Properties of the β Nerve Growth Factor Receptor of Avian Dorsal Root Ganglia

Iodination of the beta nerve growth factor protein up to 4.1 iodines per mole of nerve growth factor was achieved with no loss of biological activity. At this level of incorporation no native or monoiodo nerve growth factor remained, the major species being the tri- and tetraiodo derivatives. An (125)I-labeled beta nerve growth factor of high specific activity containing 0.5 iodine per mole of beta nerve growth factor was used to determine the specific binding of nerve growth factor to dorsal root ganglion cells of 8-day-old embryonic chicks. The specific binding of (125)I-labeled nerve growth factor reached saturation at 30-50 ng/ml and half-saturation at 7-8 ng/ml (0.26 nM). The number of receptors per responsive medio-dorsal cell was calculated to be about 2 x 10(4). The pattern of displacement of bound (125)I-labeled beta nerve growth factor by native beta nerve growth factor showed that the two proteins had identical affinities for the receptor. The specificity of the binding for beta nerve growth factor was demonstrated by the fact that only native beta nerve growth factor displaced the bound (125)I-labeled form. Partially inactivated derivatives of beta nerve growth factor retained the same fraction of their specific-binding capacity as of their biological activity. The specificity of the binding for cell type was shown by the lack of any specific component of (125)I-labeled beta nerve growth factor binding to liver or brain cells. The rate constant for association of beta nerve growth factor with its receptor, k(1), was 1.0 x 10(7) mol(-1) sec(-1) and the rate constant for dissociation, k(-1), 1.2 x 10(-8) sec(-1).