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The Effects of Aggregated Immunoglobulins on Human Blood Platelets in Relation to Their Complement-Fixing Abilities¹

II. Structural Requirements of the Immunoglobulin

From the Theodor Kocher Institute, University of Berne, Berne, Switzerland

Abstract

When the amino groups of aggregated human immunoglobulin G (IgG) were modified by acetylation or acetimidation, its ability to bind complement and to stimulate the release reaction of platelets disappeared, modification of 40% to 46% of the amino groups by acetimidation being sufficient for marked inhibition. By reaction of 26% of the amino groups with dinitrofluorobenzene, platelet stimulation disappeared completely and complement fixation was reduced by 50%. Modification of 3 to 5 tryptophane residues/molecule caused 40% reduction in both activities. There was some indication that tryptophane residues exposed by conformational changes during aggregation of IgG may be important in endowing the molecule with biologic activity.

Removal of 36% of the IgG hexose caused total loss of platelet stimulating activity, although complement fixation was hardly affected. These results are compatible with the idea that the sites of Clq binding and for platelet interaction have amino acid sequences in common. Studies of the inhibition of the normal release reaction by modified monomeric IgG indicate that tryptophane and carbohydrate, but not the amino groups, are necessary for the initial interaction of IgG with the platelet surface. The amino groups are most probably required for a subsequent step in the release process.

Footnotes

¹ This work was supported by the "Schweizerischer Nationalfonds zur Förderung der wissenschaftlichen Forschung."

² This author wishes to acknowledge a travel grant from the Postgraduate Medical Foundation, University of Sydney, Australia.