Sequestration of 125I-labeled beta nerve growth factor by sympathetic neurons

Nerve growth factor binds to cell surface membrane receptors on sensory and sympathetic neurons. The binding of radiolabeled beta nerve growth factor to sympathetic neurons is characterized by two specific binding sites with similar dissociation constants to those seen for sensory neurons. The high affinity site, type I, has an apparent dissociation constant of 1.1 X 10(-11) M. The lower affinity site, type II, has an apparent dissociation constnat of 5.0 X 10(-10) M. When radiolabeled beta nerve growth factor is incubated with sympathetic nerve cells for various lengths of time, as much as 50% of the specifically bound labeled nerve growth factor is sequestered. Sequestration is time- and energy-dependent, and appears to o ccur mainly through type I sites. These data suggest a step which is subsequent to the binding of nerve growth factor to its responsive cells and may be the first step in the process that allows nerve growth factor to be internalized.