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Biomedical subjects

C H Tung

Publications and source records attributed to C H Tung.

32 records · Page 2Linked to original sources
PubMed1993

Preparation and physical properties of conjugates of oligodeoxynucleotides with poly(delta)ornithine peptides.

Oligodeoxynucleotides (12-mers) having either a 3' aminolinker or both 3' and 5' aminolinker groups were synthesized and then reacted with N-iodoacetoxysuccinimide. Separately, a series of peptides, consisting of cysteine carboxyamide and a varying number of residues of ornithine coupled through their side-chain amino groups, was prepared, leaving on the final Fmoc protecting group. Reaction of each Fmoc-peptide with an activated oligodeoxynucleotide yielded the desired conjugates, which were purified by an Fmoc-on/Fmoc-off two-step reversed-phase HPLC procedure. On hybridization with a complementary unmodified 12-mer oligodeoxynucleotide, it was found that there is a gradual increase in melting temperature with the number of ornithine residues, whereas the appended peptide did not perturb the B form of the duplex.

Base Sequence↗
PubMed1992

Design of peptide-acridine mimics of ribonuclease activity.

A series of peptide-acridine conjugates was designed and synthesized, based on three features of the proposed catalytic mechanism of RNase A: 2'-proton abstraction by His-12, proton donation to the leaving 5'-oxygen by His-119, and stabilization of the pentacoordinated phosphorous transition state by Lys-41. The substrate binding capability of RNase A was mimicked by the intercalator, acridine. Lysine served as a linker between acridine and the catalytic tripeptide. Cleavage of target RNA was monitored by agarose gel electrophoresis and by gel-permeation chromatography. The carboxyl-amidated conjugates HGHK(Acr)-NH2, HPHK(Acr)-NH2, and GGHK(Acr)-NH2 (where Acr indicates 2-methyl-9-acridinemethylene) all had similar hydrolytic activity. The catalytic mechanism most likely involved only the abstraction of the 2'-proton and stabilization of the transition state. These RNase mimics utilized rRNA and single-stranded RNA but not double-stranded RNA and tRNA as substrates.

Acridines↗
PubMed1992

Review of convergent beam tomography in single photon emission computed tomography.

Investigation of convergent-beam single photon emission computed tomography (SPECT) is actively being pursued to evaluate its clinical potentials. Fan-beam, cone-beam, pin-hole and astigmatic collimators are being used with rotating gamma cameras having large crystal areas, to increase the sensitivity for emission and transmission computed tomography of small organs such as the thyroid, brain or heart. With new multi-detector SPECT systems, convergent-beam geometry offers the ability to simultaneously obtain emission and transmission data necessary to quantify uptake of radiopharmaceutical distributions in the heart. The development of convergent-beam geometry in SPECT requires the integration of hardware and software. In considering hardware, the optimum detector system for cone-beam tomography is a system that satisfies the data sufficiency condition for which the scanning trajectory intersects any plane passing through the reconstructed region of interest. However, the major development of algorithms has been for the data insufficient case of single planar orbit acquisitions. The development of these algorithms have made possible the preliminary evaluation of this technology and the imaging of brain and heart are showing significant potential for the clinical application of cone-beam tomography. Presently, significant research activity is pursuing the development of algorithms for data acquisitions that satisfy the data sufficiency condition and that can be implemented easily and inexpensively on clinical SPECT systems.

Brain↗
PubMedDate not supplied

Size optimization of synthetic graft copolymers for in vivo angiogenesis imaging.

Angiogenesis is a critical step in tumor development and more than 25 angiogenesis inhibitors are currently in clinical trials. Noninvasive in vivo imaging of angiogenesis represents a unique opportunity of repeatedly quantitating microvascular parameters prior to and during anti-angiogenic treatments. While several imaging tracers have been proposed for MR and nuclear imaging, there does not exist any consensus of what constitutes an ideal size of an imaging agent. A series of synthetic pegylated DOTA derivatized graft copolymers (30, 60, 120 kDa) were synthesized and their in vivo behavior tested in two breast cancer models differing in vascular endothelial growth factor (VEGF) expression. Polymers were labeled with different lanthanides (Eu, Gd, Dy) and absolute blood and tumor concentrations were determined by ICP-AES measurements. DOTA and the 30 kDa polymers underwent renal clearance resulting in low plasma levels. Slow leakage across neovasculature into tumor interstitium was clearly dependent on the molecular mass of all tested agents in MCF-7 tumors. However, a cutoff was observed with minimal extravasation occurring at and above 120 kDa in well differentiated MCF-7 tumors. VEGF overexpression caused detectable differences in extravasation of all polymers, including the 120 kDa compound. We conclude that large molecular weight contrast agents with a molecular mass of <120 kDa extravasate from experimental tumor neovasculature and may not be an accurate marker for measuring true blood volume fractions when in vivo imaging is performed in the steady state.

Animals↗
PubMedDate not supplied

Preparation of oligonucleotide-peptide conjugates.

A procedure for preparing oligonucleotide-peptide conjugates is presented. It is based on appending a maleimide group to the oligonucleotide for selective coupling to the thiol side chain of a cysteine residue in the peptide. A convenient chromatographic purification procedure, based on Fmoc-on/Fmoc-off, is described.

Amino Acid Sequence↗
PubMedDate not supplied

Synthesis of oligoarginine-oligonucleotide conjugates and oligoarginine-bridged oligonucleotide pairs.

Conjugates consisting of oligoarginine peptides linked to oligodeoxynucleotides have been synthesized, including a new type of conjugate, in which a pair of oligonucleotides is bridged by a cationic peptide. Two different 9-mer oligonucleotides were conjugated to the terminal cysteine residues of the peptide series H-Cys-(Arg)n-Cys-NH2 (n = 3, 5, 7). Different thiol protecting groups were utilized on the amino- and carboxy-terminal cysteine residues of the peptide to allow selective attachment to the 3'- or 5'-terminus of each specific oligonucleotide. The conjugates containing oligoarginine peptides were purified by anion-exchange chromatography, and their structures were confirmed by polyacrylamide gel electrophoresis and amino acid analysis.

Amino Acid Sequence↗
PubMedDate not supplied

Dual-specificity interaction of HIV-1 TAR RNA with Tat peptide-oligonucleotide conjugates.

An oligonucleotide-peptide conjugate, having dual binding capability for a designated RNA, was designed. The peptide portion of the conjugate interacts with a folded domain in the RNA, whereas the oligonucleotide portion hybridizes with a nearby single-stranded region in the RNA. The dual specificity was proven in a model HIV-1 TAR RNA system using an RNase H cleavage assay to assess antisense binding to this RNA. The peptide portion of the conjugate was shown to confer increased specificity on the oligonucleotide.

Amino Acid Sequence↗
PubMedDate not supplied

Stabilization of DNA triple-helix formation by appended cationic peptides.

We have investigated the impact of appended cationic peptides on the triplex-forming and thermal-stabilizing abilities of an oligonucleotide "third strand" using a homopurine-homopyrimidine 37-mer as the host duplex target. The family of appended cationic peptides studied here contains arginine, lysine, ornithine, and diaminobutyric acid, with the residues being linked through either their alpha-amino or side chain amino groups. On the basis of optical melting profiles, we find that peptides with different amino acid compositions, but with four positive charges in common, are able to enhance, in a similar manner, the triplex-forming capability of an oligonucleotide. We note the implications of these results for the rational design of third-strand probes.

Aminobutyrates↗
PubMedDate not supplied

High-efficiency intracellular magnetic labeling with novel superparamagnetic-Tat peptide conjugates.

A biocompatible, dextran coated superparamagnetic iron oxide particle was derivatized with a peptide sequence from the HIV-tat protein to improve intracellular magnetic labeling of different target cells. The conjugate had a mean particle size of 41 nm and contained an average of 6.7 tat peptides. Derivatized particles were internalized into lymphocytes over 100-fold more efficiently than nonmodified particles, resulting in up to 12.7 x 10(6) particles/cell. Internalized particles localized in cytoplasm and nuclear compartments as demonstrated by fluorescence microscopy and immunohistochemistry. Labeled cells were highly magnetic, were detectable by NMR imaging, and could be retained on magnetic separation columns. The described method has potential applications for in vivo tracking of magnetically labeled cells by MR imaging and for recovering intracellularly labeled cells from organs.

Amino Acid Sequence↗
PubMedDate not supplied

Preparation of a cathepsin D sensitive near-infrared fluorescence probe for imaging.

A variety of proteases are overexpressed or activated during pathogenesis and represent important targets for therapeutic drugs. We have previously shown that optical imaging probes sensitive in the near-infrared fluorescence (NIRF) spectrum can be used for in vivo imaging of enzyme activity. In the current study, we show that these probes can be designed with specificity for specific enzymes, for example, cathepsin D which is known to be overexpressed in many tumors. A NIR cyanine fluorochrome served as the optical reporter and was attached to the amino terminal of an 11 amino acid peptide sequence with specificity for cathepsin D. The peptides were subsequently attached to a synthetic graft copolymer for efficient tumoral delivery. The close spatial proximity of the multiple fluorochromes resulted in quenching of fluorescence in the bound state. A 350-fold signal amplification was observed post cleavage during in vitro testing. Cell culture experiments using a rodent tumor cell line stably transfected with human cathepsin D confirmed enzyme specific activation within cells. This sequence but not a scrambled control sequence showed enzyme specificity in vitro. We conclude that activatable NIRF optical probes can be synthesized to potentially probe for specific enzymes in living organisms.

Animals↗
PubMedDate not supplied

Macrocyclic chelators with paramagnetic cations are internalized into mammalian cells via a HIV-tat derived membrane translocation peptide.

A major obstacle to using paramagnetic MR contrast agents for in vivo cell tracking or molecular sensing is their generally low cellular uptake. In this study, we show that a paramagnetically labeled DOTA chelator derivatized with a 13-mer HIV-tat peptide is efficiently internalized into mammalian cells. Intracellular concentrations were attained that were readily detectable by MR imaging using both gadolinium and dysprosium chelates. Using this paradigm, it should be feasible to internalize a variety of chemically different agents into mammalian cells.

Amino Acid Sequence↗
PubMedDate not supplied

An acridine amino acid derivative for use in Fmoc peptide synthesis.

An acridine derivative of N-alpha-Fmoc-lysine has been prepared and used in solid phase peptide synthesis. The fluorescence properties of the acridine reporter group are retained throughout the peptide synthesis procedure. The utility of the acridine group was demonstrated by its use as an energy acceptor in a fluorescence energy-quenching assay with trypsin.

Acridines↗