Sodium dithionite

Basic toxicokinetics

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

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

Test material information

Constituent 1

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

Test material form:

not specified

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:

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.