Glutathione: Molecular Structure

Molecular Structure and Properties#

Glutathione is a peptide whose molecular structure and properties have been characterized through analytical chemistry and structural biology studies.

Amino Acid Sequence#

Molecular structure and sequence. Glutathione is γ-L-glutamyl-L-cysteinyl-glycine, in which the glutamate residue is linked to cysteine via a γ-peptide bond that uses the side-chain (γ) carboxylate of glutamate rather than the α-carboxylate; cysteine and glycine are connected by a conventional peptide bond. This γ-linkage is a defining structural feature of GSH and underlies several of its biological properties.

Amino-acid connectivity. The sequence is written γ-Glu–Cys–Gly. The γ-linkage connects Glu(γ‑COO−) to the α‑NH2 of Cys; Cys(α‑COO−) is peptide-bonded to the α‑NH2 of Gly. The sole free thiol resides on the Cys side chain and is the principal reactive center.

Physicochemical properties relevant to charge and ionization. The cysteinyl thiol has a pKa of approximately 9.6, so it is largely protonated and uncharged at neutral pH, with reactivity increasing upon deprotonation at higher pH. At physiological pH, carboxylates are predominantly deprotonated while α-amino groups are largely protonated, giving GSH an overall net negative charge under typical intracellular conditions; explicit pI values were not provided in the retrieved sources complexesof pages 1-2, ). In cells, reduced GSH is by far the dominant species, with a small fraction present as the oxidized disulfide (GSSG); typical GSH:GSSG ratios are on the order of ~100:1 under non-stressed conditions. The GSH/GSSG redox couple defines a cellular redox potential in the approximate range −260 to −150 mV depending on compartment and conditions.

Charge distribution and isoelectric point. While a numeric pI was not recovered in the gathered evidence, the qualitative distribution of charge at pH ~7.4 follows from the ionization states above: deprotonated terminal and side-chain carboxylates and protonated α‑amino groups yield a net anion, consistent with observed electrostatic interactions in the oxidized dimer (GSSG) and its metal-binding behavior (krezel2011zn(ii)complexesof pages 1-2, ). Thus, at physiological pH, GSH is expected to carry an overall negative charge with localized positive charge density on the α‑ammonium groups and negative charge density on the carboxylate oxygens.

Structural features and redox/tautomeric states. GSH exists as a reduced monomer (GSH) and an oxidized disulfide dimer (GSSG). The GSSG dimer comprises two γ‑Glu–Cys–Gly units linked via a disulfide between their cysteinyl sulfurs; at neutral pH, GSSG adopts a bent head‑to‑tail conformation stabilized by electrostatic attractions between protonated amino groups and deprotonated C‑terminal carboxylates, a geometry that also supports coordination to metal ions such as Zn(II) via amines and carboxylates (krezel2011zn(ii)complexesof pages 1-2). The unusual γ‑glutamyl linkage renders GSH resistant to most proteases and peptidases; its catabolism proceeds via γ‑glutamyltransferase (GGT), which cleaves the γ‑glutamyl bond to release cysteinylglycine as part of the γ‑glutamyl cycle. The cysteinyl thiol engages in reversible redox chemistry, forming GSSG, mixed protein–glutathione disulfides (S‑glutathionylation), and other oxidative modifications; GSSG is reduced back to GSH by glutathione reductase using NADPH, maintaining a high GSH/GSSG ratio in healthy cells.

Solubility and abundance context. Although explicit solubility constants were not retrieved, intracellular concentrations of GSH are typically in the millimolar range (∼1–10 mM), far exceeding plasma levels, reflecting high aqueous compatibility and active biosynthesis; this gradient is exploited in redox-responsive delivery systems.

Embedded summary table follows.

Limitations. Numeric values for the isoelectric point and precise solubility were not present in the gathered sources and therefore are not reported here; qualitative charge distribution and ionization behavior are provided instead based on the cited evidence (krezel2011zn(ii)complexesof pages 1-2, ).

Stability and Formulation#

We synthesized current evidence on the chemical stability of reduced glutathione (GSH) across pH and temperature, mapped key degradation pathways, and translated these into formulation guidance.

pH stability

• The cysteinyl thiol of GSH has a pKa near 9.6; thiolate formation at alkaline pH increases susceptibility to oxidation, whereas the protonated thiol at neutral/acidic pH is less reactive (mechanistic basis for higher stability at lower pH). In vitro, pure GSH is comparatively stable under acidic conditions; however, common acid deproteinization reagents (trichloroacetic, metaphosphoric, perchloric, sulfosalicylic acids) artefactually oxidize GSH by roughly 5–15% during sample workup, inflating apparent GSSG. Because cellular GSH is millimolar and GSSG micromolar, even 1% oxidation of GSH can increase measured GSSG by an order of magnitude.

Temperature sensitivity

• Elevated temperature accelerates loss of GSH and degradation of assay adducts. In HPLC method optimization, the GSH–OPA adduct was most stable at 4 °C, with time-dependent degradation at higher temperatures; net GSH loss was observed above ~20 °C. Relative to 4 °C, GSSG levels fell by ~15% at 25 °C and ~67% at 50 °C, consistent with temperature-driven degradation and assay bias; limiting derivatization to 5–10 min at 25 °C reduced losses. Cold handling therefore preserves redox state and minimizes artefacts.

Degradation pathways and kinetics

• Uncatalyzed auto-oxidation by dissolved O2 is very slow at neutral pH. For 56 μM GSH in phosphate buffer pH 7 with atmospheric O2 (~0.27 mM), loss can be as low as ~6.6 nM·min−1, and GSH remains essentially constant over 3 h when metals are chelated (±2% change) (ngamchuea2016thecopper(ii)catalyzedoxidation pages 4-6).
• Metal-catalyzed oxidation dominates in practice. With Cu2+, GSH undergoes rapid complexation followed by slower oxidation to GSSG; the oxidation rate increases with [Cu2+] and [O2] and exhibits half-order dependence on oxygen. Example step-Y rates: 56 μM GSH + 7.5 μM Cu2+ give 0.312 ± 0.008 μM·min−1 at atmospheric O2 and 0.718 ± 0.021 μM·min−1 at O2 saturation; increasing bulk GSH can paradoxically reduce the rate by forming inactive Cu–GSH species (ngamchuea2016thecopper(ii)catalyzedoxidation pages 6-10, ngamchuea2016thecopper(ii)catalyzedoxidation pages 10-14). Sub-μM to nM copper impurities can account for measurable oxidation unless chelated; 1 mM EDTA effectively suppresses Cu-catalyzed oxidation (ngamchuea2016thecopper(ii)catalyzedoxidation pages 4-6).
• Oxygen availability modulates rates: oxidation increases with dissolved O2 (rate ∝ [O2]0.5), and headspace/dissolved oxygen can materially impact thiol loss during storage/processing (ngamchuea2016thecopper(ii)catalyzedoxidation pages 6-10, ).
• Beyond oxidation to GSSG, thiol–disulfide exchange and thiyl-radical pathways operate in oxidative systems; photooxidation and peroxide-bearing excipients can further drive thiol oxidation in formulations.

Formulation and handling considerations

• pH and buffers: Maintain near-neutral to mildly acidic pH to limit thiolate formation. Avoid strong acid treatment before thiol blocking in analytical workflows; pre-block GSH with N-ethylmaleimide (NEM) to prevent artefactual oxidation/reduction during processing.
• Chelators: Include EDTA (e.g., ~1 mM) or equivalent chelation to sequester trace transition metals; EDTA prevented oxidation of 56 μM GSH over hours at pH 7 by binding Cu2+ (ngamchuea2016thecopper(ii)catalyzedoxidation pages 4-6).
• Oxygen/light control: Minimize dissolved and headspace oxygen (nitrogen/argon blanket, low-permeability containers) and protect from light to reduce ROS generation and thiol oxidation; oxidation rate scaling with [O2] supports this practice (ngamchuea2016thecopper(ii)catalyzedoxidation pages 6-10,, ngamchuea2016thecopper(ii)catalyzedoxidation pages 4-6).
• Temperature: Keep solutions and analytical derivatizations cold (≈4 °C) and short in duration; higher temperatures accelerate degradation and bias GSH/GSSG measurements.
• Excipients: Avoid or control peroxide-generating excipients (e.g., polysorbates, PEGs) and leachables that promote oxidation; consider stabilizing excipients (polyols/sugars) after compatibility testing.
• Rapid thiol masking and GR control: For biological samples, collect into tubes prefilled with NEM (about 25–40 mM final) plus EDTA to both block thiols and chelate metals, and to inhibit endogenous glutathione reductase (GR) that can rapidly reduce GSSG ex vivo; store derivatized samples at −80 °C.
• Lyophilization and trace metals: While freeze-drying reduces aqueous degradation, trace metals can compromise stability during processing or upon reconstitution, reinforcing the need for strict metal control and validated raw materials catalyzedoxidation pages 4-6).

Embedded summary

Overall, GSH is intrinsically stable toward uncatalyzed oxidation at neutral pH but is highly sensitive to trace metals, oxygen, alkaline pH (thiolate), elevated temperatures, and pro-oxidant excipients. Practical stability hinges on rigorous control of metals (chelators, clean materials), oxygen and light, temperature, and immediate thiol protection in analytical and pharmaceutical contexts (ngamchuea2016thecopper(ii)catalyzedoxidation pages 4-6, ngamchuea2016thecopper(ii)catalyzedoxidation pages 6-10,,,, ).

Pharmacokinetics#

We reviewed human pharmacokinetic data for reduced glutathione (GSH) across routes of administration and summarize quantitative parameters where available.

Absorption

• Intravenous: After 50 mg/kg infused over 10 minutes in healthy males, plasma GSH reached Cmax ≈150 µM at 10 minutes, then declined rapidly (reflecting distribution and elimination).
• Oral: A single 3.5 g oral dose produced a very low circulating signal: plasma Cmax ≈0.47 µM with median Tmax ≈1.5 h; exposure was highly variable, consistent with poor absorption and/or extensive presystemic degradation. (fanelli2018clinicalnutrition& pages 4-6)
• Intranasal: A 200 mg intranasal dose raised brain GSH measured by 1H-MRS; no increase at 8 min, but significant elevations thereafter that persisted for at least 1 hour, indicating CNS uptake by this route. Systemic absorption parameters were not reported.

Distribution

• Intravenous: Apparent volume of distribution was large (Vd ≈5.53 L/kg), implying extensive distribution beyond plasma.
• Oral and intranasal: Classical distribution parameters were not reported for systemic circulation; intranasal data demonstrate CNS exposure without systemic Vd estimation.

Metabolism

• In circulation, exogenous GSH is rapidly oxidized to GSSG and degraded via the γ‑glutamyl cycle; γ‑glutamyltransferase (γ‑GT) and peptidases generate glutamate, cysteine, and glycine, which can be used for intracellular resynthesis.

Elimination and Clearance

• Intravenous: Systemic clearance was very high at ≈309 mL·min⁻¹·kg⁻¹, with rapid disappearance from plasma, reflecting oxidation to GSSG and extracellular degradation rather than classical renal excretion of intact GSH.
• Oral and intranasal: Specific human clearance of intact GSH was not quantified; oral administration is limited by presystemic hydrolysis in the gut and at the brush border by γ‑GT. (fanelli2018clinicalnutrition& pages 4-6)

Half-life

• Intravenous: Terminal half-life ≈10.9 minutes in healthy volunteers.
• Oral: The circulating GSH signal after oral dosing is transient; one human study reported a plasma half-life signal ≈1.6 minutes for measurable plasma GSH after oral GSH, consistent with rapid extracellular turnover and measurement limitations. (fanelli2018clinicalnutrition& pages 4-6)
• Cellular context: Reviews also note short human half-life on the order of minutes, and intracellular increases from precursors may decline with a 2–3 h half-life in cells; however, this reflects cellular GSH turnover, not plasma PK of exogenous GSH. (md2022dietaryγglutamylcysteineits pages 5-7)

Bioavailability

• Intravenous: By definition, 100%.
• Oral: Absolute bioavailability of intact GSH in humans has not been directly quantified relative to IV dosing; qualitative evidence indicates it is very low/poor due to intestinal γ‑GT-mediated hydrolysis and presystemic metabolism. Plasma exposure after 3.5 g is minimal and highly variable, supporting poor F. (fanelli2018clinicalnutrition& pages 4-6, md2022dietaryγglutamylcysteineits pages 5-7)
• Intranasal: Systemic bioavailability was not determined; nevertheless, brain GSH rose for ≥1 h after 200 mg intranasal dosing, demonstrating effective CNS delivery.

Key quantitative human parameters are organized below.

Notes and caveats

• The IV dataset (Hong 2005) provides robust systemic parameters in healthy young males; applicability to other populations may vary.
• Oral studies show very limited plasma exposure to intact GSH and large interindividual variability; intestinal hydrolysis and re-synthesis pathways dominate, making absolute bioavailability of intact GSH effectively low and challenging to measure. (fanelli2018clinicalnutrition& pages 4-6, md2022dietaryγglutamylcysteineits pages 5-7)
• Intranasal data demonstrate CNS exposure by MRS but do not provide classical systemic PK; further work is needed to quantify systemic kinetics after intranasal dosing.

Related Reading#

• Glutathione overview and research guide
• Peptides similar to Glutathione
• Glutathione research evidence