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S Padmanabhan

Publications and source records attributed to S Padmanabhan.

65 records · Page 4Linked to original sources
PubMed1990

Relative helix-forming tendencies of nonpolar amino acids.

An important issue in understanding the relationship between protein sequence and structure is the degree to which different amino acids favour the formation of particular types of secondary structure. Estimates of the 'helix-forming tendency' of amino acids have been made based on 'host-guest' experiments, in which copolymers are made of the amino acid of interest (the 'guest') and a host residue (typically hydroxypropyl- or hydroxybutyl-L-glutamine). Recently, however, short alanine-based peptides were found to form stable monomeric helices in water, contrary to the result predicted from host-guest experiments. We have now measured the helix-forming tendency of five different nonpolar amino acids (Ala, Ile, Leu, Phe, Val) by substituting each in turn for alanine in a 17-residue alanine-based peptide and determining the extent of alpha-helix formation. Our results differ from those of host-guest experiments both in the degree of variation in helix-forming tendency of different amino acids, and in the rank order of the helix-forming tendency. We conclude that the helix-forming tendency of a particular amino acid depends on the sequence context in which it occurs; and the restriction of side-chain rotamer conformations is important in determining the helix-forming tendency.

Alanine↗
PubMed1988

Interaction of an N-methylated polyamine analogue, hexamethonium(2+), with NaDNA: quantitative 14N and 23Na NMR relaxation rate studies of the cation-exchange process.

The interactions of the divalent hexamethonium (Hex2+) cation with double-helical calf thymus DNA are investigated by means of 14N NMR and, indirectly, by means of 23Na NMR. During a titration of NaDNA with HexBr2, the displacement of Na+ from DNA by Hex2+ is monitored by concurrent measurements of the Lorentzian 14N signals and the bi-Lorentzian 23Na signals. The variations in the quadrupolar relaxation rates of 14N and 23Na are analyzed according to a simple two-state model for the competition between Hex2+ and Na+ associated with DNA. From this analysis parameters characterizing the exchange process are evaluated, and the following conclusions are drawn: (1) The association of one Hex2+ displaces 1.7-2.0 sodium ions from the vicinity of the DNA. (2) Cation accumulation near DNA neutralizes approximately half of the phosphate charge at all points in the titration. (3) The exchange coefficient characterizing the displacement of Na+ by Hex2+ is of the same order of magnitude as the exchange coefficients determined by NMR for other divalent cations such as Mg2+ and putrescine. These findings imply that the interaction of Hex2+ with DNA is primarily electrostatic in character. The transverse and longitudinal relaxation rates observed for 14N are analyzed under the assumption that the quadrupolar relaxation processes of 14N in Hex2+ associated with DNA can be characterized by a single-exponential correlation function with correlation time tau NB. The resulting value of tau NB, 7.8 +/- 0.8 ns, is 3 orders of magnitude greater than that estimated for Hex2+ in the absence of DNA and is only 3-4 times greater than correlation times reported for 23Na and other quadrupolar cations near DNA. These comparisons indicate that the observed enhancements in the relaxation rates of 14N are due mainly to slowing of the motions that modulate its quadrupolar interactions in Hex2+ near DNA. The magnitudes of tau NB and of the quadrupolar coupling constant of Hex2+ associated with DNA are consistent with the conclusion that this association is primarily electrostatic.

Animals↗
PubMed1987

Hepatobiliary MR contrast agents: 5-substituted iron-EHPG derivatives.

A series of iron(III) ethylenebis(2-hydroxyphenyl)glycine [Fe(EHPG)]- derivatives have been examined for their role as paramagnetic hepatobiliary contrast agents for magnetic resonance (MR) imaging. The 5-substituted complexes, Fe(5-Me-EHPG)-, Fe(5-Cl-EHPG)-, and Fe(5-Br-EHPG)-, have been compared to the parent compound in rat biodistribution and MR imaging studies; correlative in vitro parameters for the complexes, including octanol-buffer partition coefficients and albumin binding affinity, have also been obtained. The three new derivatives exhibited higher degrees of lipophilicity and albumin binding affinity and varying degrees of improvement in liver-to-blood and bile-to-liver concentration ratios measured at 30 min postinjection. The 5-Cl complex had the best overall performance in terms of these tissue ratios as well as in terms of total biliary excretion. Sequential MR images of rats after administration of the complexes revealed subtle pharmacokinetic differences among the derivatives and, in general, correlated well with and complemented the biodistribution results. This study points to the sensitivity of hepatocellular uptake and excretion to simple chemical modifications and, moreover, demonstrates the importance of screening multiple derivatives to select optimal hepatobiliary MR imaging agents.

Animals↗
PubMedDate not supplied

A comparison of cleavable and noncleavable hydrazinopyridine linkers for the 99mTc labeling of Fab' monoclonal antibody fragments.

The design and synthesis of hydrazinopyridine bifunctional chelating agents (BCA's) featuring amide, ester, and disulfide groups are described. The BCA's site-specifically react with the free thiol groups of the tumor-specific monoclonal antibody fragment C46.3 using a one-pot in situ reduction and conjugation procedure from the F(ab')2 to give Fab'-linker conjugates. Molar substitution ratios (MSR's) of the hydrazinopyridine conjugates were comparable to the theoretical (maximum) number of thiols per fragment determined by free hydrazine and residual thiol assays. The series of C46.3 Fab'-linker conjugates were 99mTc-labeled in greater than 95% radiochemical purity by incubation with 99mTc-tricine for 1 h at room temperature. In order to evaluate the conjugates for radiopharmaceutical applications, the tumor localization and biodistribution properties of the radiolabeled Fab'-linker conjugates, compared to the direct labeled fragment, were tested in nude mice bearing LS174T xenografts. Depending upon the structure of the linker connecting the radiolabeled hydrazinopyridine group to the antibody fragment, we observed a variation in kidney uptake and whole-body clearance. Diester- and monoester-linked conjugates exhibited lower kidney uptake and faster whole-body clearance than the corresponding linker containing amide groups. This result may be interpreted as evidence for rapid metabolism of ester compared to amide groups in the kidney following uptake. At 24-h postinjection, the monoester-linked conjugate 99mTc-C46.3 Fab'-BA displayed the highest tumor: blood ratio (16.2) compared to the directly labeled conjugate (6.6) and is therefore a potential clinical candidate for imaging breast and ovarian cancer.

Animals↗
PubMedDate not supplied

Genetics of hypertension: from experimental models to clinical applications.

Human essential hypertension is a complex, multifactorial, quantitative trait under a polygenic control. Over the last decade several strategies have been used to dissect the genetic determinants of hypertension. Of these strategies, the study of rare monogenic forms of hypertension has been the most successful. Attempts to identify the multiple genes involved in the more common polygenic form of hypertension has been more difficult. Many laboratories use rat models of genetic hypertension where some of the complexity of studying human hypertension can be removed. Numerous crosses between hypertensive and normotensive strains have produced several quantitative trait loci (QTL) for blood pressure and other related phenotypes such as left ventricular hypertrophy, stroke, insulin resistance and kidney failure. In this review we describe established and novel strategies to dissect the susceptibility and severity loci for human essential hypertension. We also illustrate a few successful examples of a direct translation of genetic discoveries from the experimental setting to human investigation. The use of new molecular tools such as gene 'chips' or microarrays for either gene expression profiling or single nucleotide polymorphisms (SNPs)-based total genome scanning strategies will ultimately result in new diagnostics and therapeutics for human essential hypertension.

Animals↗
PubMedDate not supplied

Identification of the principal biliary metabolite of 4'-(9-acridinylamino)methanesulfon-m-anisidide in rats.

m-AMSA [4'-(9-acridinylamino)methanesulfon-m-anisidide] labeled in either the acridine or anilino portion was used to investigate the disposition of this antitumor agent in rats. The principal biliary metabolite, which accounts for approximately 80% of the total biliary radioactivity for 90 min after administration and greater than 50% of the administered dose by 180 min after administration, had both the acridine and the anilino portions intact. Isolation and purification of the principal metabolite was achieved by preparative thin-layer chromatography on silica gel and column chromatography on Amberlite XAD-2 resin. A nuclear magnetic resonance (NMR) spectrum of the CID salt in D2O showed that the metabolite is the m-AMSA-glutathione conjugate in which the thioether linkage occurs at the 5'-position of the anilino ring. Synthesis of the metabolite was achieved by oxidizing m-AMSA with active MnO2 to -methanesulfonyl - - (9-acridinyl)-3'-methoxy - 2',5' - cyclohexadiene-1',4'-diimine (m-AQDI) followed by reaction of m-AQDI with glutathione. The 1H-NMR spectrum of the synthetic product proved identical with that of the isolated metabolite. The demonstration that the principal biliary metabolite on m-AMSA involves glutathione bound to the 9-anilino ring suggests that m-AMSA may be bioactivated in vivo to the quinoidal diimine, m-AQDI.

Acridines↗