Sequestration of125I-Labeled p Nerve Growth Factor by Embryonic Sensory Neurons

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Abstract
Nerve growth factor (NGF) binds to 2 specific receptors on sensory nerve cells. These 2 receptors are characterized by different Kd. The higher affinity (type I) receptors have a Kd of 3.3 .times. 10-11 M. The lower affinity (type II) receptors have a Kd of 1.7 .times. 10-9 M. These 2 receptors are not a result of negative cooperativity, but apparently are different receptors. At 22.degree. C the rate of association is 1 .times. 107 M-1 s-1 and the rates of dissociation are 6.5 .times. 10-4 s-1 (type I) and 3.2 .times. 10-2 s-1 (type II). After binding, a time-dependent process occurs that makes the NGF inaccessible to the external milieu (sequestered). The sequestration process is energy-dependent, but apparently temperature-independent. Evidently only the type I receptors are involved in the sequestration process. This process is similar to that observed on sympathetic neurons and may be the 1st step in the internalization of NGF by responsive cells.