Interaction of putative opioid peptides with opiate receptors.
Binding of 3H-enkephalinamide and 3H-naloxone to P2-fractions of whole rat brain homogenate displayed saturable, stereospecific binding to receptor sites with at least two binding sites for 3H-metenkephalinamide (type I: KD = 0.4 nM and Bmax = 35 fmol/mg protein; type II: KD = 5.7 nM and Bmax = 57 fmol/mg protein) and for 3H-naloxone (type I: KD = 1.5 nM and Bmax = 40 fmol/mg protein; type II: KD = 51 nM and Bmax = 255 fmol/mg protein). beta-endorphin and met- and leu-enkephalin produced a concentration-dependent inhibition of 3H-met-enkephalinamide and 3H-naloxone binding with dissociation constants in the nanomolar range, but with very different displacement curves. Purified porcine ACTH (1-39) displaced both 3H-met-enkephalinamide and 3H-naloxone with dissociation constants of 3.4 X 10(-7) M and 1.8 X 10(-6) M, respectively. The synthetic congeners, ACTH (1-32) and to a lesser extent ACTH (1-28) and ACTH (1-24) showed a similar effect, whereas other fragments of ACTH were inactive in concentrations ranging from 10(10) to 10(-6) M. In the same concentration range cholecystokinin congeners (CCK-8 and CCK-4) were without effect. Since ACTH immunoreactive nerves seem also to contain beta-endorphin and furthermore, to show a partially overlapping distribution with the enkephalinergic systems it is possible that the binding of ACTH fragments to opiate receptors is of physiological relevance.