Albuminomics

Cysteinylation

3 min read

Cysteinylation

Definition

Cysteinylation is the formation of a mixed disulfide bond between a free protein cysteine (–SH) and free cysteine in plasma. The result is a protein–S–S–Cys adduct.

On HSA, this occurs at Cys34 — the only free thiol in the mature protein and the most reactive thiol in human plasma. Cys34 is the primary extracellular antioxidant site in blood.

Mechanism

Protein–Cys–SH  +  HS–Cys (free)  →  Protein–Cys–S–S–Cys  +  H₂
  • Reaction is reversible (mixed disulfide, can be reduced back by thioredoxin/GSH)
  • Occurs spontaneously under oxidative conditions
  • Free cysteine in plasma: ~250–300 µmol/L
  • Also: glutathionylation (HSA–S–S–glutathione, Δmass +305 Da) and homocysteinylation (Δmass +135 Da)

Mass spectrometry signature

  • Δmass: +119.004 Da (cysteinylation: +Cys −2H)
  • Glutathionylation: +305.068 Da
  • Sulfinylation (Cys-SO₂H, irreversible): +32 Da
  • Sulfonylation (Cys-SO₃H, irreversible): +48 Da
  • Detected on: Cys34 of HSA primarily
  • By top-down MS: produces a characteristic +119 Da shift on the intact protein mass

Biological significance

  • Redox sensor: Cys34 is oxidized/modified early in oxidative stress — HSA acts as a sacrificial antioxidant
  • Proportion reflects redox status: in healthy adults ~70–80% HSA is cysteinylated (HSA–Cys), ~20–25% native (free thiol)
  • Under oxidative conditions: free thiol fraction decreases; irreversible sulfinyl/sulfonyl forms appear
  • Drug binding: Cys34 can also bind reactive drug metabolites → pharmacological relevance

Clinical relevance

  • Liver disease: altered HSA redox state (decreased free thiol, increased oxidized forms) is a marker of hepatic synthetic function and oxidative stress
  • DILI: covalent drug-HSA adducts via Cys34 are a mechanism of immunogenic drug reactions
  • Sepsis, CKD, aging: all show progressive oxidation of HSA Cys34
  • Pre-analytical challenge: Cys34 oxidizes rapidly ex vivo → sample must be stabilized immediately (N-ethylmaleimide alkylation or rapid freezing)

In our research

Cysteinylation at Cys34 is a key PTM tracked in the ALBOM study and CQFD-PTM pipeline. The ratio of native:cysteinylated:oxidized HSA may serve as a liver function index.

In chronic liver disease (ALBOM data — el-balkhi-2025)

Cysteinylation of HSA at Cys34 (HSA+CYS, Δmass +119 Da) shows a biphasic pattern across CLD stages:

  • Controls: 8.7 g/L (absolute HSA+CYS)
  • F4_A (compensated cirrhosis): peak at 11.1 g/L — reflecting rising systemic oxidative stress as liver disease progresses
  • F4_B: 7.9 g/L, F4_C: 6.8 g/L — falling as native HSA substrate collapses in decompensation

The ratio HSA+CYS/Native better captures the diagnostic signal:

  • Sensitivity 65%, Specificity 99% for F4_C vs controls
  • Moderate sensitivity for earlier stages when used alone; best combined with glycated isoform ratios
  • Significant increase only in F4_B and F4_C stages (not early fibrosis)

HSA+CYS+2GLYC (doubly glycated + cysteinylated, Δmass +443 Da): progressive, monotonic increase across all cirrhosis sub-classes; near-undetectable in healthy controls — best single candidate for end-stage liver damage marker.

HSA+SO₃H (sulfonylated, irreversible Cys34 oxidation, Δmass +48 Da): decreases in F4_B and F4_C — reflects depletion of the oxidizable substrate pool under severe hepatic failure.

⚠️ Pre-analytical note: Cys34 is extremely reactive ex vivo — rapid freezing or NEM alkylation required to preserve isoform distribution. Pre-analytical standardization is a prerequisite for multicenter deployment.

Key references

  • el-balkhi-2025 — quantitative data on HSA cysteinylation across CLD stages (controls to F4_C)

Graph View

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