Thiol groups of normal human immunoglobulin G

Area of Science:

• Biochemistry
• Immunology
• Protein Chemistry

Background:

• Human immunoglobulin G (IgG) structure and function are critical in immunology.
• Disulfide bonds play a key role in IgG structural integrity.
• Previous estimates suggested a small but consistent level of reduced thiol groups in IgG.

Purpose of the Study:

• To investigate the oxidation state of disulfide bonds in normal human IgG.
• To identify and quantify the reduced cysteine residues within IgG.
• To reconcile existing estimates of free thiol groups in IgG.

Main Methods:

• Treatment of normal human IgG with radioactive N-ethylmaleimide under denaturing conditions.
• Thermolytic digestion of heavy and light chains.
• Isolation and sequencing of radioactive peptides to identify reaction sites.
• Quantification of radioactivity in peptides to determine the proportion of reduced half-cystine residues.

Main Results:

• Six radioactive peptides from the light chain and ten from the heavy chain were identified.
• All sequenced peptides corresponded to half-cystine residues involved in disulfide bonds.
• The proportion of reduced half-cystine residues ranged from 0.57% to 2.54% across different sites.
• These findings support the existence of partially reduced disulfide bonds in normal IgG.

Conclusions:

• Normal human IgG disulfide bonds are not fully oxidized, with a measurable proportion of reduced cysteine residues.
• The observed level of reduced thiols is consistent with previous estimates of free SH groups in IgG.
• Variable cysteine residues, if present, are likely rare in normal immunoglobulins.