Characterization of [3H][2-D-penicillamine, 5-D-penicillamine]-enkephalin binding to delta opiate receptors in the rat brain and neuroblastoma--glioma hybrid cell line (NG 108-15).

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Abstract
Specific binding properties of the tritium-labeled .delta. opiate receptor agonist [3H][2-D-penicillamine, 5-D-penicillamine]enkephalin ([3H](D-Pen2, D-Pen5]enkephalin) were characterized in the rat brain and in a mouse neuroblastoma-rat glioma hybrid cell line (NG 108-15). Saturation isotherms of [3H][D-Pen2,D-Pen5]enkephalin binding to rat brain and NG 108-15 membranes gave apparent Kd values of 1-6 nM. The Bmax [maximum binding capacity] value in NG 108-15 membranes was 235.3 fmol/mg of protein. An apparent regional distribution of [3H][D-Pen2,D-Pen5]enkephalin binding was observed in the rat brain. A number of enkephalin analogs inhibited [3H][D-Pen2,D-Pen5]enkephalin binding with high affinity (IC50 [mean inhibitory concentration] values of 0.5-5.0 nM) in both NG 108-15 and rat brain membranes. Putative .mu. receptor-selective ligands such as morphine, [D-Ala2,MePhe4,Gly5-ol]enkephalin, [MePhe3,D-Pro4]morphiceptin, and naloxone were less effective inhibitors of [3H][D-Pen2,DPen5]enkephalin binding in both systems tested. [3H][D-Pen2,D-Pen5]enkephalin is a potent and selective ligand for the .delta. opioid receptor.