Protein A from Staphylococcus aureus The Effect of Iodination on the Reactivity with Immunoglobulin G

Protein A, a cell‐wall protein from Staphylococcus aureus containing 4 residues each of histidine and tyrosine, was iodinated at 0°C and pH 8.6 with about a 35‐fold molar excess of KI3. The incorporation of iodine as followed with 125I was comparatively slow, i.e. 8 atoms were incorporated in about 14 h and 12 atoms in about 12 days. An electrophoretically homogeneous fraction of iodo‐protein A containing 3.5 iodine atoms was isolated and shown to contain only 0.68 residues of mono‐iodotyrosine and 0.12 residues of di‐iodotyrosine. However, iodohistidine was identified in enzymatic digests, and thus most probably the histidine residues are iodinated faster than the tyrosine residues. Catalytic dehalogenation of iodoprotein A in tritium was unsuccessful. The circular dichroism spectrum of iodoprotein A was compared with that of native protein A at different pH values and the changes seen could be fully related to the increased acidic properties of iodotyrosine. The reactivity of protein A towards the Fc part of immunoglobulin G was decreased as a consequence of the iodination. However, the reactivity towards specific anti‐protein A serum was unchanged.