Four Free Cysteine Residues Found in Human IgG1 of Healthy Donors

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N. M. Gevondyan^*, A. M. Volynskaia, and V. S. Gevondyan

Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, ul. Miklukho-Maklaya 16/10, 117997 Moscow, Russia; fax: (7-495) 335-7103; E-mail: nmgevondyan@mail.ru

^* To whom correspondence should be addressed.

Received August 9, 2005; Revision received October 4, 2005

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Modifications with different thiol reagents demonstrated that 28 of 32 cysteine residues of human IgG1 are involved in the formation of disulfide bonds, and four cysteines remain free. So IgG1 is a protein possessing both free SH-groups and disulfide bonds. Only one of the four SH-groups is accessible for silver or mercury ions and hydrophobic reagents, whereas the remaining three SH-groups are masked and can be revealed only after deep denaturation of the protein. Detection of the masked cysteine residues was shown to depend on the kinetics of intramolecular changes occurring during denaturation of the protein and on the method of the assay of the SH-groups.

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KEY WORDS: human IgG, readily accessible SH-groups, masked SH-groups, protein denaturation, antibodies

DOI: 10.1134/S0006297906030072

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