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Biomedical subjects

I M Armitage

Publications and source records attributed to I M Armitage.

At least 55 records · Page 3Linked to original sources
PubMed1986

1H NMR studies on bovine cyclophilin: preliminary structural characterization of this specific cyclosporin A binding protein.

High-field 1H NMR spectroscopy has been used to study the conformation of the cytosolic cyclosporin A binding protein cyclophilin. For the drug-free form of cyclophilin, spectral editing methods in conjunction with a pH titration were used to identify all four His residues present in the protein, and two-dimensional COSY and RELAY spectroscopy was used to elucidate the scalar connectivities in the aromatic and upfield methyl regions of the spectrum. From these scalar connectivities, it was possible to distinguish between inter- and intraresidue dipolar interactions within the aromatic and upfield methyl regions of cyclophilin in the NOESY spectrum. The results of this analysis showed extensive interresidue cross-relaxation among and between these latter spectral regions indicative of the proximal relationships of several of these residues and the presence of a hydrophobic core within cyclophilin.

Animals↗
PubMed1985

Location of polar substituents and fatty acyl chains on lipid A precursors from a 3-deoxy-D-manno-octulosonic acid-deficient mutant of Salmonella typhimurium. Studies by 1H, 13C, and 31P nuclear magnetic resonance.

Eight anionic disaccharide precursors of lipid A accumulate at 42 degrees C in 3-deoxy-D-manno-octulosonic acid-deficient temperature-sensitive mutants of Salmonella typhimurium. These compounds comprise a series of lipids based on the minimal structure, O-[2-amino-2-deoxy-N2,O3-bis(3-hydroxytetradecanoyl)-beta-D-glucopyranos yl] -(1----6)-2-amino-2-deoxy-N2, O3-bis(3-hydroxytetradecanoyl)-alpha-D-glucopyranose 1,4'- bisphosphate (designated lipid IVA) that differ from each other by the presence of an additional phosphoethanolamine moiety (IIIA), or an aminodeoxypentose moiety (IIA), or both (IA). A homologous set of metabolites is further derivatized with a palmitoyl function; these are designated IVB, IIIB, IIB, and IB (Raetz, C. R. H., Purcell, S., Meyer, M. V., Qureshi, N., and Takayama, K. (1985) J. Biol. Chem. 260, 16080-16088). The attachment of the palmitoyl moiety, known to be on the reducing terminal GlcN residue by mass spectrometry, was determined to be O-beta of the N2-linked beta-hydroxymyristoyl group of that residue of IVB by 13C NMR and two-dimensional 1H chemical shift correlation spectroscopy experiments. 31P NMR indicated the presence of diphosphodiester moieties in IIIA, IIIB, and IA and monophosphodiester moieties in IIA and IA. Selective 1H decoupling of the 31P spectrum of IIIA demonstrated that the O-diphosphoethanolamine moiety is attached to the O4' position in IIIA. On the basis of the observed 31P chemical shifts it was concluded that the aminodeoxypentose is located at position 1 in IIA and IA, while diphosphoethanolamine is most likely located at O-4' in IA and IIIB, as in IIIA.

Acylation↗
PubMed1985

Selective detection of 3-deoxymannooctulosonic acid in intact lipopolysaccharides by spin-echo 13C NMR.

The 3-deoxy-D-mannooctulosonic acid (KDO) region of lipopolysaccharides (LPS) from the heptoseless mutant Salmonella minnesota R595 and inner core and heptoseless mutants derived from Escherichia coli K12 was studied by 13C NMR spectroscopy. A spin-echo spectral editing technique was employed for the selective detection of the quaternary anomeric carbon of ketosidically linked KDO. Only two quaternary carbon resonances attributable to KDO were detected in the anomeric carbon spectral region of each LPS from heptoseless mutants E. coli D31m4 (99.7 and 100.8 ppm) and S. minnesota R595 (100.0 and 100.9 ppm). Integrated signal intensities from fully relaxed normal 13C spectra showed that equivalent molar quantities of KDO and glucosamine (i.e. 2 mol of each) were present in each of these samples. Similarly, only two KDO anomeric carbon resonances were detected in the LPS from the inner core mutants E. coli D21f1 (100.8 and 101.2 ppm) and E. coli D21e7 (100.8 and 101.2 ppm). These data confirm the presence of a KDO disaccharide structure rather than a trisaccharide as determined by others using thiobarbituric acid-based assays. The LPS of E. coli D21 (complete inner core oligosaccharide) exhibited four quaternary anomeric carbon resonances (99.4, 100.7, 101.8, and 102.7 ppm). The unequal intensities of these resonances, however, demonstrated that significant heterogeneity exists with respect to KDO substitution in this LPS. A third KDO moiety present in substoichiometric amounts could be consistent with this observation. However, this possibility could not be distinguished from other modes of substitutional heterogeneity involving only 2 KDO residues.

Escherichia coli↗
PubMed1985

m-Fluorotyrosine substitution in beta-galactosidase; evidence for the existence of a catalytically active tyrosine.

The pH profiles of beta-galactosidase, having tyr replaced by m-fluorotyrosine, were compared to those of normal enzyme. The inflection point on the alkaline side was lowered about 1.5 pH units in the fluoro-enzyme, corresponding to the difference in the phenolic pKa values of m-fluorotyrosine and tyr. When glycosidic bond breakage was rate-limiting, the Vm at pH 7.0 was higher for the fluoro-enzyme. When hydrolysis was rate-limiting or when acceptors which made transgalactosylis rate-limiting were used, the Vm was lower for the fluoro-enzyme. This shows that a tyr in beta-galactosidase is a general-acid catalyst in the glycosidic bond breaking reaction and a tyr (probably the same one) is a general-base catalyst in the hydrolytic reaction.

Drug Stability↗
PubMed1984

Studies of factors affecting the design of NMR contrast agents: manganese in blood as a model system.

Some factors affecting the performance of paramagnetic ions as contrast agents for proton NMR imaging have been studied. It is demonstrated that the relaxation rate of an aqueous solution of the ion is not reliably predicted by its magnetic moment, but that significant relaxation enhancement may result when the ion is complexed with large molecules, which increases the dominant correlation time. This enhancement in turn can be altered by factors such as pH and competition for binding. Chelation of the paramagnetic ion, which may be implemented to lower its toxicity, can considerably reduce its efficacy by not only limiting its access to water but also by preventing the enhancement from associations and macromolecules. For manganese the ratio T1/T2 is a useful parameter which is sensitive to the degree of metal binding. These features of paramagnetic relaxation enhancement in tissue are demonstrated in a series of experiments on systems consisting of blood components and manganese.

Contrast Media↗
PubMed1984

NMR analysis of the structure and metal sequestering properties of metallothioneins.

113Cd-NMR studies have been used to elucidate the structure of the metal-binding sites in mammalian and invertebrate ( Scylla serrata) metallothioneins (MTs). Chemical shift data have shown that all Cd ions are tetrahedrally coordinated to four cysteine thiolate ligands with single cysteinyl sulfurs bridging adjacent metals. Homonuclear decoupling experiments have shown that the 7 g-atoms of metal bound per mole of mammalian protein are located in a three- and a four-metal cluster while the 6 g-atoms of metal in the invertebrate MT are located in two three-metal clusters. The different metal binding affinities of the two mammalian clusters have been determined by 113Cd-NMR. The three-metal cluster prefers Cu greater than Zn greater than Cd whereas exactly the reverse order applies in the four-metal cluster. Proteolytic cleavage of the protein produced a 32-residue fragment which contained the four-metal cluster and demonstrated the presence of two separate domains in the protein. 500 MHz 1H-NMR has been employed to elucidate the arrangement of these metal clusters in the tertiary structure of the protein. The 1H resonances were assigned from their scalar and dipolar connectivities obtained from extensive one and two-dimensional NMR experiments. A specific application of 2D correlation spectroscopy ( COSY ) to the assignment of the 1H resonances in crab MT-1 is discussed. A molecular model, representing the three-dimensional solution structure of this protein, has been constructed based on an analysis of all these data. Detailed structural features of this model are discussed, with particular emphasis on their relationship to the function and evolution of the protein.

Animals↗
PubMed1984

Nuclear magnetic resonance studies on the antibiotic avoparcin, Conformational properties in relation to antibacterial activity.

Avoparcin is a commercially important glycopeptide antibiotic which is active against Gram-positive bacteria. Recently, beta-avoparcin, the major component of the avoparcin mixture, and several other analogues have been structurally characterized and tested for their antibacterial activity. Varying degrees of activity were observed for only minor structural differences. For example, beta-avoparcin and epi-beta-avoparcin, which differ only in the stereochemistry at position 1' of the NH2-terminal phenylsarcosine subunit, exhibit a 10- to 100-fold difference in antibacterial activity. Following the supposition that the conformational properties of these molecules may explain the differences in their antibacterial activity, we have analyzed the conformations of beta-avoparcin, epi-beta-avoparcin, and other avoparcin analogues in aqueous solutions using NMR methods. On the basis of an analysis of the 1H chemical shifts, 1H-1H nuclear Overhauser enhancement data, and pH titration experiments, it was concluded that the conformations of all of the analogues are similar at the COOH terminus. However, for beta-avoparcin and epi-beta-avoparcin, conformational differences were observed in the NH2-terminal region of the molecules. In this paper, we present a detailed description of the over-all conformations of these two glycopeptides as deduced from an in-depth analysis of their corresponding 1H NMR spectra and discuss the possible relationships this may have with their markedly different antibacterial activities.

Anti-Bacterial Agents↗
PubMed1984

Nuclear magnetic resonance studies on the interaction of avoparcin with model receptors of bacterial cell walls.

On the basis of nuclear Overhauser enhancement and 1H chemical shift data obtained in aqueous solution, a model is proposed for the interaction of beta-avoparcin and epi-beta-avoparcin with acetyl-D-alanyl-D-alanine (Ac-D-Ala-D-Ala) and diacetyl-L-lysyl-D-alanyl-D-alanine (Ac2-L-Lys-D-Ala-D-Ala). For the beta-avoparcin: Ac2-L-Lys-D-Ala-Ala complex, the COOH-terminal end of the tripeptide is located near the NH2 terminus of the antibiotic with the tripeptide extending across the peptide backbone of beta-avoparcin toward its COOH-terminal end. In our proposed structure, the three amino acid residues of the peptide span the entire length of the antibiotic, and the aliphatic side chain of the lysine residue extends over the D-ring of beta-avoparcin. The structure of the epi-beta-avoparcin:Ac2-L-Lys-D-Ala-D-Ala complex was found to be similar to the beta-avoparcin complex at the binding site for the lysine residue at the COOH-terminal end of the antibiotic, but differed in the interactions at the NH2 terminus. These results are consistent with the similarities in the COOH-terminal conformations and the differences in conformations at the NH2 terminus found for beta-avoparcin and epi-beta-avoparcin which were described in the preceding paper [Fesik, S. W., I. M. Armitage, G. A. Ellestad, and W. J. McGahren, Mol. Pharmacol. 25:275-280 (1984)]. The association constants (measured by UV methods) for both beta-avoparcin:peptide complexes were greater than those measured for epi-beta-avoparcin and correlated with their differences in antibacterial activity. Epi-beta-avoparcin exhibited no measurable binding to the dipeptide: however, a significant affinity was measured for the tripeptide, indicating that the interactions with the NH2 terminus of the antibiotics provide binding energy for the antibiotic peptide complex but that the COOH-terminal end of the antibiotics also plays an important role in the binding interaction. These results are interesting in light of the similarities in the structural and conformational features at the COOH terminus for all of the glycopeptide antibiotics.

Anti-Bacterial Agents↗
PubMed1983

Structure and metal-binding properties of lipopolysaccharides from heptoseless mutants of Escherichia coli studied by 13C and 31P nuclear magnetic resonance.

The structure and metal-binding properties of lipopolysaccharides (LPS) from heptoseless mutants of Escherichia coli were studied by 13C and 31P NMR techniques. Carbon-13 NMR spectra were used to determine the linkages and configurations of the saccharide backbone and the types and locations of fatty acyl groups in E. coli LPS. Resonance assignments for native LPS were made by chemical shift correlation with model compounds, deacylated LPS, lipid A, deacylated lipid A, and fatty acids released from LPS by mild alkaline hydrolysis. The 3-deoxy-D-manno-octulosonate (KDO) disaccharide was tentatively assigned the structure KDO alpha 2 leads to 5KDO alpha 2 leads to. The presence of amide- and ester-linked 3-hydroxy and 3-acyloxy fatty acids in native LPS was confirmed directly from the 13C spectrum and evidence is presented for a labile acyl ester at C-3' (GlcNII) of the lipid A moiety. A significant finding was that the KDO disaccharide is linked to the C-6' position of the lipid A moiety, rather than C-3', as previously reported. The effects of binding Ca2+, Cd2+, Yb3+, Gd3+, and La3+ on the 31P NMR spectrum of LPS indicated that the glycosidic diphosphate moiety participates in a high affinity metal-binding site.

Cadmium↗
PubMed1983

Characterization of lipopolysaccharide from a heptoseless mutant of Escherichia coli by carbon 13 nuclear magnetic resonance.

Lipopolysaccharide (LPS) isolated from Escherichia coli D31m4, a heptoseless mutant, was studied by 13C and 31P NMR spectroscopy. Modified isolation and purification procedures are described which permitted high resolution NMR spectra to be obtained from samples of intact LPS. 31P NMR was used to monitor the purity and native heterogeneity of LPS samples. The anomeric carbon region of the 13C NMR spectrum taken at pH 7 contained five resonances that were assigned on the basis of chemical shift correlation, 13C-1H couplings, and T1 relaxation times. Two resonances, at 99.9 and 100.8 ppm, were attributed to two residues of 3-deoxy-D-manno-octulosonate (KDO) of which both were tentatively assigned to the alpha configuration. The Lipid A moiety gave rise to resonances at 94.0 and 94.9 ppm, both assigned to GlcNI, and a resonance at 103.1 ppm, assigned to GlcNII. The two anomeric carbon resonances observed for GlcNI reflected the variable substitution of C-1 with monophosphate or diphosphate groups. GlcNI and GlcNII were ascertained to be of the alpha and beta anomeric configuration, respectively, through chemical shift comparisons with model saccharides. The accepted KDO linkage site at C-3' of GlcNII appears not to be supported by the 13C chemical shift data.

Carbohydrates↗
PubMed1983

Model for mammalian metallothionein structure.

The results of physicochemical studies of mammalian metallothioneins are summarized and used to propose a model of the protein. The primary structures of all mammalian metallothioneins are very homologous; there are 38 invariant residues and 20 of them are cysteines. The results of UV and CD optical studies indicated that all 20 cysteines are involved in the ligation of 7 mol of metal per mol of metallothionein and that the protein does not contain any alpha-helix structure. A theoretical analysis by the Chou-Fasman method has predicted 11 beta-bends, each one involving at least one cysteine residue. The most significant structural data, provided by 113Cd NMR, demonstrated that the 7 mol of bound Cd2+ are arranged in two separate metal clusters, one containing four metal ions and the other containing three, with all Cd2+ tetrahedrally coordinated to cysteine thiolate ligands. The 11 cysteine residues of the carboxyl-terminal portion of the metallothionein chain (residues 30-61) are ligated to the 4-metal cluster as shown by 113Cd NMR of this enzymatically cleaved fragment. The remaining cysteine residues from the amino-terminal polypeptide portion (residues 1-29) form the 3-metal cluster. Such a division of the chain is consistent with the presence of an intron in the mouse metallothionein-1 gene corresponding to residue 32 in the polypeptide chain. A two-domain molecular model has been constructed based on an analysis of all the available data and is described in detail. The accuracy of this model was tested by 1H NMR at 500 MHz and the data are in agreement with our proposed structure.

Amino Acid Sequence↗
PubMed1982

113Cd NMR study of a metallothionein fragment. Evidence for a two-domain structure.

A 32-residue polypeptide fragment, designated alpha I, of rat liver metallothionein obtained by subtilisin digestion was studied by 113Cd NMR. The amino acid composition of the fragment corresponded to residues 30-61 of the metallothionein primary structure, and it contained 3.4 g atoms of Cd2+/mol of alpha I-fragment. Four 113Cd resonances were observed, three of which had identical chemical shifts to those assigned to the four-metal cluster in human liver metallothionein-2 under the same pH and buffer conditions. The 5-ppm chemical shift difference between the remaining resonance assigned to the four-metal cluster in the intact protein can be explained to result from the removal of the NH2-terminal polypeptide fragment containing the three-metal cluster. These results provide unambiguous evidence for the two-domain structure of metallothionein, containing a separate three- and a four-metal cluster.

Animals↗
PubMed1982

Evidence for site-selective metal binding in calf liver metallothionein.

Two isoproteins of calf liver metallothionein (MT) have been isolated, purified, and characterized by atomic absorption, ultraviolet absorption, electron spin resonance, and 113Cd nuclear magnetic resonance spectroscopy. Native calf liver MT was found to contain both Cu+ and Zn2+ in a mole ratio of approximately 0.75. Selective replacement of the native Zn2+ with 113Cd2+ can be accomplished in vitro by adding 113CdCl2 to the homogenate before chromatography. Both isoproteins of metallothionein thus prepared contain approximately 3.9 g atoms of Cd2+ and 2.6 g atoms of Cu+/mol of protein. No ESR signal was found, indicating that either Cu+ or antiferromagnetically coupled Cu2+ is the form of copper present. Arguments in support of the former state are presented. Unlike the native 113Cd,Zn MT from rabbit liver, calf liver 113Cd,Cu MT exhibits a remarkably simple 113Cd NMR spectrum. Four major resonances were found for each isoprotein, in the same positions as the resonances assigned to the metals in the four-metal cluster A of rabbit liver metallothionein. This conclusion was confirmed by homonuclear decoupling experiments. This result in conjunction with the stoichiometry of bound metal ions found in the native protein suggests that Cu+ is bound selectively to the three-metal cluster B sites, and that one homogeneous protein fraction predominates. Three minor resonances to higher field are observed in the 113Cd NMR spectrum of calf liver MT-1 and one in calf liver MT-2, which may be attributed to a small fraction of cluster B with one Cu+ replaced by 113Cd2+. The possible biological significance of the different metal ion specificities of cluster A versus cluster B is discussed.

Animals↗