A study of the relationship between biological activity and prolyl amide isomer geometry in oxytocin using 5-tert-butylproline to augment the Cys(6)-Pro(7) amide cis-isomer population

Three [5-t-BuPro(7)]oxytocin analogues were synthesized by substituting (2S ,5R)-5-tert-butyl-proline for proline in oxytocin, [Mpa(1)]oxytocin, and [d Pen(1)]oxytocin. Relative to oxytocin, [5-t-BuPro(7)]oxytocin and [Mpa(1),5 -t-BuPro(7)]oxytocin exhibited strongly reduced binding affinity to the rec eptor; however, both peptides maintained the pharmacophore characteristics responsible for signal transfer evoking the same maximal response as oxytoc in in the single-dose procedure and exhibiting partial agonistic activity i n the cumulative dose-response procedure. Although [dPen(1)]oxytocin exhibi ted inhibitory as well as partial agonistic activity, [dPen(1),5-t-BuPro(7) ]oxytocin exhibited only inhibitory potency with a similar in vitro pA(2) v alue of 7.50 in the absence of magnesium. In the presence of magnesium, [dP en(1),5-t-BuPro7]oxytocin exhibited stronger inhibitory potency than [dPen( 1)]oxytocin and no partial agonism. Assignment of the proton signals for th e 5-tert-butylprolyl amide cis- and trans-isomers by two-dimensional NMR ex periments in water indicated that the Cys(6)-Pro(7) peptide bond cis-isomer population was augmented relative to the prolyl peptides and measured resp ectively at 35%, 33%, and 20% in the 5-tert-butylproline(7) analogues of ox ytocin, [Mpa(1)]oxytocin and [dPen(1)]oxytocin. Although caution must be ta ken when relating the increase in cis-isomer population with an influence o n biological activity in [5-t-BuPro7(])oxytocin analogues, the synthesis an d evaluation of analogues 1-3 have provided additional evidence that can be used to support the hypothesis that the prolyl amide cis-isomer may favor antagonism and the trans-isomer is necessary for agonist activity.