Calorimetric study of rheumatoid factor binding to human IgG

The binding of human IgM monoclonal rheumatoid factor (RF) to native human IgG was studied in a flow calorimeter. Thermal titration gave a enthalpy of binding of 5.5 X 10(4) J/mole site for the reaction, with 1 mole of RF binding to 2.5 moles of IgG; this corresponds to one RF subunit per IgG heavy chain. Similar experiments performed with heat-aggregated human IgG gave almost identical results. If the IgG molecule undergoes a conformational change upon heating, this is not such as to affect the enthalpic contribution of binding.