[CYS(O2NH2)(2)]ENKEPHALIN ANALOGS AND DALARGIN - SELECTIVITY FOR DELTA-OPIOID RECEPTORS

To investigate the structure-activity relationships for potent and sel ective action of enkephalins at the delta-opioid receptors, two newly synthesized analogues, [Cys(O2NH2)(2),Leu]enkephalin and [Cys(O2NH2)(2 ),Met(5)]enkephalin and the hexapeptide [D-Ala(2),Leu(5)]enkephalyl-Ar g (dalargin) were tested and compared with [Leu(5)]enkephalin and [Met (5)]enkephalin, for their effectiveness to inhibit electrically evoked contractions of the mouse vas deferens (predominantly enkephalin-sele ctive delta-opioid receptors) and the guinea pig ileum (mu- and kappa- opioid receptors). The mouse vas deferens assays included evaluation o f the effects of opioid agonists on the first, purinergic, and the sec ond, adrenergic, components of electrically evoked biphasic responses (10 Hz and 20 Hz) and on ATP- or noradrenaline-evoked, tetrodotoxin-re sistant responses. The opioids tested inhibited in a similar manner: ( i) the purinergic and the adrenergic components of the electrically ev oked contractions; and (ii) the ATP- and noradrenaline-induced postjun ctional responses of the mouse vas deferens. Extremely low IC50 values (of 2-5 orders) were found for [Cys(O2NH2)(2),Leu(5)]enkephalin, whos e relative potency was between 239 and 1316 times higher than that of [Leu(5)]enkephalin. The order of potency for the other peptides in thi s tissue was: [Cys(O2NH2)(2),Met(5)]enkephalin > [Leu(5)]enkephalin > dalargin > [Met(5)]enkephalin. The highest IC50 values in the guinea p ig ileum assays, indicating the lowest affinity for mu/kappa-opioid re ceptors, were obtained for the cysteine sulfonamide analogues, while d alargin showed a potency four times higher than that of [Met(5)]enkeph alin. The order of potency in this tissue was: dalargin > [Met(5)]enke phalin > [Leu(5)]enkephalin > [Cys(O2NH2)(2),Met(5)]enkephalin > [Cys( O2NH2)(2),Leu(5)]enkephalin. The ratio, IC50 in guinea pig ileum:IC50 in mouse vas deferens, indicating selectivity of the respective peptid e for delta-opioid receptors, was extremely high for [Cys(O2NH2)(2),Le u(5)]enkephalin and especially for the adrenergic component of the res ponses. This ratio for [Cys(O2NH2)(2),Met(5)]enkephalin was higher tha n the ratios for dalargin, [Leu(5)]enkephalin and [Met(5)]enkephalin, which were about 3 orders of magnitude lower. The results suggest that incorporation of hydrophilic Cys(O2NH2) in the enkephalin molecule gr eatly increases the potency and selectivity of the analogues at delta- opioid receptors, while both D-Ala(2) substitution and lengthening of the peptide chain by Arg(6) in the molecule of [Leu(5)]enkephalin decr ease them.