Gupta, "Sulphur containing amino acids in chronic renal failure with particular reference to homocysteine and cysteine-homocysteine mixed disulfide
," European Journal of Clinical Investigation, vol.
This fact strongly suggests that in the GSSG reduction pathway by TGR, the Grx-like domain is able to catalyze only a thiol-disulfide exchange reaction with either GSSG or protein-glutathione mixed disulfides
. In this proposal, the N-terminal cysteine residue of such motif will be involved in the nucleophilic attack on the disulfide bond of GSSG, leading to the formation of a glutathioneenzyme mixed disulfide
DISCUSSION: Homocysteine is a sulfhydryl amino acid that is readily oxidized to homocysteine and homocysteine-cysteine mixed disulfide
in the plasma.
At the more acidic pH of the Golgi the C-terminal tail opens, unveiling the active site cysteine and the surrounding hydrophobic region for interaction and formation of a mixed disulfide
bond with the client proteins.
During these experiments, we gained knowledge about the catalytic properties of the enzyme, which enabled us to construct an enzymic assay for homocysteine in rat and mouse tissues as well as in plasma from mice (1).When this methodological work was being carried out in 1982, data on homocysteine were sparse and addressed limited aspects of homocysteine in health and disease, such as homocyst(e)ine in patients with the inborn error of homocystinuria, increased homocyst(e)ine (often the homocysteine-cysteine mixed disulfide
) in patients with cardiovascular disease (CVD), and the so-called methionine dependence of some cancer cells (2).
Prx1 also mediated the disulfide-linked activation of the apoptosis signaling kinase ASK1 by forming a mixed disulfide
intermediate with ASK1 in the peroxide-treated cells .
The remaining unbound HCY combines by oxidation, either with itself to form the dimer homocystine or with cysteine to form the mixed disulfide
More than 80% of Hcy is found in plasma, mostly conjugated to proteins through disulfide bonding or as symmetrical disulfide homocystine, as mixed disulfide
Hcy-Cys, or as a free thiol (<2%) (1,5).
Hcy occurs in the circulation in multiple forms, including Hcy linked via disulfides to albumin (-70%), as a mixed disulfide
with Cys (25%), as a disulfide-linked dimer (<5%), as the free reduced amino acid (<5%), and as a thiolactone (trace) (5-6).
This reaction was necessary to break the disulfide linkages in plasma proteins and to reduce the lowmolecular weight disulfides homocystine and homocysteine-cysteine mixed disulfide
The latter include homocystine, the homocysteine-cysteine mixed disulfide
, and protein-bound homocysteine.
Subsequent plasma and urine amino acid analysis in specimens promptly delivered to our laboratory, however, revealed the presence of homocystine and the mixed disulfide
in addition to the increase in methionine.