COMPARISON OF THE BINDING CHARACTERISTICS OF TRITIATED OPIATES AND OPIOID PEPTIDES

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Abstract
Binding assays on homogenates of guinea‐pig brain showed that the maximal number of binding sites was different for different tritiated ligands interacting with the opiate receptors. At 25°C the binding capacity of morphine or dihydromorphine was only about 3 pmol/g fresh brain whereas etorphine and d‐Ala2‐l‐Leu5‐ and d‐Ala2‐l‐Met5‐enkephalin amide had capacities of 13 to 15 pmol/g brain. d‐Ala2‐d‐Leu5‐enkephalin had an intermediate capacity of about 6 pmol/g brain. The binding capacities of the natural methionine‐ and leucine‐enkephalins measured at 0°C were 5 to 6 pmol/g brain. At this temperature, the binding capacity of dihydromorphine, d‐Ala2‐d‐Leu5‐enkephalin and of the two enkephalin amides was only slightly lower than at 25°C. In assays in which unlabelled ligand competed with the same labelled ligand, the inhibition constants (K1) were equal to or not more than twice as large as the equilibrium dissociation constant (Kd) determined in saturation assays. In contrast, the K1 of unlabelled dihydromorphine against [3H]‐d‐Ala2‐d‐Leu5‐enkephalin or of unlabelled d‐Ala2‐d‐Leu5‐enkephalin against [3H]‐dihydromorphine were about 20 times higher than the respective Kd values. When for a given compound the ratio of the K1 value against [3H]‐d‐Ala2‐d‐Leu5‐enkephalin to the K1 value against [3H]‐dihydromorphine (discrimination ratio) is calculated, a high value indicates selectivity in favour of the μ‐receptor and a low value selectivity in favour of the δ‐receptor. The most selective μ‐agonist known so far is normorphine with a discrimination ratio of 70 and the most selective δ‐agonist is d‐Ala2‐d‐Leu5‐enkephalin with a ratio of 0.11. The selectivity of the known antagonists is in favour of the μ‐receptor, since their discrimination ratios are larger than 1, varying between 10 for naloxone and 4 for Mr 2266.

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