Apparent cooperativity in radioimmunoassay of human chorionic gonadotropin

Eight of 11 guinea pig antisera to human chorionic gonadotropin (hCG) examined in radioimmunoassay showed enhanced binding of labeled hCG after addition of small increments of unlabeled hCG. Gel chromatography of one antiserum revealed that this effect was attributable to antibodies on the 7S class. Mild papain digestion of the anti—hCG produced a modified antibody, possibly divalent F(ab)2, which showed enhanced binding and univalent Fab, which did not. These data indicated that the effect was related to the multivalence of antibodies, and tended to exclude a variety of other possible explanations. Various binding analyses suggested that the data were consistent with positive cooperativity between the two combining sites of a small population of antibodies. However, it is not clear whether the apparent cooperativity of the multivalent antibodies results from allosterism, or is related to another mechanism of enhanced binding. (Endocrinology92: 1250, 1973)