[Binding sites for 3H-Leu-enkephalin in rat straitum]

The binding of 3H-Leu-Enkephalin to a particulate fraction from Rat striatum has been investigated in the presence of 20 mum bacitracin which prevents its hydrolysis. After deduction of a low-affinity, "non-saturable" component, the saturation kinetics at equilibrium provide evidence for two distinct saturable sites. The first exhibits Michaelis kinetics with a Kd value of 2.7 +/- 0.1 nM; both morphine and naloxone compete with the binding of 3H-Leu-Enkephalin on this site but not on the second one. The marked decrease in binding of 3H-Leu-Enkephalin observed in the presence of high concentration of sodium ions indicates that these two pentapeptides have the same properties as morphinomimetic analgesics. Taken together, these data indicate that the first site is identical to the "opiate receptor", while the nature of the second saturable site remains to be established.