Disulfide bridge formation between C1q and IgG in vitro

Area of Science:

• Biochemistry
• Immunology

Background:

• The globular heads of complement protein C1q contain free sulfhydryl (SH) groups.
• These SH groups are typically concealed in the native C1q molecule.

Purpose of the Study:

• To investigate the structural and functional changes in C1q upon interaction with dialysis membranes, immunoglobulin (Ig) aggregates, and during iodination.
• To elucidate the mechanism behind these C1q modifications, particularly the role of free sulfhydryl groups.

Main Methods:

• Exposure of C1q SH groups through interaction with a dialysis membrane.
• Oxidation of exposed sulfhydryls during iodination, leading to disulfide-linked C1q aggregates.
• Analysis of C1q bound to immunoglobulin aggregates, including resistance to elution and requirement for reducing agents.

Main Results:

• Dialysis membrane interaction exposes C1q's concealed free-SH groups.
• Iodination oxidizes these SH groups, forming disulfide-linked C1q aggregates.
• A portion of C1q bound to Ig aggregates resists standard elution, requiring reducing agents, indicating disulfide bond formation.

Conclusions:

• Dialysis, Ig adsorption, and iodination induce significant structural and functional alterations in C1q.
• Disulfide bridging between C1q and IgG in vitro suggests a potential physiological role for C1q's conserved free cysteines in immune complex binding.