11x4
.Vot f'h
Journal ofhledicznal Chemzstp, 197,3. Vol 16, N o . 10
Notes Solid-Phase Synthesis of [3-Proline,8-isoleucine]-, [ I-Sarcosine,3-proline.X-isoleucine]-, a n d [4-Phenylalanine,X-isoleucine]angiotensin I1 as t h e Antagonists of t h e P a r e n t Hormone
Table I. Comparative Pressor and Antagonistic Properties of Analogs of Angiotensin IIa,b
M .( ~ Khosla. . Ivl. 51. Hall, R. R. Smeby. and F. >I. Bumpusx Kesearch Dicihion. l'he CleL'eland Clinic Foundation. C'lecPland, Ohio 11106.Receiced M a r c h 8 , 197.1
Continuation of our studies to investigate the factors which influence antagohistic properties of angiotensin I1 a n t a g o n i s t ~ l -has ~ led us to determine the effect of variations in positions 3 a n d 4 of [8-isoleucine]angiotensin 11. We reported earlier5 t h a t replacement of valine (position 3 ) with proline in angiotensin I1 reduced the pressor activity to 53% of that of the parent hormone. Recently, we observed t h a t [Pro3]angiotensin I1 when present in high concentration (100 ng/ml) in t h e bath inhibited the myotropic response of rabbit aortic strips to angiotensin 11. This was the first 3-substituted analog of angiotensin I1 in which antagonistic properties were observed without a simultaneous modification in the 8 position. Since a n aliphatic residue in position 8 of angiotensin I1 produced good antagonistic activity and sarcosine in the one position enhanced this effect, we synthesized [Pro3.IleS]- and [Sar1,Pro3,Ile8]angiotensinI1 to determine whether these substitutions enhanced the antagonistic properties of these analogs. However, compared to [Ile8]angiotensin I1 (Table 1): both these compounds were found to have l o a antagonistic activity against angiotensin 11. Replacement of tyrosine (position 4) with alanine reduced the antagonistic activity of [Ile8]angiotensin II.4 In order to determine if the phenolic hydroxyl group is important in this position, we synthesized [Phe4,11e8]angiotensin 11. The antagonistic activity of this compound (Table I) was found t o be between t h a t of [Ile8]angiotensin I1 and [Ala4,1les]angiotensin 11. This indicates that both the aromatic ring and the phenolic hydroxyl group in position 4 are required. The phenolic group of tyrosine in angiotensin I1 is not involved in intramolecular hydrogen bonding and is therefore available for hydrogen bonding to a suitable group on the receptor.6.7.t With the limited evidence available for t h e antagonistic peptide, [Ileslangiotensin II.x it is reasonable t o assume t h a t the same situation prevails with regard to the phenolic group of this peptide. The results available suggest t h a t an angiotensin I1 a n a log is agonistic or antagonistic to myotropic response depending on the nature of t h e C-terminal amino a ~ i d l - ~ while the functional groups in the remainder of t h e chain play an important role in determining the potency of the peptide. This may be because of their role in determining the peptide's conformation, in altering the binding onto the receptor, or in increasing resistance to metabolism or it may be due to all of these phenomena. We synthesized the C-terminal tetrapeptide of [Ile8]angiotensin I1 (IleHis-Pro-Ile) and found it t o have very weak inhibitory activity. t 5 . Fermandjian, J . L. Morgat, P. F'iomageot. C . Lutz. and .J. n a m . unpublished results.
F' Leick-
Analog of angiotensin I1
Log K ,
Pressor activityc
[Ilea [Pro3,11eS1[Sar1,Pro:ZleB ][Phe4,11e81[Pro3] - A
9.21 i 1.02c 6.92 i 0.18 (6Id 7.17 + 0.05 (6) 7.17 f 0.15 (6) 6.80 f 0.11 (6)
0.80 1.50 0.01 53 .OOf
.___
-
Ile-His-Pro-Ile caused inhibition of angiotensin I1 at a dose level of 1 wg'ml. This tetrapeptide did not inhibit response of epinephrine. T h e antagonistic activity was determined on rabbit aortic strips? and, for comparison, has been expressed as log I
