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W C Chu

Publications and source records attributed to W C Chu.

30 records · Page 2Linked to original sources
PubMed1997

Convergent flow phenomenon mimics the appearance of venous thrombosis in gradient-echo images with or without the presence of a contrast agent.

We have observed signal voids at the junction of the renal vein and the inferior vena cava in the Spoiled Grass images. They mimicked the magnetic resonance appearance of an intraluminal thrombus with and without the presence of a contrast agent. The patency of the vessels was unveiled by fast Spoiled Grass sequence with reduced echo time as well as by Doppler ultrasound. Phantom studies revealed patterns of counterrotating vortices at the confluence. The cause of this image artifact was subsequently deduced as the intravoxel spin phase dispersion arising from the impinging flows of the renal vein and inferior vena cava. It is concluded that in regions where complex flow patterns reside, fast imaging sequences that reduce spin phase variations should always be conducted in addition to other routine sequences to exclude uncertainties in image interpretation.

Aged↗
PubMed1996

An evaluation of the accuracy of magnetic-resonance-guided Gamma Knife surgery.

An evaluation of the systematic accuracy of magnetic resonance (MR)-guided Gamma Knife surgery was performed. In two experiments, a cylinder phantom filled with dosimeter gel containing ferrous sulfate was fixed to a stereotactic frame. The gel phantom was irradiated with the Gamma Knife with a single shot using 4-mm collimators. The target point was set at the frame center of the stereotactic system giving coordinate values of X = 100, Y = 100, Z = 100. The maximum target dose was 15 Gy. MR imaging was undertaken immediately after the irradiation, using a superconductive 1.5-T MR scanner. Spin echo T1-weighted images, with transaxial, coronal, and sagittal views, were obtained. On the images, points with the highest signals were defined as the target point which received the maximum dose. Within the dose range of the experiment, this definition is based on a linear relationship between the dose to the gel and the T1 relaxation shortening after irradiation. The distances between the frame center and the target point defined on the MR images in the experiments were 0.12 mm (0.2375 pixels) and 0.43 mm (0.8515 pixels), respectively. Both are within the mechanical accuracy of the Gamma Knife. The imaging study confirms the accuracy of the Gamma Knife surgery used in the institution.

Evaluation Studies as Topic↗
PubMed1995

The use of arch index to characterize arch height: a digital image processing approach.

Attempts to evaluate foot arch types from footprint parameters have yielded conflicting results in the past. This could be caused by the uncertainty inherent in the definition of some footprint parameters and the inaccuracy during the footprint acquisition and the parameter calculation phases of the traditional methods. In order to avoid these problems, digital image processing methods were used to acquire and to calculate the Arch Index (AI), a parameter which is robust in its definition. A significant correlation (r = -0.70, p < 0.0001) was found between AI and arch height. Therefore this study confirms that foot arch type does correlate with the footprint parameter, AI. This was further revealed by a new parameter, the Modified Arch Index (MAI), which incorporates foot pressure information in the evaluation. MAI not only correlated well with arch height (r = -0.71, p < 0.0001) but appeared to characterize abnormal foot types better than AI.

Adult↗
PubMed1994

Enzymatically active truncated cat brain glutamate decarboxylase: expression, purification, and absorption spectrum.

The DNA encoding the sequence for glutamate decarboxylase from cat brain was recloned into the Escherichia coli expression vector pET11a. The N-terminal 77- to 84-amino acid residues encoded by the cloned gene had been deleted from the protein which was purified to near homogeneity in 20-mg batches. The truncated protein is a dimer with a subunit molecular mass of about 59 kDa. This protein is enzymatically active and has a Km for L-glutamate of 1.37 mM and a turnover number of 7 s-1 at its optimal pH of 6.6. The absorption spectrum, resulting from the bound coenzyme, pyridoxal phosphate, showed pH-dependent bands at 338 and 420 nm with an isosbestic point at 356 nm. A spectrophotometric pKa value of 6.92 was evaluated for the bound coenzyme. The pH-dependent kinetic data suggest the presence of two dissociable groups in the free enzyme with pKa values of about 6.45 and 7.05 and pKa value of the enzyme-substrate complex of about 6.82 in phosphate buffer. Structures for the coenzyme in the active site of brain glutamate decarboxylase are proposed.

Animals↗
PubMed1992

Fluorine-19 nuclear magnetic resonance as a probe of the solution structure of mutants of 5-fluorouracil-substituted Escherichia coli valine tRNA.

In order to utilize 19F nuclear magnetic resonance (NMR) to probe the solution structure of Escherichia coli tRNAVal labeled by incorporation of 5-fluorouracil, we have assigned its 19F spectrum. We describe here assignments made by examining the spectra of a series of tRNAVal mutants with nucleotide substitutions for individual 5-fluorouracil residues. The result of base replacements on the structure and function of the tRNA are also characterized. Mutants were prepared by oligonucleotide-directed mutagenesis of a cloned tRNAVal gene, and the tRNAs transcribed in vitro by bacteriophage T7 RNA polymerase. By identifying the missing peak in the 19F NMR spectrum of each tRNA variant we were able to assign resonances from fluorouracil residues in loop and stem regions of the tRNA. As a result of the assignment of FU33, FU34 and FU29, temperature-dependent spectral shifts could be attributed to changes in anticodon loop and stem conformation. Observation of a magnesium ion-dependent splitting of the resonance assigned to FU64 suggested that the T-arm of tRNAVal can exist in two conformations in slow exchange on the NMR time scale. Replacement of most 5-fluorouracil residues in loops and stems had little effect on the structure of tRNAVal; few shifts in the 19F NMR spectrum of the mutant tRNAs were noted. However, replacing the FU29.A41 base-pair in the anticodon stem with C29.G41 induced conformational changes in the anticodon loop as well as in the P-10 loop. Effects of nucleotide substitution on aminoacylation were determined by comparing the Vmax and Km values of tRNAVal mutants with those of the wild-type tRNA. Nucleotide substitution at the 3' end of the anticodon (position 36) reduced the aminoacylation efficiency (Vmax/Km) of tRNAVal by three orders of magnitude. Base replacement at the 5' end of the anticodon (position 34) had only a small negative effect on the aminoacylation efficiency. Substitution of the FU29.A41 base-pair increased the Km value 20-fold, while Vmax remained almost unchanged. The FU4.A69 base-pair in the acceptor stem, could readily be replaced with little effect on the aminoacylation efficiency of E. coli tRNAVal, indicating that this base-pair is not an identity element of the tRNA, as suggested by others.

Acylation↗
PubMed1992

Correlations between fluorine-19 nuclear magnetic resonance chemical shift and the secondary and tertiary structure of 5-fluorouracil-substituted tRNA.

To complete assignment of the 19F nuclear magnetic resonance (NMR) spectrum of 5-fluorouracil-substituted Escherichia coli tRNA(Val), resonances from 5-fluorouracil residues involved in tertiary interactions have been identified. Because these assignments could not be made directly by the base-replacement method used to assign 5-fluorouracil residues in loop and stem regions of the tRNA, alternative assignment strategies were employed. FU54 and FU55 were identified by 19F homonuclear Overhauser experiments and were then assigned by comparison of their 19F NMR spectra with those of 5-fluorouracil-labeled yeast tRNA(Phe) mutants having FU54 replaced by adenine and FU55 replaced by cytosine. FU8 and FU12, were assigned from the 19F NMR spectrum of the tRNA(Val) mutant in which the base triple G9-C23-G12 substituted for the wild-type A9-A23-FU12. Although replacement of the conserved U8 (FU8) with A or C disrupts the tertiary structure of tRNA(Val), it has only a small effect on the catalytic turnover number of valyl-tRNA synthetase, while reducing the affinity of the tRNA for enzyme. Analysis of the 19F chemical shift assignments of all 14 resonances in the spectrum of 5-fluorouracil-substituted tRNAVal indicated a strong correlation to tRNA secondary and tertiary structure. 5-Fluorouracil residues in loop regions gave rise to peaks in the central region of the spectrum, 4.4 to 4.9 parts per million (p.p.m.) downfield from free 5-fluorouracil. However, the signal from FU59, in the T-loop of tRNA(Val), was shifted more than 1 p.p.m. downfield, to 5.9 p.p.m., presumably because of the involvement of this fluorouracil in the tertiary interactions between the T and D-loops. The 19F chemical shift moved upfield, to the 2.0 to 2.8 p.p.m. range, when fluorouracil was base-paired with adenine in helical stems. This upfield shift was less pronounced for the fluorine of the FU7.A66 base-pair, located at the base of the acceptor stem, an indication that FU7 is only partially stacked on the adjacent G49 in the continuous acceptor stem/T-stem helix. An unanticipated finding was that the 19F resonances of 5-fluorouracil residues wobble base-paired with guanine were shifted 4 to 5 p.p.m. downfield of those from fluorouracil residues paired with A. In the 19F NMR spectra of all fluorinated tRNAs studied, the farthest downfield peak corresponded to FU55, which replaced the conserved pseudouridine normally found at this position.

Acylation↗
PubMed1991

Fluorine-19 NMR studies of the thermal unfolding of 5-fluorouracil-substituted Escherichia coli valine transfer RNA.

19F NMR spectroscopy was used to monitor the thermal unfolding of E. coli tRNAVal labeled by incorporation of 5-fluorouracil (FUra). With rising temperatures, resonances in the 19F NMR spectrum of (FUra)tRNAVal gradually shift towards the central region of the spectrum and merge into a single broad peak above 85 degrees C. FU55 and FU12 are the first to shift, beginning at temperatures below 40 degrees C, which suggests that the initial steps of thermal denaturation of tRNAVal involve disruption of the tertiary interactions between the D- and T-arms. The acceptor stem and the FU64-G50 wobble base pair in the T-stem are particularly stable to thermal denaturation. A temperature-dependent splitting of the 19F resonance assigned to FU64, at temperatures above 40 degrees C, suggests that the T-arm of (FUra)tRNAVal exists in two conformations in slow exchange on the NMR time scale.

Base Sequence↗
PubMed1991

Recognition of Escherichia coli valine transfer RNA by its cognate synthetase: a fluorine-19 NMR study.

Interactions of 5-fluorouracil-substituted Escherichia coli tRNAVal with its cognate synthetase have been investigated by fluorine-19 nuclear magnetic resonance. Valyl-tRNA synthetase (VRS) (EC 6.1.1.9), purified to homogeneity from an overproducing strain of E. coli, differs somewhat from VRS previously isolated from E. coli K12. Its amino acid composition and N-terminal sequence agree well with results derived from the sequence of the VRS gene [Heck, J.D., & Hatfield, G.W. (1988) J. Biol. Chem. 263, 868-877]. Apparent KM and Vmax values of the purified VRS are the same for both normal and 5-fluorouracil (FUra)-substituted tRNAVal. Binding of VRS to (FUra)tRNAVal induces structural perturbations that are reflected in selective changes in the 19F NMR spectrum of the tRNA. Addition of increasing amounts of VRS results in a gradual loss of intensity at resonances corresponding to FU34, FU7, and FU67, with FU34, at the wobble position of the anticodon, being affected most. At higher VRS/tRNA ratios, a broadening and shifting of FU12 and of FU4 and/or FU8 occur. These results indicate that VRS interacts with tRNAVal along the entire inside of the L-shape molecule, from the acceptor stem to the anticodon. Valyl-tRNA synthetase also causes a splitting of resonances FU55 and FU64 in the T-loop and stem of tRNAVal, suggesting conformational changes in this part of the molecule. No 19F NMR evidence was found for formation of the Michael adduct between VRS and FU8 of 5-fluorouracil-substituted tRNAVal that has been proposed as a common intermediate in the aminoacylation reaction.

Base Sequence↗
PubMed1989

19F NMR of 5-fluorouracil-substituted transfer RNA transcribed in vitro: resonance assignment of fluorouracil-guanine base pairs.

5-Fluorouracil is readily incorporated into active tRNA(Val) transcribed in vitro from a recombinant phagemid containing a synthetic E. coli tRNA(Val) gene. This tRNA has the expected sequence and a secondary and tertiary structure resembling that of native 5-fluorouracil-substituted tRNA(Val), as judged by 19F NMR spectroscopy. To assign resonances in the 19F spectrum, mutant phagemids were constructed having base changes in the tRNA gene. Replacement of fluorouracil in the T-stem with cytosine, converting a FU-G to a C-G base pair, results in the loss of one downfield peak in the 19F NMR spectrum of the mutant tRNA(Val). The spectra of other mutant tRNAs having guanine for adenine substitutions that convert FU-A to FU-G base pairs all have one resonance shifted 4.5 to 5 ppm downfield. These results allow assignment of several 19F resonances and demonstrate that the chemical shift of the 19F signal from base-paired 5-fluorouracil differs considerably between Watson-Crick and wobble geometry.

Anticodon↗