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.

The content of free SH groups and disulfide bonds in the purified pig kidney Na+,K+-ATPase was determined by ammetric titration with silver nitrate. In the native enzyme, most of the free SH groups are masked due to their location in the polypeptide chain regions poorly accessible to SH reagents. Denaturation with 5% SDS and 8 M urea makes these regions accessible thus revealing 22 free SH groups/mol of the protein. After complete blocking of free SH groups with silver ions, 8 SH groups/mol of the protein are being released upon sulfitolysis which indicates the presence of four disulfide bonds in the enzyme. At least one disulfide bridge is located in the a-subunit whereas the @-subunit contains three disulfide bonds.

The experimental evidences for photochemical mechanism of light activation of immune system are presented. The experiments were made in vitro using serums from normal blood, from blood of people with secondary immunodeficiericy containing low-avidity IgG (up to 75-90%), and pwe IgG buffer solutions obtained from the same serums. LED arrays and halogen lamps with narrow spectral filters from UV to JR were used as optical monochromatic sources. We have discovered that light can activate immune system by IgG transformation from low-avidity state to high-avidity one. This change has multistage irreversible character and depends on light wavelength and intensity. The estimations of optical effects were made by determination of JgG functional activity and quaternary antibody structure through the number of accessible functional protein residues in the IgG buffer solutions. Both methods showed very correlated results. Each IgG avidity transformation stage correlated with definite change of antibody spatial structure that, to our opinion, corresponds to the phenomenon pass through the several intermediate metastable forms, which are maintained by different intermolecular bonds. The bioaction spectrum ofdiscovered effects is also presented.

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