Leu In Position Research Articles

Adaptive substitutions at two amino acids of HCPro modify its functional properties to separately increase the virulence of a potyviral chimera.

We had previously reported that a plum pox virus (PPV)-based chimera that had its P1-HCPro bi-cistron replaced by a modified one from potato virus Y (PVY) increased its virulence in some Nicotiana benthamiana plants, after mechanical passages. This correlated with the natural acquisition of amino acid substitutions in several proteins, including in HCPro at either position 352 (Ile→Thr) or 454 (Leu→Arg), or of mutations in non-coding regions. Thr in position 352 is not found among natural potyviruses, while Arg in 454 is a reversion to the native PVY HCPro amino acid. We show here that both mutations separately contributed to the increased virulence observed in the passaged chimeras that acquired them, and that Thr in position 352 is no intragenic suppressor to a Leu in position 454, because their combined effects were cumulative. We demonstrate that Arg in position 454 improved HCPro autocatalytic cleavage, while Thr in position 352 increased its accumulation and the silencing suppression of a reporter in agropatch assays. We assessed infection by four cloned chimera variants expressing HCPro with none of the two substitutions, one of them or both, in wild-type versus DCL2/4-silenced transgenic plants. We found that during infection, the transgenic context of altered small RNAs affected the accumulation of the four HCPro variants differently and hence, also infection virulence.

Angiotensin II Analogues with Sulphur-containing Side-chains in Position 5

Four sets of angiotensin II (AngII) analogues with position 5 modifications, two agonist series with either Asp or Sar in position 1 and L-Phe in position 8, and two antagonist series with again Asp or Sar in position 1 and Leu in position 8 were synthesized. Modifications in positions 5 were introduced successively: Ile, Nle, Met, S-ethyl Cys, S-n-propyl-Cys, S-n-butyl Cys, S-t-butyl Cys and S-benzyl Cys in all four series. The study was undertaken in order to investigate the 5-position residue of AngII by replacing the hydrophobic side-chain by another containing an electrophilic moiety. The analogues were synthesised by solid phase synthesis using the Boc/Bzl or Fmoc/But strategy. All analogues were evaluated by their binding properties to the AT1 receptor on bovine adrenocortical membranes (bAT1). The results indicate that AngII analogues bind, irrespective of their agonistic or antagonistic nature or of their position 1 modification, in a similar manner and that position 5 modifications without beta-branching behave in an additive manner towards their affinity.

The two tryptophans of β2-microglobulin have distinct roles in function and folding and might represent two independent responses to evolutionary pressure

BackgroundWe have recently discovered that the two tryptophans of human β2-microglobulin have distinctive roles within the structure and function of the protein. Deeply buried in the core, Trp95 is essential for folding stability, whereas Trp60, which is solvent-exposed, plays a crucial role in promoting the binding of β2-microglobulin to the heavy chain of the class I major histocompatibility complex (MHCI). We have previously shown that the thermodynamic disadvantage of having Trp60 exposed on the surface is counter-balanced by the perfect fit between it and a cavity within the MHCI heavy chain that contributes significantly to the functional stabilization of the MHCI. Therefore, based on the peculiar differences of the two tryptophans, we have analysed the evolution of β2-microglobulin with respect to these residues.ResultsHaving defined the β2-microglobulin protein family, we performed multiple sequence alignments and analysed the residue conservation in homologous proteins to generate a phylogenetic tree. Our results indicate that Trp60 is highly conserved, whereas some species have a Leu in position 95; the replacement of Trp95 with Leu destabilizes β2-microglobulin by 1 kcal/mol and accelerates the kinetics of unfolding. Both thermodynamic and kinetic data fit with the crystallographic structure of the Trp95Leu variant, which shows how the hydrophobic cavity of the wild-type protein is completely occupied by Trp95, but is only half filled by Leu95.ConclusionsWe have established that the functional Trp60 has been present within the sequence of β2-microglobulin since the evolutionary appearance of proteins responsible for acquired immunity, whereas the structural Trp95 was selected and stabilized, most likely, for its capacity to fully occupy an internal cavity of the protein thereby creating a better stabilization of its folded state.

Influence of N‐terminus modifications on the biological activity, membrane interaction, and secondary structure of the antimicrobial peptide hylin‐a1

Recently the peptide Hy-a1 (IFGAILPLALGALKNLIK), with antimicrobial activity, was isolated from the skin secretion of the frog Hypsiboas albopunctatus. The aim of the present work was to evaluate four analogues with introduction of acetyl group, Asp or Lys at the N-terminus of antimicrobial peptide Hy-al to supply information about the relationship of structure-biological activity. The antimicrobial activities were assayed by measuring growth inhibition of four species of bacteria and four species of fungus. The hemolytic activity was also tested. The peptide containing Trp instead of Leu in position 6 (for fluorescence studies) presented MIC values comparable to wild type sequence: 32 μmol L(-1) , 32 μmol L(-1) , 8 μmol L(-1) , and 2 μmol L(-1) for E. coli, P. aeruginosa, S. aureus, and B. subtilis, respectively. Two peptides with this modification and containing one acetyl group or Asp residue at the N-terminal region showed activities only against Gram-positive bacteria. Different results were observed when the residue added was Lys. In this case, the activity against the microorganisms was sustained or increased. Conformational properties were investigated by CD techniques in water, TFE, and in zwitterionic micelles (LPC). The CD experiments demonstrated that, in water, the peptides had a random structure, but in TFE and LPC solutions they acquired an ordered structure, composed mainly by α-helix. However, these data have no relationship with activity against Gram-positive bacteria. These results showed that the N-terminal region of the peptide Hy-a1 has key roles in its antibacterial action toward different types of bacteria.

Linear basic peptides for targeting interferon-gamma-glycosaminoglycan interactions: synthesis and inhibitory properties.

Glycosaminoglycans (GAGs) play an important role in inflammatory responses due to their ability to interact with cytokines and chemokines, resulting in the localization of these mediators to specific anatomical sites, where they function to direct leukocyte recruitment and activation. Targeting GAG-cytokine/chemokine interactions might may thus have therapeutic applications as anti-inflammatory or immunomodulatory therapy in vivo. Peptides that mimic the heparin-binding domains of cytokines may have a potential use as inhibitors of GAG-cytokine interactions. A linear octapeptide (MC-2) derived from the conserved heparin-binding region of interferon-gamma (IFN-gamma) was synthesized along with four analogs featuring a substitution of Phe for Leu in position 1 and varying number of positive charges on the octapeptide molecule. The relative abilities of the synthesized peptides to inhibit the interactions between IFN-gamma and GAGs were compared. From the results, it follows that the inhibitory potency of the octapeptide analogs was related to the number of positive charges in the molecule, while increased hydrophobicity had no significant effect.

Glycogenosis type II: identification and expression of three novel mutations in the acid α-glucosidase gene causing the infantile form of the disease

Glycogenosis type II (GSDII) is an autosomal recessive disorder due to the deficiency of the lysosomal enzyme acid α-glucosidase (GAA). We identified three novel point mutations, C399A, T1064C, and C2104T, in three unrelated Italian patients with the infantile form of the disease. The C399A mutation was present in homozygosity in proband 1. The C > A transition introduces a premature stop signal in exon 2 resulting in no enzyme production that is correlated with the severe clinical phenotype in this patient. The other two nucleotide changes were missense mutations. The T1064C mutation, which changes Leu in position 355 into Pro, was carried in homozygosity by proband 2. The C2104T nucleotide change, which substitutes Arg 702 into Cys, was present in proband 3 in combination with a known severe mutation ΔI17-18. The in vitro expression in COS-1 cells of T1064C and C2104T constructs demonstrated no enzymatic activity with respect to the negative control cells. Western blot analysis revealed that both T1064C and C2104T mutant proteins produced in COS-1 cells migrated in SDS–PAGE as the GAA inactive precursor of 110 kDa. Immunofluorescence detection of mutant α-glucosidases showed enzyme localization primarily in the ER-Golgi compartment, suggesting that T1064C and C2104T mutations could affect the normal processing and stability of the enzyme. In vitro studies demonstrated that the same degree of deficiency in T1064C and C2104T mutations, which is in contrast with patient phenotype. A better correlation was observed with the in vivo studies since proband 2, with a less severe phenotype, presented with low residual enzyme activity while in proband 3, with a classic severe infantile onset GSDII, fibroblast enzyme activity was completely absent.

Differences in the Specificities of the Highly Alkalophilic Proteinases Savinase and Esperase Imposed by Changes in the Rigidity and Geometry of the Substrate Binding Sites

Savinase and Esperase are closely related highly alkalophilic proteinases produced by Bacillus lentus. They are suitable couple for investigating the structural basis of proteinase specificity due to the identity of the catalytic and the differences in the substrate binding sites. Two of the substitutions in these sites are very important: T129P and G131P. The two prolines provide an extra rigidity of the Savinase-binding site. The substitutions S166N and Q191T in the S1 recognition loop change the binding geometry of the substrate P1 residue. The geometry of S1 in Esperase is more favorable for binding and catalysis in comparison to that in Savinase. Differences in P3 specificity are probably created by the substitution V104L, which influences the conformation of S3. Leu in position 104 is more favorable for the binding of Phe to S4 than Val. The lower affinity and catalytic efficiency as well as more narrow proteolytic specificity of Savinase in comparison to those of Esperase are explained with the extra rigidity and unfavorable changes in geometry of the substrate binding site of the first enzyme.

A detailed analysis of the C5a anaphylatoxin effector domain: selection of C5a phage libraries on differentiated U937 cells.

We have used a phage-display-based system to investigate the effect of simultaneous substitutions within the C5a effector domain. Two different libraries were constructed. In library I, known binding positions 67, 68, 72 and 74 of human complement C5a (hC5a) and in library II, positions 69-73 of hC5a without C-terminal Arg74 (des-Arg74-C5a) were randomly mutated. In more than 80% (position 72) or 90% (positions 68 and 74) of all cases, the original residues of hC5a were selected from library I, demonstrating that the phage system can be used to define binding points within the C5a molecule. Surprisingly, in more than 90% of all clones, a Phe residue was enriched at position 67 instead of the original His residue which, however, did not affect the binding affinity or the signalling activity. In library II, Leu was preferentially selected at positions 70-72 and Tyr at position 73, while no enrichment of an individual amino acid was observed at position 69. Mutants with (a) Leu in positions 71 and 72 (b) Ser or Leu in position 70 and (c) Arg or Tyr in position 73, showed a 4-10-fold higher binding affinity as compared to des-Arg74-[Ala27, Phe67]C5a. The binding affinity was indistinguishable from that of hC5a. In consequence, not only position 72 but also positions 70, 71 and 73 are able to interact with the C5a receptor, whereas position 69 is not. Intriguingly, one mutant with a high binding affinity but without signalling activity was selected. Thus, random mutagenesis of phage-displayed C5a was proven to be a powerful strategy to define receptor-binding points and to select C5aR antagonists based on the structure of the natural ligand.

Peptides control the gain and loss of allele specificity by mutated MHC class I molecules.

To analyze the molecular basis of MHC class I allele-restricted peptide recognition, a set of eight Ld/Lq mutants was constructed and tested for peptide recognition by allele-restricted and peptide-specific CTL. The MHC molecules H-2Ld and H-2Lq differ at six amino acid positions (95, 97, 107, 116, 155, 157) located within the alpha 2 domain of the molecule. Both molecules present the lymphocytic choriomeningitis virus (LCMV) nucleoprotein-derived peptide RPQASGVYM and the murine cytomegalovirus (MCMV) pp89-derived peptide YPHFMPTNL to the respective virus-specific CD8+ CTL is a strictly allele-restricted fashion. All mutated MHC class I molecules did still bind the LCMV peptide and seven of eight mutants retained MCMV peptide binding. The exchange Arg-->Trp at position 97 of Lq in pocket C of the peptide binding groove prevented binding of the MCMV ligand and this loss was compensated by the additional exchange of Ile-->Leu in position 95 (pocket F). Within the Lq molecule, single mutations at either position 97 on the floor of the groove or position 155 of the wall sufficed for a gain of LCMV peptide recognition by Ld-restricted CTL. Altogether, six of eight mutants resulted in a gain of recognition by CTL specific for the other allele. Thus, six of the eight mutants lost MHC-restricted recognition and were accepted by both Ld- as well as Lq-restricted CTL when presenting the LCMV peptide. Only one case of simultaneous recognition of the MCMV peptide by both Ld- as well as Lq-restricted CTL was noted. In other mutations, a gain of recognition by Ld-restricted CTL was associated with a loss of recognition of Lq-restricted CTL. Analysis of extracted MCMV peptide from mutant molecules excluded quantitative differences in presented MCMV peptide as a reason for the lack of CTL recognition. Altogether, the results show that, rather than aminoacids at certain residue positions, individual peptides govern MHC allele specificity of CTL recognition.

A single arginine residue determines species specificity of the human growth hormone receptor.

Although growth hormone (GH) receptors (GHRs) in many species bind human (h) GH as well as their own GH, the hGHR only binds primate GH. Arg43 in hGHR interacts with Asp171 of hGH. Nonprimates have a His in the position equivalent to residue 171 of primate GH and a Leu in position 43 of primate GHR. To determine whether Arg43 accounts for the species specificity of the hGHR, point mutations that changed Leu43 to Arg were introduced into the cDNAs encoding the bovine (b) GHR or the rat GH binding protein (GHBP) and these mutants or their wild-type (WT) counterparts were expressed in mouse L cells. Binding of hGH or bGH to transfected cells or to GHBP secreted into the incubation medium was assessed by displacement of 125I-labeled hGH. WT and mutant bGHR bound hGH with similar affinity, but the affinity of the mutant receptors for bGH was reduced 200-fold. Likewise, WT and mutant GHBP bound hGH with equal affinity, but only WT GHBP bound bGH. Cross-linking of 125I-labeled hGH to WT or mutant GHR produced a 141-kDa labeled complex whose appearance was blocked by unlabeled hGH, but bGH blocked cross-linking only to WT receptors. Both hGH and bGH stimulated tyrosine phosphorylation of a 95-kDa protein in cells transfected with WT GHR, but bGH was less effective in cells expressing mutant GHR. We conclude that incompatibility of Arg43 in the hGHR with His171 in nonprimate GH is the major determinant of species specificity.

New Gaba-containing analogues of human growth hormone releasing hormone (1-30)-amide: II. Detailed in vivo biological examinations.

Analogues of human growth hormone-releasing hormone-(1-30)-amide [GH-RH(1-30)-amide] were tested for their ability to stimulate GH release in vivo by injecting the peptides intravenously (iv), subcutaneously (sc), and intramuscularly (im). The analogues involved derivatization with Nle27 and Gaba substituents at the C-terminus with or without D-amino acid(s) in the peptide chain. The potency of the analogues was compared to that of GH-RH(1-29)-amid testing their ability to release GH at 5, 15 and 30 min after the administration. In iv test the potency of the analogues was 1.2-2 times higher than that of the GH-RH(1-29)-amide, and no significant differences were detected between the potencies of the analogues with or without D-amino acid. In the sc test the analogue with D-Ala2, Nle27, and Gaba30 substitutions expressed 8.0-51.7 times higher potency than the GH-RH(1-29)-amide, however, the analogue with similar modifications but with L-Ala2 showed the same low potency (1.2-2.1) as in the iv test. Results from the im experiments were similar to those of SC test. The most potent analogues were those which had D-Ala2, Nle27, and Gaba30 substitutions with Gly15 or Leu15. Circular dichroism (CD) spectra of the analogues showed that Leu in position 15 increased the stability of the predominant alpha-helix conformation, which improved the absorption of the molecule.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of Insulin Autoantibodies in Relatives of Patients With Type I Diabetes

We studied in detail the anti-insulin autoantibodies in 29 nondiabetic relatives of patients with type I diabetes. The affinity of the autoantibodies for [125I]human insulin was high (1.34 x 10(9)-20.71 x 10(9) L/mol), and the capacity was low (0.84 x 10(-12)-37.80 x 10(-12) M). The product of affinity x capacity of each relative's antibodies directly correlated (r = 0.99) with the level of antibodies determined in our standard radioassay. The autoantibodies from each of the subjects studied had the same rank order of affinities for insulin from different species. Guinea pig, fish insulin, and insulin containing Trp rather than Leu in position 13 of the A-chain inhibited minimally the human insulin binding. Human proinsulin, insulin containing Gln rather than Glu in position 17 of the A-chain, and desoctapeptide insulin (des B23-30) all inhibited binding effectively. Insulin autoantibodies in relatives of patients with type I diabetes share common epitope(s), which suggests a common pathogenic mechanism for production of such antibodies. The epitopes from this initial analysis appear to include amino acids B1-B3 and A8-A13. The region recognized can be distinguished from the insulin receptor binding domain.

Structure-function relationships in an antifreeze polypeptide. The effect of added bulky groups on activity

We have proposed that the antifreeze activity of an alanine-rich alpha-helical antifreeze polypeptide (AFP) is due in part to side-by-side hydrophobic interactions of AFP molecules bound on the ice surface (Wen, D., and Laursen, R. A. (1992b) Biophys. J. 63, 1659-1662). To test this hypothesis and to assess the importance of a hydrophobic surface on the outward facing (non-ice-binding) portions of the helix, we synthesized several AFP analogs with up to four Ala-->Gln or Ala-->Leu replacements and determined the effect of these changes on antifreeze activity and helix stability. Although Gln replacements caused some helix destabilization and resultant reduction of antifreeze activity, they were in general well tolerated, suggesting that the bulk hydrophobicity of the non-ice-binding faces of the AFP, per se, is probably not a major factor determining AFP activity. However, placement of either Gln or Leu in position 17 on one side of the helix completely abolished activity, demonstrating that the specific location of bulky groups can dramatically alter activity. We conclude from molecular modeling studies that the effect seen by placing bulky groups in position 17 is due to steric hindrance that prevents effective association or packing of AFP molecules on the ice surface, in support of our hypothesis.

Influenza viruses differ in recognition of 4- O-acetyl substitution of sialic acid receptor determinant

Equine α 2 macroglobulin (EM), known to contain both Neu5Ac and Neu4,5Ac2 sialic acid residues, was treated with Vibrio cholerae sialidase for the selective removal of Neu5Ac and was compared with the untreated EM for its binding by a panel of influenza viruses. Type A H3N2 virus strains having Leu in position 226 of their hemagglutinin (HA) changed the affinity for sialidase-treated EM only slightly, if at all, indicative of their ability to bind the 4- 0-Ac-substituted Neu5Ac receptor determinant. At the same time, all B and H1 N1 viruses, some H2N2 variants, as well as H3N2 strains with 226 Gin studied were unable to recognize Neu4,5Ac 2 moieties of EM. Molecular modeling based on the known 3-D structure of H3 HA complexed with sialyllactose (Weis et al. (1988) Nature 333, 426–431) predicts that the 4- 0-Ac substituent of sialic acid would protrude with its carbonyl oxygen inside the receptor-binding site of HA, thus possibly interfering with binding.

Peptide synthesis catalyzed by native proteinase K in water-miscible organic solvents with low water content

Reaction of Ac-Tyr-OEt with HBr.Gly-NH2, catalyzed by free proteinase K in various water-miscible organic solvents in the presence of triethylamine and 5 vol.% of water, was studied. Some aliphatic alcohols and acetonitrile proved to be suitable solvents. The effect of water content (2%-20%) on the synthesis of Ac-Tyr-Gly-NH2 was studied using acetonitrile as solvent. Lowering of the water content to 5% or 2% led to almost 100% yield of the desired dipeptide; higher water content accelerated the reaction, reducing at the same time the yield of Ac-Tyr-Gly-NH2 due to the concurrent hydrolysis of the ester Ac-Tyr-OEt. No reaction was observed in the absence of base (triethylamine), whereas an excess of base only retarded the reaction. The enzyme is capable of catalyzing the peptide bond synthesis with N-acylamino acids or N-acyl peptides as acylating components, which may contain all types of L-amino acid residues (except Pro) in the P1 position. However, the peptide bond synthesis depends strongly on the amino component composition, particularly on the amino acid residue in the P'1 position. Only amides of glycine and of hydrophilic amino acids were acylated with Ac-Tyr-OEt; amides of hydrophobic amino acids enter the reaction only reluctantly or not at all. The presence of Leu or Phe in position P'2 and Leu in position P'3 has not so negative effect on acylation of the amino component as has its presence in the P'1 position. The choice of protecting groups for the α-carboxyl of the amino component is restricted only to amide and in some cases its undesired enzymatic removal was observed. Unprotected peptides seem to be suitable amino components.

Isolation, characterization, and corticotropic activity of rabbit adrenocorticotropin.

Rabbit (r) ACTH was extracted from 600 pituitaries, and 2 forms of immunoreactive ACTH were identified with the least polar form accounting for approximately 90% of the total. Peptide mapping and sequence analysis indicated that three tryptic peptides had retention times identical to those obtained from human (h) ACTH. The least polar tryptic fragment from rACTH had a shorter retention time than the corresponding one from hACTH. Sequence analysis indicated that rACTH differed from hACTH at three different loci, namely, an Asn in place of Asp in position 29; a Val in place of Leu in position 37, and a Val in place of Phe in position 39. Biological activity of the ACTH was compared with synthetic hACTH in 2 bioassays with adrenals from 10-day-old pups, the first using dispersed rabbit adrenal cells and the second using monolayer adrenal cells in culture. The biological potencies of the two ACTH preparations were identical with respect to corticosterone (B) release in the short term bioassay, with an ED50 value of 1.67 X 10(-10) M. The ED50 value for cortisol (F) release for rACTH and hACTH were 1.1 X 10(-10) M and 1.67 X 10(-10) M, respectively, which were not statistically different. The biological potency of rACTH in the monolayer adrenal cell system for both F and B was significantly greater than the hACTH, and the ED50 values were 4.4 X 10(-10) M and 8.9 X 10(-10) M, respectively. There was a progressive decrease in the B/F ratios with increasing concentrations of ACTH in both the bioassay systems suggesting that ACTH stimulated the 17 alpha-hydroxylase activity even when the exposure of cells to ACTH was as short as 2 h.

Comparison of bovine and mouse pituitary glycoprotein hormone pre-α subunits synthesized in vitro

Poly(A)-containing RNA isolated from bovine and mouse pituitaries and a mouse pituitary thyrotropic tumor was translated in a wheat germ cell-free biosynthetic system. A precursor of the glycoprotein hormone alpha subunit, "pre-alpha," was immunoprecipitated from the translation mixtures with antiserum against bovine luteinizing hormone (LH; lutropin) alpha. The specificity of the immunoprecipitation was shown by competition with authentic bovine LHalpha and lack of competition with bovine thyroid-stimulating hormone (TSH; thyrotropin) beta. Bovine and mouse pre-alpha subunits migrated identically in sodium dodecyl sulfate gradient polyacrylamide slab gels with an apparent molecular weight of about 17,000. Pre-alpha comprised 2-3% and 20-30% of the total proteins translated with pituitary and pituitary tumor mRNA, respectively. Microanalysis of amino acid sequence of the pre-alpha subunits containing various radiolabeled amino acids gave the following partial sequence for mouse tumor pre-alpha: [Formula: see text] Met was also found in positions 1, 14, and 17 in mouse pituitary pre-alpha but only in residue 1 of the bovine pituitary pre-alpha subunit. Leu was found in identical positions in bovine pituitary pre-alpha, with an additional Leu in position 17. Leu in the common positions (12, 15, 19, and 22) has also been found in human choriogonadotropin pre-alpha subunit [Birken, S., Fetherston, J., Desmond, J., Canfield, R. & Boime, I. (1978) Biochem. Biophys. Res. Commun. 85, 1247-1253]. The data demonstrate that pituitary as well as placental glycoprotein hormone alpha subunits are synthesized with an amino-terminal hydrophobic extension, in accord with the "signal hypothesis" for secreted proteins. Furthermore, the positions of the hydrophobic amino acid Leu have been strictly conserved in pre-alpha subunits from various species and in two different tissues, the pituitary and placenta.

Role of the N-terminal amino acid for the biological activities of angiotensin and inhibitory analogues.

Aspartic acid was replaced in position 1 of angiotensin II (ATII) with several amino acids, to assess the possible influence of the N-terminal amino acid for (a) the intrinsic activity, (b) the affinity, and (c) the metabolic degradation of agonist analogues of ATII. Some of the substitutions in position 1 were used in combination with replacement of Phe by Gly or Leu in position 8, to obtain the corresponding antagonist.The compounds were tested in vivo (rat blood pressure) and in two in vitro preparations (rat stomach and rabbit aorta strips). The oil immersion technique, described by Kalsner and Nickerson (1968) (Can. J. Physiol. Pharmacol. 46, 719–730), was used to study the disposition of the peptides by vascular smooth muscles (rabbit aorta strips). Degradation of the peptides by purified aminopeptidases was evaluated in vitro by measuring the fragments on paper chromatography. Potency of antagonists was estimated in vivo (ID50) and in vitro (pA2 values): duration of action was established by infusing the inhibitors intravenously into anesthetized rats and testing the effect of standard doses of angiotensin before and after.The results indicate that replacement of Asp with other amino acids does not influence the intrinsic activity, but can either increase or decrease the affinity in vitro or the potency in vivo. 1-Sar-ATII, and 1-D-Ala-ATII are more potent and longer acting than 1-Asp-ATII on isolated intestinal and vascular smooth muscles, but not in vivo. On the contrary, 1-β-Asp-ATII and 1-β-D-Asp-ATII are more potent than 1-Asp-ATIIin vivo, but not on rabbit aorta strips. Rate of relaxation of rabbit aorta strips suspended in oil, after contraction with submaximal doses of several analogues of ATII, are significantly slower than relaxation after 1-Asn2-ATII and 1-Asp-ATII. A close parallelism between the diminution of the relaxation rate in oil and the degradation by aminopeptideses in vitro was observed, suggesting that metabolic degradation may be the major factor determining relaxation of rabbit aorta in oil after contraction with one of these peptides. Potencies of antagonists in vivo and in vitro are increased by replacing Asp with Sar. Substitution of Asp with β-Asp or β-D-Asp brings about a slight increase of potency in vivo but not in vitro. It appears that firm binding and prolonged occupation of receptors by sarcosyl derivatives are the primary factors contributing to increase the potency and to prolong the duration of action of antagonists, while prevention or reduction of metabolic breakdown by aminopeptidases is much less efficient.