Cysteine/cystine couple is a newly recognized node in the circuitry for biologic redox signaling and control

Abstract

Redox mechanisms function in control of gene expression, cell proliferation, and apoptosis, but the circuitry for redox signaling remains unclear. Cysteine and methionine are the only amino acids in proteins that undergo reversible oxidation/reduction under biologic conditions and, as such, provide a means for control of protein activity, protein-protein interaction, protein trafficking, and protein-DNA interaction. Hydrogen peroxide and other reactive oxygen species (ROS) provide a mechanism to oxidize signaling proteins. However, oxidation of sulfur-containing side chains of cysteine and methionine by ROS can result in oxidation states of sulfur (e.g., sulfinate, sulfonate, sulfone) that are not reducible under biologic conditions. Thus, mechanisms for oxidation that protect against over-oxidation of these susceptible residues and prevent irreversible loss of activity would be advantageous. The present study shows that the steady-state redox potential of the cysteine/cystine couple (Eh = -145 mV) in cells is sufficiently oxidized (>90 mV) relative to the GSH/GSSG (-250 mV) and thioredoxin (Trx1, -280 mV) redox couples for the cysteine/cystine couple to function as an oxidant in redox switching. Consequently, the cysteine/cystine couple provides a means to oxidize proteins without direct involvement of more potent oxidants. A circuitry model incorporating cysteine as a redox node, along with Trx1 and GSH, reveals how selective interactions between the different thiol/disulfide couples and reactive protein thiols could differentially regulate metabolic functions. Moreover, inclusion of cysteine/cystine as a signaling node distinct from GSH and Trx1 significantly expands the redox range over which protein thiol/disulfide couples may operate to control physiologically relevant processes.

Dissection of mitogenic and neurodegenerative actions of cystine and glutamate in malignant gliomas

“…We show that neither glutamate nor AMPA possesses any mitogenic effects on glioma cells even under growth factor-reduced or low-cystine conditions. However, disrupting glutamate secretion by interfering with system X c À activity attenuates glioma cell proliferation, which is because of impaired cystine incorporation required for glutathione synthesis (Jones et al, 2004;Chung et al, 2005) and not because of decreased extracellular levels of glutamate. All glioma cell lines investigated appear to be insensitive to AMPA receptormediated signaling either by solid downregulation of all AMPA receptor subunits or disproportionately high expression of the fully edited GluR2 subunit, which blocks Ca 2 þ permeability of the AMPA receptor complex.…”

Radical-free biology of oxidative stress

“…Cytoplasmic pH was determined by distribution of the pH indicator [ 14 C]dimethadione (92). Errors due to sequestration by mitochondrial and secretory compartments were estimated by digitonin fractionation (83,86). The results showed that the E h of the cytoplasmic GSH/ GSSG couple was about Ϫ260 mV under proliferative conditions, about Ϫ220 mV under differentiated conditions, transiently oxidized and then reduced following treatment with a GSH synthesis inducer (6), and oxidized to about Ϫ170 mV during apoptosis (21,82,91).…”

Redox-based regulation of signal transduction: Principles, pitfalls, and promises

“…Thus the notion is emerging that cells have evolved with multiple and independently regulated systems, consisting of cysteine/cystine, GSH/GSSG, and Trx-SH/Trx-SS to control the redox tone [114], a concept that also warrants inclusion of measurements of NO/SNO homeostasis (nitroso-redox homeostasis). Furthermore, the existence of multiple reversible cysteine modifications in proteins warrants a redefinition of the general concept of "oxidative and nitrosative stresses" to include the identity of particular oxidative event (culprit protein, Cys identity, oxidative or nitrosative modification), and its contribution to cellular or tissue (patho-) physiology.…”