Experiments with rabbit aortic strips and pithed rats indicated that l-Sar-8-AJa-angiotensin II (Sar-Arg-Val-Tyr-Val-His-Pro-Ala) is a specific competitive antagonist of the vascular action of angiotensin II. Norepinephrine-induced hypertension was not affected by infusions of l-Sar-8-Ala-angiotensin II which evoked a dose-related reduction of angiotensin II-induced hypertension in conscious rats. Infusions of l-Sar-8-Ala-angiotensin II that caused a doserelated reduction of the blood pressure of conscious rats in the acute phase of unilateral renal hypertension were ineffective during the chronic phase of such hypertension. Infusions of l-Sar-8-Ala-angiotensin II lasting 1 hour did not reduce the blood pressure of normotensive, spontaneously hypertensive, or metacorticoid hypertensive conscious rats. These data indicate that, under certain experimental conditions, the blood pressure of rats during the acute phase of unilateral renal hypertension is maintained by the endogenous angiotensin II previously demonstrated to be present in the plasma in supernormal concentrations. The blood pressure of normotensive, spontaneously hypertensive, metacorticoid hypertensive, and chronic unilateral renal hypertensive rats previously shown to have normal plasma levels of renin and angiotensin II appeared to be independent of the pressor action of endogenous angiotensin II. This is additional direct evidence implicating the pressor action of angiotensin II in the etiology of renal hypertension. KEY WORDSspecific competitive inhibition renal hypertension metacorticoid hypertension spontaneous hypertension pep tide synthesis rabbit aortic strips pithed rats conscious rats From the Pharmacology Section and Polypeptide

Peptides that contain difluorostatine and difluorostatone residues have been shown to be potent inhibitors of the aspartyl protease renin. The readily hydrated fluoro ketone is proposed to mimic the tetrahedral intermediate that forms during the enzyme-catalyzed hydrolysis of a peptidic bond. It is suggested that the sp3-hybridized ketal acts as a transition-state analogue renin inhibitor. The fluoro ketone is shown to be a much more effective inhibitor than the corresponding nonfluorinated ketone, which acts as a pseudosubstrate. More lipophilic side chains at the P1 site can enhance the inhibitory potency of the difluorostatine analogue, but this cannot be demonstrated in the difluorostatone series. Additionally, high renin specificity has been shown for a difluorostatone-containing peptide.

The effect of 1-Asn-8-Ala-angiotensin II (Asn-Arg-Val-Tyr-Val-His-Pro-Ala) on the vascular action of angiotensin II was studied with rabbit aortic strips, pithed rats, and conscious rats. Dose-response (contraction) curves for angiotensin II on rabbit aortic strips shifted progressively to the right with increasing bath concentrations of 1-Asn-8-Ala-angiotensin II. The pA 2 value (an indication of the relative affinity of a competitive antagonist for an agonist's receptor site) was 6.84 ± 0.03. Dose-response curves for norepinephtine, serotonin, and histamine were relatively unaffected. No pA 3 values could be obtained with these agonists in the concentration range tested. Partial dose-response (pressor) curves for angiotensin II in pithed rats shifted to the right during infusions of 1-Asn-8-Ala-angiotensin II at 20 and 60 µg/kg min -1 Partial dose-response curves for vasopressin, phenylephririe, and tyramine were unaffected. Infusions of 1-Asn-8-Ala-angiotensin II counteracted the pressor effect of angiotensin II infusions in pithed and conscious rats. Infusions of 1-Asn-8-Ala-angiotensin II (200 µ/ kg min -1 ) effective in reducing the blood pressure of conscious angiotensin II-infused rats caused only small pressor effects in conscious normotensive rats but reduced the blood pressure in 75% of the conscious rats with acute unilateral renal hypertension. The data presented are consistent with the conclusion that 1-Asn-8-Ala-angiotensin II is a specific competitive antagonist of the vascular action of angiotensin II both in vitro and in vivo.

A model of the conformation of the enzyme-bound inhibitor of human renin suggested the possibility of a gamma-lactam conformational restriction at the P2-P3 site. Synthetic routes to these gamma-lactam dipeptide isosteres and their incorporation into potential renin inhibitors are described. Peptide VIa,b with a gamma-lactam conformational constraint and a hydroxyethylene isostere at the cleavage site inhibited human plasma renin with an IC50 value of 6.5 nM. The flexibility of these syntheses should make available a number of potential enzyme inhibitors with this structural feature for the study of enzyme-bound conformers.

The preparations of sodium 4(S)-[(tert-butyloxycarbonyl)amino]-2,2-difluoro-3(S)- and -3(R)-[(4-methoxyphenyl)amino]-6-methylheptanoates (7a and 7b) from sodium 4(S)-[(tert-butyloxycarbonyl)amino]-2,2-difluoro-3(R)- and -3(S)-hydroxy-6-methylheptanoates (1a and 1b) are described. The key step involves the stereospecific intramolecular displacement via a Mitsunobu reaction for the conversion of a beta-hydroxy hydroxamate to a beta-lactam ring. Compounds 7a and 7b are useful as synthetic intermediates for the preparation of enzyme inhibitors that contain 3(S),4(S)- and 3(R),4(S)-diamino-2,2-difluoro-6-methylheptanoic acid inserts. Angiotensinogen analogues VII and VIII that contain these novel amino analogues of difluorostatine were shown to be inhibitors of the enzyme renin. The alpha,alpha-difluoro-beta-aminodeoxystatine-containing compounds were shown to be weaker inhibitors than the corresponding difluorostatine-containing congeners.