Heavy Metal Complexation of Thiol-Containing Peptides from Soy Glycinin Hydrolysates

Overview

Ding and colleagues at Jiangnan University (Wuxi, China) hydrolysed soy glycinin (the 11S storage protein of soybean) with three commercial proteases — alcalase, papain, and pepsin — to three degrees of hydrolysis (DH 5%, 15%, 25%), enriched the thiol-containing peptide (TCP) sub-population from each hydrolysate by Thiopropyl-Sepharose 6B covalent chromatography, and measured the stability constants (lgβ₁ and lgβ₂) of the resulting TCPs against Hg²⁺, Cd²⁺, and Pb²⁺ by potentiometric pH titration. The TCPs from the 25% DH papain digest reached lgβ₂ values of 33.4 mercury (Hg), 15.2 cadmium (Cd), and 21.1 lead (Pb), comparable to or exceeding the corresponding glutathione values measured in the same study. The paper frames TCPs as candidate functional-food ingredients for “natural detoxication of heavy metals,” not as a measurement of heavy metals in soy or in any food product.

Why this matters

• This is one of a small set of papers in the HMI peptide-metal-binding corpus that measures stability constants for food-protein-derived peptides against Pb²⁺, Cd²⁺, and Hg²⁺ on a common (potentiometric, 0.1 M KCl, 25 °C) platform, with glutathione as an in-study comparator. The published data make soy-glycinin TCP affinities directly comparable to glutathione without cross-paper buffer/temperature/instrument correction. Companion papers in the same source-retrieval folder include Biomining: A biological approach to recycling elemental components from end-of-life electronics (ITC characterisation of natural and rationally-designed Cu/Zn/Ni-binding peptides), Phytochelatins, the heavy-metal-binding peptides of plants, are synthesized from glutathione by a specific gamma-glutamylcysteine dipeptidyl transpeptidase (phytochelatin synthase) (the discovery paper on plant phytochelatins), and Phytochelatins and Metallothioneins: Roles in Heavy Metal Detoxification and Homeostasis (review of PC/MT biology).
• The paper provides no contamination data on any food matrix. Soy glycinin here is a purified protein starting material, not a measured soy product. The wiki should not record Pb/Cd/Hg occurrence values for soy on the basis of this paper.
• The functional-food framing in the paper’s discussion (“TCPs could be used as ingredients in the formulation of functional foods to control and manage diseases associated with heavy metal accumulation”) is a hypothesis the authors put forward, not a demonstrated in-vivo or in-clinical effect. No animal-model or human-clinical chelation data are presented. The chemistry establishes that the peptides bind the metals in vitro; whether oral intake of such peptides would reduce body-burden of Pb/Cd/Hg in humans is not tested here.
• The paper’s Hg²⁺-glutathione comparators are internally inconsistent with one literature source (Cardiano et al. 2011, reference [32] in the paper) but consistent with another (Oram et al. 1996, reference [44]). The authors note this and report their own GSH-Hg lgβ values as 19.2 (β₁) and 30.5 (β₂) in 0.1 M KCl at 25 °C. A future synthesis page that uses Hg-glutathione lgβ as a benchmark should treat this disagreement explicitly rather than averaging across the GSH literature.

Key numbers

Stability constants of TCP-metal complexes at 25 °C, I = 0.1 M KCl (Table 4, p. 8048; mean ± SE, triplicate measurement). Sample codes: Alc/Pap/Pep = alcalase/papain/pepsin; 05/15/25 = degree of hydrolysis in % (the hydrolysate code therefore identifies the protease and the DH level).

Glutathione comparators measured in this study (text, pp. 8048-8049): lgβ₁ = 19.2, lgβ₂ = 30.5 for Hg²⁺-GSH; lgβ₁ = 10.1, lgβ₂ = 15.4 for Cd²⁺-GSH (consistent with Leverrier et al. 2007 values of 8.5 and 12.4 for Cd-GSH and Cd-GSH₂ respectively, paper text).

Acid dissociation constants of glutathione measured in this study (Table 2, p. 8047, “Experimental Values” row; I = 0.1 M KCl, 25 °C): pK₁ = 2.3, pK₂ = 3.6, pK₃ = 8.9, pK₄ = 9.8 (literature values [35]-[37] also tabulated, ~0.2 pK units lower across the range).

Acid dissociation constants of TCPs by hydrolysate (Table 3, p. 8047). For DH 5% TCPs: alcalase gave six titratable pK values (pK₁ = 11.6, pK₂ = 9.9, pK₃ = 7.1, pK₄ = 4.5, pK₅ = 3.5, pK₆ = 2.4); papain gave four (pK₁ = 10.7, pK₂ = 7.5, pK₃ = 4.4, pK₄ = 3.1); pepsin gave six. For DH 25% TCPs: alcalase gave three (pK₁ = 11.4, pK₂ = 8.0, pK₃ = 3.1); papain gave four; pepsin gave four. The number of titratable protons decreased with increasing DH for alcalase and pepsin TCPs but remained constant at four for papain TCPs across all DH levels. Lowest pK values (range 1.8–3.6 across all TCPs) are assigned to the C-terminal carboxylic-acid; values 7.4–9.1 to cysteine sulfhydryl and N-terminal amine groups (which cannot be distinguished in pK alone); values >10 to lysine and arginine side chains.

Total sulfhydryl content of SGHs and enriched TCPs (Figure 2, p. 8044, qualitative reading of axes). Total sulfhydryl content of SGHs was approximately 100–120 µmol/g protein across all enzymes and DH levels (i.e., proteolysis itself caused little loss of -SH). After Thiopropyl-Sepharose enrichment, sulfhydryl content of TCPs ranged from ~400 µmol/g (DH 5%) to ~950 µmol/g (Alc25), with enrichment factors of 4.2–8.2× (alcalase), 4.0–4.8× (papain), and 3.5–4.8× (pepsin) over the unenriched hydrolysate.

Order of TCP-metal stability among the three metals (text, p. 8049):

• Alcalase TCPs and pepsin TCPs: Hg²⁺ > Cd²⁺ > Pb²⁺
• Papain TCPs: Hg²⁺ > Pb²⁺ > Cd²⁺
• The authors note this order does not follow the inverse-ionic-radius rule (Cd²⁺ < Hg²⁺ < Pb²⁺ in ionic radius), so the relative-affinity order is “dependent upon the experimental conditions used” rather than a simple ionic-radius prediction.

Correlations between sulfhydryl group content and lgβ (Figure 5, pp. 8049-8050; Pearson r):

The authors interpret the systematically larger r-values for lgβ₂ than for lgβ₁ as evidence that the sulfhydryl group is more important in stabilising the ML₂ (two-ligand) complex than the ML (one-ligand) complex, consistent with a linear two-S-coordinated Cys–M–Cys binding geometry being a preferred low-strain mode for soft-acceptor metals.

Correlations between TCP molecular-weight-fraction content and lgβ (Table 5, p. 8051; Pearson r):

The authors interpret the much stronger Mw-dependent signal for Pb²⁺ than for Hg²⁺ or Cd²⁺ as evidence that steric accessibility of the binding groups matters more for Pb²⁺ than for the softer metals. No fraction-III (1–3 kDa) correlation reached significance for any metal.

Methods (brief)

Starting material. Defatted soy flakes (Shandong Gushen Industrial, China; 53.13% protein, 87% nitrogen solubility index). Soy glycinin (11S) prepared by the method of Wolf (1993) without further purification: flakes suspended in 30 mM Tris-HCl, pH 8.0, with 10 mM 2-mercaptoethanol; centrifuged; supernatant brought to pH 6.4 with 2 M HCl to precipitate proteins; precipitate redissolved at pH 8.0, dialysed against distilled water, and freeze-dried. Final 11S protein content: 95.3 ± 1.6% (w/w) by micro-Kjeldahl (N × 6.25); purity 90 ± 3.1% by SDS-PAGE.

Hydrolysis. Alcalase (Novozymes 2.4 L FG, 180,000 uranium (U)/mL), papain, and pepsin (Sigma) used at E/S ratio 5:100 (w/w). Conditions: alcalase 50 °C pH 8.0; papain 50 °C pH 7.0; pepsin 37 °C pH 2.0. pH-stat method with 0.5 M NaOH (alcalase, papain) or 1 M HCl (pepsin). Hydrolysis stopped at 5%, 15%, and 25% DH by boiling 5 min. Supernatant lyophilised.

TCP enrichment. Hydrolysate at 50 g/L reduced with 30 mM DTT, pH 8.0, 50 °C, 30 min. pH adjusted to 3.0; loaded on Sephadex LH-20 to remove excess DTT (eluted with 1 mM HCl). Eluate pH-adjusted to 7.5, brought to 0.5% SDS, and applied to Thiopropyl-Sepharose 6B beads. Bound peptides liberated with 25 mM DTT in 10 mM Tris-HCl pH 7.5. Excess DTT and 2-thiopyridine removed on a C-18 column.

Sulfhydryl content. 4,4’-Dithiodipyridine (4-DPS) method (Riener et al. 2002): 0.3 mL TCP solution (~25 µmol/L sulfhydryl) + 2.7 mL 0.1 M citrate-Na₂HPO₄ buffer pH 4.5 with 1% SDS + 125 µL 4-DPS (4 mM); A₃₂₄ read after 30 min ambient; ε = 21,400 M⁻¹ cm⁻¹. Total sulfhydryl after NaBH₄ reduction of disulfides (Cavallini et al. 1966 modified).

Molecular-weight distribution. SEC on TSK gel 2000 SW_XL (Tosoh, 300 × 7.8 mm) at 0.5 mL/min in acetonitrile/water/TFA = 45/55/0.1 (v/v/v); detection at 214 nm. Calibration: cytochrome C (12,500 Da), bacitracin (1450 Da), tetrapeptide GGYR (451 Da), tripeptide GGG (189 Da).

Potentiometric titration. Lab-assembled cell: 50 mL thermostatted, Teflon-stoppered reactor with high-precision pH probe (Mettler-Toledo FE 20) and inert-gas (N₂) atmosphere. Buffers pH 4.00, 6.86, 9.18 used for calibration. Titrations at 25 ± 0.1 °C, pH range 2.0–12.0, with 0.1 M KOH as titrant. Sample volume 25 mL; TCP concentration 4 mM (assumed equal to sulfhydryl concentration on the basis of one –SH per TCP molecule); metal-ion (Hg²⁺, Cd²⁺, or Pb²⁺) concentration 1 mM where present.

Stability-constant calculation. Bjerrum method. Protonation constants β_j^H calculated from titrations in absence of metal ions by non-linear least-squares (equations 1–3 in the paper, p. 8053). Average ligand-per-metal coordination number n̄ obtained by the half-integral method by plotting against pL (=−log[L]); stepwise stability constants Kₙ read from the formation curve at the pL where n̄ = n − ½; overall stability constants βₙ computed as K₁·K₂·…·Kₙ.

Statistics. ANOVA with SPSS; LSD test at 95% confidence; triplicate runs reported as mean ± SD; Pearson correlation coefficients reported in Figures 5 and Table 5.

What this paper does not measure. No Pb, Cd, or Hg occurrence in soy beans, soy flakes, soy protein isolate, soy-based infant formula, or any food product. No in-vivo or in-vitro digestion model; no animal or human chelation outcome. The “functional food” framing in the conclusion is forward-looking; no human or animal data on whether oral TCP intake reduces body-burden of these metals are presented.

Implications

Microbiome: Not applicable. No microbiota or gut-microbial measurements.

Related evidence

• Mercury, Total
• Cadmium
• Lead

Update history

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