Table 1.

Effect of depleting rapidly binding dimers on the ability of the remainder to initiate phosphorylation of receptor

Time, min	Unfractionated preparation			Depleted preparation
	Bound dimer, cpm	PY, OD units	PY per bound dimer	Bound dimer, cpm	PY, OD units	PY per bound dimer
1	1483 ± 43	501 ± 27	0.34 ± 0.02	ND	ND
3	2840 ± 144	1757 ± 3.8	0.62 ± 0.03	1421 ± 59	829 ± 55	0.58 ± 0.05
9	5467 ± 200	2760 ± 8.9	0.50 ± 0.02	2616 ± 233	1501 ± 286	0.57 ± 0.12
15	ND	ND		3495 ± 185	1778 ± 315	0.51 ± 0.09

RBL-2H3 cells at 6.25 × 10⁶/ml (≈3 × 10⁻⁹ M receptors) were reacted with 0.3 μg/ml of ¹²⁵I-labeled rat IgE oligomer (≈1.5 × 10⁻⁹ M total IgE, of which ≈55% was determined to be bindable in a separate experiment) in a total volume of 15 ml. At the times indicated, duplicate samples of 0.8 ml were taken for immunoprecipitation and duplicate 0.05 ml samples were taken for measuring the amount of bound IgE. After the sampling at 9 min, the remaining suspension was centrifuged and the supernatant was added to freshly prepared cells. This second incubation was estimated to contain the same concentration of receptors as in the first, but only 1.05 × 10⁻⁹ M of total IgE based on the radioactive counts. Phosphotyrosines on the β and γ subunits of the receptors were quantitated (see Materials and Methods). The data are the averages of two samples and the range is shown. The values have not been corrected for the small amount of non-specifically bound oligomers; the relatively low value for the 1-min time point in the initial incubation (top of column 4) is likely accounted for by the fact that this value in particular would be affected by the nonspecifically bound material. ND, not determined.