Cystine

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics

Type of information:

not specified

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Data from handbook or data collection

Data source

Reference

Reference Type:

review article or handbook

Title:

Amino acids from chemical group 34, Flavouring Group Evaluation 26, Revision 1

Author:

AFC (EFSA)

Year:

2008

Bibliographic source:

The EFSA Journal, 790, 1-51

Materials and methods

Test guideline

Qualifier:

no guideline followed

Test material

Test material information

Constituent 1

Reference substance name:

Cystine

EC Number:

200-296-3

EC Name:

Cystine

Cas Number:

56-89-3

Molecular formula:

C6H12N2O4S2

IUPAC Name:

cystine

Results and discussion

Main ADME results

Type:

other:

Results:

L-Cystine metabolic pathway

Applicant's summary and conclusion

Conclusions:

L-Cystine is converted to L-cysteine through cystine reductase, which requires NADH as cofactor.
L-Cysteine is the central compound in sulfur metabolism in the human body. In proteins the for-mation of disulfide bonds between the thiol groups of cysteine plays an important role for tertiary structure and enzymatic activity, Cysteine is however always incorporated in the polypeptide chain as cysteine.
L-Cysteine is degraded to pyruvate in two steps, one is removal of sulphur and the other is a transamination. Cysteine can be metabolised to form taurine and carbon dioxide through the cys-teinsulfinate pathway, where the initial step is oxidation of cysteine to cysteine sulfinate. This step is catalysed by cysteine dioxygenase. Cysteine sulfinate may then be decarboxylated to form taurine or it may be metabolised via the putative intermediate beta-sulfinylpyruvate to pyruvate and sulfite and then to carbondioxide and sulfate (Stipanuk, 1986). D-Cysteine is not metabolised to taurine (Cavallini et al., 1958; Krijgsheld et al., 1981).

Executive summary:

L-Cystine is converted to L-cysteine through cystine reductase, which requires NADH as cofactor.

L-Cysteine is the central compound in sulfur metabolism in the human body. In proteins the for-mation of disulfide bonds between the thiol groups of cysteine plays an important role for tertiary structure and enzymatic activity, Cysteine is however always incorporated in the polypeptide chain as cysteine.

L-Cysteine is degraded to pyruvate in two steps, one is removal of sulphur and the other is a transamination. Cysteine can be metabolised to form taurine and carbon dioxide through the cys-teinsulfinate pathway, where the initial step is oxidation of cysteine to cysteine sulfinate. This step is catalysed by cysteine dioxygenase. Cysteine sulfinate may then be decarboxylated to form taurine or it may be metabolised via the putative intermediate beta-sulfinylpyruvate to pyruvate and sulfite and then to carbondioxide and sulfate (Stipanuk, 1986). D-Cysteine is not metabolised to taurine (Cavallini et al., 1958; Krijgsheld et al., 1981).