Properties and Specificity of Binding Sites for 125I-Nerve Growth Factor in Embryonic Heart and Brain

The specific binding of 125I-nerve growth factor (125I-NGF) to the low speed pellet fraction of dissociated 13-day chick embryo heart and brain is a complex, nonsaturable process exhibiting a range of affinities (heart,Ka = 2.9 x 109 to 6 x 106 liters per mole and brain, 2.5 x 109 to 8 x 106 liters per mole). The binding to both tissues at 24° is rapid (k+1 = 7 x 106 m-1 s-1) and reversible (k-1 = 7 x 10-4 s-1) and the dissociation of bound 125I-NGF is accelerated by 8 x 10-8 m native NGF (k-1 = 4 x 10-3 s-1). Insulin and proinsulin inhibit the binding of 125I-NGF 20 to 30% at concentrations of 10-9 to 10-8 m, lower or higher concentrations being less effective. Bovine growth hormone, human placental lactogen and glucagon inhibit 125I-NGF binding only to the extent of 5 to 8%, and this only at high concentrations. The regional distribution of specific 125I-NGF binding in heart correlates with regions of sympathetic innervation. The properties of 125I-NGF binding to these specific sites in embryonic heart and brain are very similar to those of the NGF receptor of sympathetic and dorsal root ganglia, the recognized target tissues for NGF.