Photoreactive enkephalin analogue: [D-Ala2, p-N3-Phe 4-Met 5] enkephalin

Synthesis of a biotinylated, iodinatable, and photoactivatable probe for angiotensin receptors

We propose here a biotinyl-aminohexanoyl-[Ala1, Phe(4N3)8]angiotensin II analog as a radioiodinatable and photoactivatable probe for covalent labeling, detection and isolation of angiotensin receptors. A combination of solid phase and minimum-protection segment-coupling strategy using hexafluorophosphate of (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium (BOP) as a coupling reagent is proposed for the synthesis of this probe. Optimized yields were obtained by HPLC monitoring of all reactions. A complete n.m.r. study suggests an extended conformation of this molecule, allowing a simultaneous recognition of receptor and avidin. The probe binds with high affinity (Kd = 2 nM) to angiotensin II receptors from rat liver membranes.

Photoaffinity labeling of opioid receptor of rat brain membranes with 125I(D-Ala2, p-N3-Phe4-Met5)enkephalin

A photoreactive (D-Ala2, p-N3-Phe4-Met5)enkephalin derivative was prepared, iodinated with carrier-free 125I, and then purified by high-performance liquid chromatography. The purified radioactive photoprobe was monoiodinated at the amino terminal tyrosine residue. This radioactive photoprobe was used to photoaffinity label membranes prepared from the rat brain (minus cerebellum) and the spinal cord. The photolabeled membranes were analyzed by sodium dodecyl sulfate gel electrophoresis. A 46,000-Da protein was specifically photolabeled in these membrane preparations. The photolabeling of this protein was inhibited by peptides related to enkephalin but not by unrelated substance P or gastrin tetrapeptide. A concentration-dependent inhibition of the photolabeling of the 46,000-Da protein was observed in the presence of competing ligands specific for the mu-, delta-, and kappa-opioid receptors. These data demonstrate that the radioactive photoprobe labels the mu-, delta-, and kappa-opioid receptors. Although there is no evidence available to show that the 46,000-Da protein is identical in all the cases, our data strongly suggest that it is a binding protein common to all of the opioid receptor subtypes.

Identification of mammalian brain and spinal cord opioid receptor by photoaffinity labeling

A radioiodinated photoreactive enkephalin derivative, 125I(D-Ala2, p-N3-Phe4-Met5)enkephalin, was used to photoaffinity label the opioid receptor from the membranes of four mammalian brains (without cerebellum) and spinal cords. These included the cat, rabbit, guinea pig and mouse. The photolabeled membranes were analyzed by sodium dodecyl sulfate gel electrophoresis. A 43,000-daltons protein was specifically photolabeled in all the membranes tested, as the specific labeling of this protein was inhibited in the presence of 14.5 microM of (D-Ala2, Met5)enkephalin. These data suggest that the 43,000-daltons protein is a binding protein of the opioid receptor in the different mammalian neural tissues.