Binding properties of IgE receptors on normal mouse mast cells

Normal rat mast cells and mouse mast cells were purified, and the binding of rat IgE and mouse IgE to IgE receptors was measured. When normal rat mast cells were saturated with either rat IgE or mouse IgE, comparable numbers of IgE molecules bound to the cells. The number of the receptors on rat mast cells was approximately 3 x 10⁵/cell. The forward rate constant, k₁, dissociation constant, k_-1, and equilibrium constant, K(A), for rat IgE and mouse IgE were almost similar; K(A) for rat IgE and mouse IgE were 8 x 10⁹ M^-1 and 2.5 x 10⁹ M^-1, respectively. The results indicated that rat IgE and mouse IgE bind to the same IgE receptors on rat mast cells with comparable affinity. It was also found that IgE from the two species bound to mouse mast cells with high affinity. Forward rate constant, K₁, for the binding of mouse IgE and rat IgE to IgE receptors on mast cells from BALB/c mice were 1.9 x 10⁵ M^-1sec^-1 and 1.5 x 10⁵ M^-1sec^-1, respectively. Mouse IgE and rat IgE dissociate from the receptors with comparable rate (approximately 10^-4 sec^-1). However, mouse mast cells appear to have two distinct types of IgE receptors. One type binds both rat IgE and mouse IgE with comparable affinity, whereas the second type binds only mouse IgE. This type of receptor comprises about one-third to one-half of IgE receptors on mast cells of CBA/J mice, and about one-half to two-thirds of IgE receptors in BALB/c mice. Although the binding of rat IgE to these receptors was not detected, the presence of rat IgE along with ¹²⁵I-mouse IgE interferred with the binding of the latter protein to the receptors. It was suggested that rat IgE might associate with the second type receptors with a forward rate constant comparable to those of mouse IgE, but dissociate rapidly from the receptors.