Binding of Leu5-enkephalin and Met5-enkephalin to a particulate fraction from rat cerebrum.

Leucine-enkephalin and methionine-enkephalin are two pentapeptides with morphine like properties, naturally present in mammalian brain. They were first characterized by Hughes et al. [ 1 ] as H-TyrGlyGlyPheLeu-OH and H-TyrGlyGlyPheMet-OH respectively and found to behave as potent, although short lived opiates in a variety of pharmacological tests in vivo [2]. They do interact with the so-called opiate receptor (OpR) in vitro as evidenced by their ability to inhibit the binding to OpR of a variety of non-peptide opiate agonists and antagonists [3,4]. We have made use of radioactive enkephalins to demonstrate directly their interaction with the opiate receptor in vitro. It is shown in this report that both enkephalins bind with nearly equal affinities to one class of non-interacting sites in a particulate fraction from rat cerebrum. Binding of both enkephalins cannot be distinguished on the basis of its inhibition by non-peptide opiates such as levorphanol or naloxone, but is differentially affected by a variety of cations. These results are compared with those that have recently appeared in the litterature and which favor a multiplicity of enkephalin binding sites in similar preparations [2,.5,6].