Binding sites for 125I-neuropeptide Y (NPY) on membranes from bovine adrenal medulla

Bovine adrenal medulla membranes were examined for the presence of specific 125I-neuropeptide Y (125I-NPY) binding sites using rapid centrifugation to measure the amount of bound ligand. Specific binding was determined from the difference between 125I-NPY bound in the presence and absence of 10(-7) M unlabeled NPY. The binding was saturable and reached equilibrium within 5 min at 0 degrees C. Analysis of specific 125I-NPY binding using the LIGAND computer program indicated a best fit for a two site model with a Kd of 0.26 nM and a Bmax of 12 fmol/mg protein for the high affinity site and a Kd of 170 nM and a Bmax of 6 pmol/mg protein for the low affinity site. The rate of dissociation (k-1) was 0.071/min with a t1/2 of 9 min. Displacement curves for avian or human pancreatic polypeptide revealed that these peptides displaced 125I-NPY from both sites with IC50 values greater than 10 nM.