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K Chandrasekhar

Publications and source records attributed to K Chandrasekhar.

At least 19 recordsLinked to original sources
PubMed2003

Non-Watson Crick base pairs might stabilize RNA structural motifs in ribozymes -- a comparative study of group-I intron structures.

In recent decades studies on RNA structure and function have gained significance due to discoveries on diversified functions of RNA. A common element for RNA secondary structure formed by series of non- Watson/Watson Crick base pairs, internal loops and pseudoknots have been the highlighting feature of recent structural determination of RNAs. The recent crystal structure of group-I introns has demonstrated that these might constitute RNA structural motifs in ribozymes, playing a crucial role in their enzymatic activity. To understand the functional significance of these non-canonical base pairs in catalytic RNA, we analysed the sequences of group-I introns from nuclear genes. The results suggest that they might form the building blocks of folded RNA motifs which are crucial to the catalytic activity of the ribozyme. The conservation of these, as observed from divergent organisms, argues for the presence of non-canonical base pairs as an important requisite for the structure and enzymatic property of ribozymes by enabling them to carry out functions such as replication, polymerase activity etc. in primordial conditions in the absence of proteins.

Animals↗
PubMed2003

Influence of rifampicin pretreatment on the pharmacokinetics of celecoxib in healthy male volunteers.

The effect of rifampicin pretreatment on the pharmacokinetics of celecoxib was investigated in 12 healthy male human volunteers. After an overnight fast, celecoxib 200 mg was administered to the volunteers, either alone or after 5 days pretreatment with once daily dose of 600 mg rifampicin. Serum concentrations of celecoxib were estimated by reverse phase HPLC. Pharmacokinetic parameters were determined based on non-compartmental model analysis using the computer program KINETICA. A significant difference was observed in AUC(0-1) (4531.28 +/- 2147 vs 1629.1 +/- 1006 ng x h x ml(-1), p < 0.0001), AUC(0-infinity) (4632.42 +/- 2221.75 vs 1629.46 +/- 1012.61 ng x h x ml(-1), p = 0.0006), Cmax (544.89 +/- 273.91 vs 238.61 +/- 146.34 ng/ml, p = 0.04), t(1/2) (9.3 +/- 3.58 vs 4.0 +/- 1.43 h, p = 0.0317) and Cl/f (43.14 +/- 36.23 vs 122.85 +/- 95 l x h(-1), p < 0.0001) of celecoxib administered before and after rifampicin pretreatment. However, time to reach peak concentration, tmax (4 +/- 0.88 vs 4 +/- 0.83 h) and volume of distribution Vd/f (583 +/- 251 vs 710 +/- 690 l/kg) were not affected significantly. Rifampicin pretreatment reduced the AUC of celecoxib by 64% and increased the clearance by 185%. This may be due to increased metabolism of celecoxib due to the induction of cytochrome P4502C9 (CYP2C9) in liver. This interaction has a significant clinical relevance and may warrant dosage adjustment when celecoxib is co-administered with rifampicin in chronic treatment conditions, such as tuberculosis, leprosy and other infections of joints, bones, etc.

Adult↗
PubMed2002

Effect of cephalexin on the pharmacokinetics of metformin in healthy human volunteers.

The purpose of this study was to assess the effect of a single dose of cephalexin on the pharmacokinetics of metformin in healthy human volunteers. A 2 x 2 double blind randomized crossover study was conducted in 12 healthy human volunteers. Each subject received orally either 500 mg of metformin with a placebo or a combination of 500 mg of metformin and 500 mg of cephalexin. Serum and urine levels of metformin were estimated by a validated HPLC method. The systemic disposition of metformin was altered by the co-administration of cephalexin. Cephalexin increased Cmax and AUC by an average of 34% and 24%, respectively, and reduced renal clearance to 14%. The renal clearance of metformin was reduced in a time-dependent manner in the presence of cephalexin. Hence, it is concluded that cephalexin inhibits the renal tubular secretion of metformin resulting in higher circulating serum concentrations.

Area Under Curve↗
PubMed2002

Validated HPLC method for the determination of celecoxib in human serum and its application in a clinical pharmacokinetic study.

A simple high performance liquid chromatographic method using UV detection for the determination of celecoxib, a specific COX 2 inhibitor, in serum was developed. Serum samples containing the internal standard, tolbutamide, are eluted through a C18, Wakosil column. After extracting with dichloromethane, the eluent is monitored at 250 nm. The mobile phase comprised of 10 mM potassium dihydrogen ortho phosphate (pH 3.2) and acetonitrile (50:50 v/v) with a flow rate of 1 ml/min. Retention times of celecoxib and tolbutamide were 9.6 and 3.5 min, respectively. The mean absolute recovery value was about 70-80%, while the intra day and inter day coefficient of variation and percent error values of the assay method were less than 10%. The calibration curve was linear over a concentration range of 10-1000 ng/ml.

Adult↗
PubMed1997

De novo methylation of the proto-oncogene, c-fos, during development occurs step-wise and directionally in the laboratory mouse.

We have analyzed the ontogenic initiation and maintenance of methylation of certain Hpall (m), Hhal (H), Hincll (Hc), and Sall (SI)-specific CpG sites in the coding region of the proto-oncogene, c-fos, through testicular cells, sperm, and fetal, neonatal, and adult somatic tissues. The results show that 1) sperm-derived methylated sites get demethylated in early development. However, unlike other studied genes, they remain so at least up to day 13.5 post coitum (pc); 2) de novo methylation proceeds unidirectionally in a step-wise, site-specific manner between m5-m3 sites; 3) the mature, tissue-specific, adult methylation pattern is established between day 0 and day 20 of neonatal development; 4) the Hc and SI sites (CGTCGAC), occurring at an interval of one nucleotide, are only partially methylated in all the tissues; and 5) m3 and H1 sites, which occur close to an Sp1 motif, escape methylation in most of the tissues. The present study on the embryonic gene, c-fos, thus provides a novel pattern of de novo methylation in development. Also, it suggests that close proximity of CpGs may prevent methylation.

Animals↗
PubMed1996

Periodic intermittent electromechanical dissociation: hemodynamic correlate of a malfunctioning mechanical prosthetic valve.

We describe a patient who developed the unusual haemodynamic phenomenon of periodic intermittent electromechanical dissociation during regular sinus tachycardia about 20 hours after mitral valve replacement with a tilting disc mechanical prosthesis. Echocardiographic confirmation allowed prompt and life saving corrective surgery for a stuck prosthetic valve disc.

Adult↗
PubMed1994

Effect of disulfide bridge formation on the NMR spectrum of a protein: studies on oxidized and reduced Escherichia coli thioredoxin.

As a prelude to complete structure calculations of both the oxidized and reduced forms of Escherichia coli thioredoxin (M(r) 11,700), we have analyzed the NMR data obtained for the two proteins under identical conditions. The complete aliphatic 13C assignments for both oxidized and reduced thioredoxin are reported. Correlations previously noted between 13C chemical shifts and secondary structure are confirmed in this work, and significant differences are observed in the C beta and C gamma shifts between cis- and trans-proline, consistent with previous work that identifies this as a simple and unambiguous method of identifying cis-proline residues in proteins. Reduction of the disulfide bond in the active-site Cys32-Gly-Pro-Cys35 sequence causes changes in the 1H, 15N and 13C chemical shifts of residues close to the active site, some of them quite far distant in the amino acid sequence. Coupling constants, both backbone and side chain, show some differences between the two proteins, and the NOE connectivities and chemical shifts are consistent with small changes in the positions of several side chains, including the two tryptophan rings (Trp28 and Trp31). These results show that, consistent with the biochemical behavior of thioredoxin, there are minimal differences in backbone configuration between the oxidized and reduced forms of the protein.

Amino Acid Sequence↗
PubMed1993

Comparison of backbone and tryptophan side-chain dynamics of reduced and oxidized Escherichia coli thioredoxin using 15N NMR relaxation measurements.

The backbone and tryptophan side-chain dynamics of both the reduced and oxidized forms of uniformly 15N-labeled Escherichia coli thioredoxin have been characterized using inverse-detected two-dimensional 1H-15N NMR spectroscopy. Longitudinal (T1) and transverse (T2) 15N relaxation time constants and steady-state (1H)-15N NOEs were measured for more than 90% of the protonated backbone nitrogen atoms and for the protonated indole nitrogen atoms of the two tryptophan residues. These data were analyzed by using a model free dynamics formalism to determine the generalized order parameter (S2), the effective correlation time for internal motions (tau e), and 15N exchange broadening contributions (Rex) for each residue, as well as the overall molecular rotational correlation time (tau m). The reduced and oxidized forms exhibit almost identical dynamic behavior on the picosecond to nanosecond time scale. The W31 side chain is significantly more mobile than the W28 side chain, consistent with the positions of W31 on the protein surface and W28 buried in the hydrophobic core. Backbone regions which are significantly more mobile than the average include the N-terminus, which is constrained in the crystal structure of oxidized thioredoxin by specific contacts with a Cu2+ ion, the C-terminus, residues 20-22, which constitute a linker region between the first alpha-helix and the second beta-strand, and residues 73-75 and 93-94, which are located adjacent to the active site. In contrast, on the microsecond to millisecond time scale, reduced thioredoxin exhibits considerable dynamic mobility in the residue 73-75 region, while oxidized thioredoxin exhibits no significant mobility in this region. The possible functional implications of the dynamics results are discussed.

Amino Acid Sequence↗
PubMed1991

Solution conformational preferences of immunogenic peptides derived from the principal neutralizing determinant of the HIV-1 envelope glycoprotein gp120.

With standard one- and two-dimensional proton NMR techniques, a common structural motif has been identified in water solutions of short peptide sequences derived from the envelope glycoprotein gp120 of HIV-1. Three peptides of lengths 12, 24, and 40 residues (termed RP342, RP142, and RP70, respectively) were synthesized, each containing a central amino acid sequence common to many HIV-1 isolates. In addition, RP70 contained a disulfide bond between cysteine residues close to the ends of the molecule, forming a loop that is thought to constitute an important structural and immunological component of the intact glycoprotein. Peptides RP70 and RP142 showed evidence for the presence of a significant population of conformations containing a beta-turn in the conserved sequence Gly-Pro-Gly-Arg. Strong nuclear Overhauser effect (NOE) connectivities were observed between the amide protons of the arginine and the adjacent glycine. A weak NOE connectivity was observed between the C alpha H of the proline residue and the NH of the Arg [a d alpha N(i,i + 2) NOE connectivity], confirming the presence of a conformational preference for a turn conformation in this sequence. The remainder of the peptide showed evidence of conformational averaging: no NMR evidence for a uniquely folded structure was obtained for any of the peptides in water solution. Circular dichroism (CD) spectra indicated that no ordered helix was present in water solutions of RP70, although a CD spectrum that indicated the presence of approximately 30% helix could be induced by the addition of trifluoroethanol.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1991

Two-dimensional proton-NMR studies on a hybrid peptide between cecropin A and melittin. Resonance assignments and secondary structure.

A hybrid peptide of cecropin A and melittin was investigated by two-dimensional 1H-NMR at pH 5.8 in aqueous solution with 30% (by vol.) hexafluoroisopropanol. The peptide contains 26 amino acids, is a combination of the first 13 residues of each of the two parent peptides, CA(1-13)M(1-13) identical to CAM(1-26) and has an amidated C terminal. This peptide was recently synthesized [Boman, H.G., Wade, D., Boman, I.A., Wåhlin, B. & Merrifield, R.B. (1989) FEBS Lett. 259, 103-106] and shown to have strong antibacterial activity but to be harmless towards erythrocytes. All resonances of the main chain and side chain beta-protons are assigned except for those of the N-terminal lysine. Several medium range NOE connectivities were observed showing two separated alpha-helices, involving residues 4-12 and 16-26. The JNH alpha-coupling constants in these sections support the conclusion. From the exchange rates of the NH protons it is concluded that the alpha-helix of residues 16-26 is much more stable than the other helix. The circular dichroism data indicates about 30% less alpha-helix character than the NMR data. A reduced contribution to the ellipticity from the unstable helix is suggested. The chemical-shift differences between the two parts of the hybrid and the respective parent peptides are larger for the cecropin part than for the melittin part. For the latter, residues 17-26 of the hybrid are proposed to have a secondary structure very similar to that of residues 4-13 of melittin.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1991

Assignment of the 15N NMR spectra of reduced and oxidized Escherichia coli thioredoxin.

As a necessary first step in the use of heteronuclear correlated spectra to obtain high resolution solution structures of the protein, assignment of the 15N NMR spectra of reduced and oxidized Escherichia coli thioredoxin (Mr 12,000) uniformly labeled with 15N has been performed. The 15N chemical shifts of backbone amide nitrogen atoms have been determined for both oxidation states of thioredoxin using 15N-1H correlated and two-dimensional heteronuclear single-quantum coherence (HSQC) TOCSY and NOESY spectra. The backbone assignments are complete, except for the proline imide nitrogen resonances and include Gly33, whose amide proton resonance is difficult to observe in homonuclear 1H spectra. The differences in the 15N chemical shift between oxidized and reduced thioredoxin, which occur mainly in the vicinity of the two active site cysteines, including residues distant in the amino acid sequence which form a hydrophobic surface close to the active site, are consistent with the differences observed for proton chemical shifts in earlier work on thioredoxin.

Amino Acid Sequence↗
PubMed1990

Direct methods with single isomorphous replacement data. I. Reduction of systematic errors.

The direct-methods procedure for single isomorphous replacement (SIR) data [Hauptman (1982). Acta Cryst. A38, 289-294], as modified by Fortier, Moore & Fraser [Acta Cryst. (1985), A41, 571-577] has been implemented and tested with a large number of known structures. It was found that the modified procedure greatly reduces the bias toward 'unresolved' SIR invariant values associated with estimates of 0 or pi, but does not remove it entirely. If the heavy atoms are not in a centrosymmetric array the centroid of the distribution of invariant estimates is not centered on true protein values, but is biased toward conventional SIR values by up to 15 degrees, thus errors in the estimates are not random but systematic. When the heavy atoms are in a centrosymmetric array (or single heavy-atom site in space group P21), the distribution of estimates is often sharply bimodal, with peaks centered at both true invariant values and pure 'unresolved' SIR values. Simple procedures are given which can be applied in both situations to reduce significantly the bias with no overall loss of accuracy. An additional correction factor is then described which can be used to remove nearly all of the bias, and improve the accuracy as well. The result is that errors in the corrected invariant estimates are small in magnitude, but are now also random instead of systematic. Since the number of estimates greatly exceeds the number of phases, the remaining random errors should have little impact in phasing processes.

Bence Jones Protein↗
PubMed1985

Preliminary crystallographic study of an L-asparaginase from Vibrio succinogenes.

Crystals of an L-asparaginase from Vibrio succinogenes were obtained with the hanging drop method from ammonium sulphate-containing solutions. The crystals belong to the orthorhombic space group P22(1)2(1) with unit cell dimensions of a = 71.3 A, b = 85.8 A, c = 114.0 A, and contain two tetrameric enzyme molecules per unit cell. There are two subunits in the asymmetric unit; a molecular dyad is coincident with the crystallographic dyad. The crystal lattice is similar to that reported for an Escherichia coli asparaginase. Rotation function calculations have revealed that the V. succinogenes enzyme has 222 point group symmetry in the crystal. The second and third molecular dyads differ, however, from the corresponding E. coli asparaginase dyads by approximately 40 degrees. The crystals diffract to at least 2.2 A resolution and are suitable for X-ray crystallographic structure determination.

Asparaginase↗
PubMed1980

Artificial spawning effected in the fresh water teleost, Cyprinus carpio by clomiphene citrate.

Triweekly i.m. injections of clomiphene citrate (group 1, 25 micrograms/0.5 ml and group II, 50 micrograms/1.0 ml) were administered for a period of 3 months during the preparatory period to female fresh water teleosts exhibiting ovarian recrudescence, while a control group received 0.5 ml of physiological saline throughout the period of experimentation i.e., from February through April. 50 micrograms clomiphene citrate treatment brought about a steady increase in ovarian size, and oocytes began to enlarge and mature and finally ovulation took place in April. This is 4 months ahead of their normal occurrence.

Animals↗
PubMed1979

Light and electron microscopical investigations on the tanycyte differentiation during the perinatal period in the rat.

The differentiation of tanycytes was studied light and electron microscopically during the perinatal period in rats, the time when functional connections between hypothalamus and hypophysis are established. The 3rd ventricle is slit-like between 16 and 18 days of the prenatal period. Its wall is formed by intensively proliferating matrix cells with apical processes, ovoid perikarya and a basal process. The ventral region of the 3rd ventricle becomes funnel-shaped on the 20th day of the prenatal period. As the cells differentiate, the apical process becomes shorter and broader. Moreover, on day 20 of prenatal life cells without apical processes appear. Their number increases during the postnatal period. The concentration of endoplasmic reticulum, mitochondria, polysomes, lipid droplets, dense bodies (lysosomes), lamellated and multivesicular bodies increases. Initially the cells are similar but from the 3rd day of postnatal life differentiation occurs in different regions of the infundibular recess. After the 5th day, there are no marked changes in the structure and distribution of these cells.

Animals↗