A kinetic study of human IgG binding to placental Fc gamma-receptor.

Various methods for studying the IgG interaction with its placental receptor have been examined to establish kinetic parameters. IgG binding has an average rate constant k1 of (2.57 +/- 0.94) x 10(7)/M per min. However, high displacement doses (1000 micrograms) of unlabelled IgG resulted in a curvilinear dissociation curve with two binding sites: one with a fast dissociation rate constant, k2 of 0.100/min and a slow one with k2 of 0.010/min. IgG binding was associated with a high average affinity of (3.70 +/- 1.65) x 10(8)/M and a total number of receptor binding sites of (2.07 +/- 0.93) x 10(14) sites/mg of membrane protein, in close agreement with previous results. The stoichiometry of IgG to Fc gamma-receptor was established as a 4:1 binding ratio from log dose-response curves as opposed to a 1:1 binding ratio from previous reports.