phorbol myristate was added to polymorphonuclear leukocytes and sulfite, an EPR spectrum compatible with the presence of sulfur trioxide radicals as well as hydroxyl adducts with DMPO is observed.

Any other information on results incl. tables

The interaction of sulfite and the superoxide radical anion (O2^-) formed during the oxidative burst in the leukocytes was also investigated. The formation of O2^- was estimated by measuring the reduction of cytochrome C. Sulfite efficiently inhibited the reduction of cytochrome C by cells with low sulfite oxidase activity. For instance in the presence of 1 mM sulfite the inhibition was more than 80%. The reason is possible an interaction between O2^- and sulfite leading to sulfur trioxide radical formation. The effect on cytochrome C reduction by sulfite in leukocytes with high sulfite oxidase activity is minor. Due to the very rapid enzymatic oxidation of sulfite to sulfate only limited interction between sulfite and O2^- occurred.

Applicant's summary and conclusion

Conclusions:
Two different oxidation routes of sulfite to sulfate have been identified in the human polymorphonuclear leukocytes. Besides the pathway via sulfite oxidase another route of oxidation via an one electron oxidation step with an intermediate formation of sulfur trioxide radicals has been identified.
The contribution of the different pathways is expected to vary substantially due to the great interindividual variation in sulfite oxidase activity. The contribution of the trioxide radical pathway is expected to be high in individuals with low sulfite oxidase activity.