N-TERMINAL AND C-TERMINAL STRUCTURE-ACTIVITY STUDY OF ANGIOTENSIN-II ON THE ANGIOTENSIN AT(2) RECEPTOR

The predominant angiotensin II receptor expressed in the human myometr ium is the angiotensin AT(2) receptor. This preparation was used for a structure-activity relationship study on angiotensin II analogues mod ified in positions 1 and 8. The angiotensin AT(2) receptor present on human myometrium membranes displayed a high affinity (pK(d) = 9.18) an d was relatively abundant (53-253 fmol/mg of protein). The pharmacolog ical profile was typical of an angiotensin AT(2) receptor with the fol lowing order of affinities: (angiotensin III greater than or equal to angiotensin II > angiotensin I > PD123319 > angiotensin-(1-7) > angiot ensin-(1-6) approximate to angiotensin IV >> Losartan). Modifications of the N-terminal side chain and of the primary amine of angiotensin I I were evaluated. Neutralisation of the methylcarboxylate (Asp) to a m ethylcarboxamide (Asn) or to a hydroxymethyl (Ser) or substitution for a methylsulfonate group (cysteic acid) improved the affinity. Extensi on from methylcarboxylate (Asp) to ethylcarboxylate (Glu) did not affe ct the affinity. Introduction of larger side chains such as the bulky p-benzoylphenylalanine (p-Bpa) or the positively charged Lys did not s ubstantially affect the affinity. Complete removal of the side chain ( angiotensin III), however, resulted in a significant affinity increase . Removal or acetylation of the primary amine of angiotensin II did no t noticeably influence the affinity. Progressive alkylation of the pri mary amine significantly increased the affinity, betain structures bei ng the most potent. It appears that quite important differences exist between the angiotensin AT(1) and AT(2) receptors concerning their pha rmacological profile towards analogues of angiotensin II modified in p osition 1. On position 8 of angiotensin II, a structure-activity relat ionship on the angiotensin AT(2) receptor was quite similar to that ob served with angiotensin AT(1) receptor. Bulky, hydrophobic aromatic re sidues displayed affinities similar to or even better than [Sarcosine( 1)]angiotensin II. Aliphatic residues, especially those of reduced siz e, caused a significant decrease in affinity especially [Sarcosine(1), Gly(8)]angiotensin II who showed a 30-fold decrease. Introduction of a positive charge (Lys) at position 8 reduced the affinity even furthe r. Stereoisomers in position 8 (L --> D configuration) also induced lo wer affinities. The angiotensin AT(2) receptor display a structure-act ivity relationship similar to that observed on the AT(1) receptor for the C-terminal position of the peptide hormone. Position 1 structure-a ctivity relationships are however fundamentally different between the angiotensin AT(1) and AT(2) receptor. (C) 1998 Elsevier Science B.V.