Dissociation kinetics of 125I-nerve growth factor from cell surface receptors. Acceleration by unlabeled ligand and its relationship to negative cooperativity.

Abstract

The dissociation kinetics of 125I-labeled nerve growth factor (NGF) from dissociated cells of the 9-day chick embryo dorsal root ganglion were studied following preincubation for 2 h at 25 degrees C with 15 to 30 pM125I-NGF. Subsequent dilution in the presence of 4 nM NGF resulted in greater initial dissociation than occurred with dilution alone. This effect was largely limited to the first half-hour after addition of unlabeled NGF; thereafter, cell-associated radioactivity declined at the same rate for both dilution conditions. In addition, dissociation kinetics did not follow a single first order rate law in the presence or absence of excess unlabeled NGF. These data are not consistent with a model of a single class of high affinity NGF receptors, whether or not they are noninteracting or negatively cooperative. Alternative models based on multiple noninteracting or negatively cooperative receptors might explain these results, but the current data do not allow clear discrimination among them.