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Biomedical subjects

X Ji

Publications and source records attributed to X Ji.

At least 163 records · Page 9Linked to original sources
PubMed1993

Further studies on the plasma-sprayed amorphous phase in hydroxyapatite coatings and its deamorphization.

Plasma-sprayed hydroxyapatite coatings contain a quite large amount of amorphous phase. Infrared analysis shows that the plasma-sprayed amorphous phase is an oxyapatite and the coating predetermined as a hydroxyapatite is proved to be an oxyhydroxyapatite with a small quantity of hydroxyapatite. Heat treatment promotes the transformation of amorphous oxyapatite into a crystalline hydroxyapatite structure and reduces the dissolution rate of the coatings. Further studies focus on the properties of the amorphous phase, showing that amorphous oxyapatite is more soluble in 0.15 M lactic acid at 25 degrees C and that water molecules can accelerate the amorphous/crystalline transformation of the hydroxyapatite structure during the plasma-spraying process.

Crystallization↗
PubMed1993

[Preliminary mechanism studies of siwenmycin on the inhibition of nucleic acid syntheses].

Siwenmycin, isolated from a streptomyces culture, is a new member of aclacinomycin analogues. It exhibited a remarkable inhibitory effect on the biosynthesis of DNA and RNA in vivo. In order to recognize the mechanism of the inhibitions, the reactions of siwenmycin to DNA template, DNA polymerase I and RNA polymerase were studied. Experimental results showed that siwenmycin could intercalate DNA, but it did not inhibit DNA polymerase I-mediated DNA repair replication and T7RNA polymerase-mediated DNA transcription. This indicates that siwenmycin is not a damage to the DNA template function, nor will it inhibit DNA polymerase I and T7RNA polymerase, though it can intercalate DNA.

Aclarubicin↗
PubMed1993

Depleting cell cholesterol alters calcium-induced assembly of tight junctions by monolayers of MDCK cells.

A role for lipids in the formation of tight junctions (TJ) has been proposed. Attempts to relate changes in whole cell phospholipid composition to the formation of TJs, however, have yielded equivocal results. The object in the present study was to relate changes in TJ of MDCK cells more specifically to alterations in plasma membrane lipids. Cholesterol, which resides primarily in the plasma membrane, was reduced by 25% after incubation of cell monolayers for 24 h in a low Ca2+ medium supplemented with (1-2 microM) Lovastatin, an inhibitor of hydroxymethylglutarylcoenzyme A (HMG-CoA) reductase. This was associated with a halving of the time required for Ca2+ to induce TJ formation as monitored by transepithelial electrical resistance (TER). [3H]Mannitol flux, and morphometric measurements made on freeze fracture replicas confirm that the effects on TER reflect changes in the characteristics of the paracellular pathway. Peak and steady state values of TER were also elevated over control values. The changes in cholesterol content and the time course for TJ assembly were apparent at levels of Lovastatin which do not affect prenylation of proteins, and were prevented if 5 mM mevalonate was present along with Lovastatin. Paradoxically, despite a decrease of approximately 1/3 in the Ca concentration required to yield maximum rates of TJ assembly, 45Ca2+ uptake was actually reduced after cholesterol depletion. The data suggest that cholesterol may modulate the properties of membrane proteins and/or phospholipids which interact with Ca2+, possibly on the exoplasmic leaflet, during TJ assembly.

Animals↗
PubMed1992

The three-dimensional structure of a glutathione S-transferase from the mu gene class. Structural analysis of the binary complex of isoenzyme 3-3 and glutathione at 2.2-A resolution.

The crystal structure of a mu class glutathione S-transferase (EC 2.5.1.18) from rat liver (isoenzyme 3-3) in complex with the physiological substrate glutathione (GSH) has been solved at 2.2-A resolution by multiple isomorphous replacement methods. The enzyme crystallized in the monoclinic space group C2 with unit cell dimensions of a = 87.98 A, b = 69.41 A, c = 81.34 A, and beta = 106.07 degrees. Oligonucleotide-directed site-specific mutagenesis played an important role in the solution of the structure in that the cysteine mutants C86S, C114S, and C173S were used to help locate the positions of mercuric ion sites in nonisomorphous derivatives with ethylmercuric phosphate and to align the sequence with the model derived from MIR phases. A complete model for the protein was not obtained until part of the solvent structure was interpreted. The dimer in the asymmetric unit refined to a crystallographic R = 0.171 for 19,298 data and I > or = 1.5 sigma (I). The final model consists of 4150 atoms, including all non-hydrogen atoms of 434 amino acid residues, two GSH molecules, and oxygen atoms of 474 water molecules. The dimeric enzyme is globular in shape with dimensions of 53 x 62 x 56 A. Crystal contacts are primarily responsible for conformational differences between the two subunits which are related by a noncrystallographic 2-fold axis. The structure of the type 3 subunit can be divided into two domains separated by a short linker, a smaller alpha/beta domain (domain I, residues 1-82), and a larger alpha domain (domain II, residues 90-217). Domain I contains four beta-strands which form a central mixed beta-sheet and three alpha-helices which are arranged in a beta alpha beta alpha beta beta alpha motif. Domain II is composed of five alpha-helices. Domain I can be considered the glutathione binding domain, while domain II seems to be primarily responsible for xenobiotic substrate binding. The active site is located in a deep (19-A) cavity which is composed of three relatively mobile structural elements: the long loop (residues 33-42) of domain I, the alpha 4/alpha 5 helix-turn-helix segment, and the C-terminal tail. GSH is bound at the active site in an extended conformation at one end of the beta-sheet of domain I with its backbone facing the cavity and the sulfur pointing toward the subunit to which it is bound.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1992

Modular mutagenesis of exons 1, 2, and 8 of a glutathione S-transferase from the mu class. Mechanistic and structural consequences for chimeras of isoenzyme 3-3.

Exons 1 and 2 and exon 8 of the mu class GSH transferases from rat encode sequence-variable regions 1 and 4 of mu class isoenzymes, respectively. These two of four variable regions are located at the N- and C-termini of this isoenzyme class and impinge on the active site. In order to assess the influence of these variable regions on the catalytic diversity of the class mu isoenzymes, seven chimeric isoenzymes were constructed by transplantation of the variable regions of the sequence of the type 4 subunit into the corresponding regions of the type 3 subunit. The chimeric isoenzymes exhibit unique catalytic properties. Replacement of all, or part, of variable region 4 of the type 3 subunit with that of the type 4 subunit results in chimeric catalysts with higher turnover numbers in nucleophilic aromatic substitution reactions. Analysis of the crystal structure of isoenzyme 3-3 [Ji, X., Zhang, P., Armstrong, R. N., & Gilliland, G. L. (1992) Biochemistry (preceding paper in this issue)] suggests that interaction of the flexible C-terminal tail with the N-terminal domain helps limit the rate of product release from the active site of isoenzyme 3-3 in this type of reaction. Substitution of all, or part, of the sequence-variable region 1 of subunit 3 with that of subunit 4 results in chimeric isoenzymes that mimic the high stereoselectivity but not the catalytic efficiency of isoenzyme 4-4 toward alpha,beta-unsaturated ketones.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1992

Structure of lansimide 2, a product from Clausena lansium.

The natural product lansimide 2 is a 1:1 mixture of two different cyclic amides, C18H17NO2.C18H19NO3. The mixture crystallizes as a molecular pair in the centrosymmetric space group P21/n. M(r) = 576.69, monoclinic, a = 20.151 (2), b = 6.2984 (4), c = 24.051 (2) A, beta = 104.339 (8) degrees, V = 2957.4 A3, Z = 4, Dx = 1.30 g cm-3, Cu K alpha, lambda = 1.54178 A, mu = 6.1 cm-1, F(000) = 1224, T = 163 (1) K, R = 0.033, wR = 0.034 for 5002 observed reflections.

Lactams↗
PubMed1992

Structure of 1,1-dichloro-2-(4-methoxyphenyl)-2,3-diphenylcyclopropane.

C22H18Cl2O, M(r) = 369.3, monoclinic, P2(1)/a, a = 16.585(1), b = 17.328(1), c = 13.192(3) A, beta = 107.443(8) degrees, V = 3616.8 A3, Z = 8, Dx = 1.356 g cm-3, lambda(Mo K alpha) = 0.71069 A, mu = 3.2 cm-1, F(000) = 1536, T = 138 K, R = 0.039 for 5450 observed reflections. The structural features of the two independent molecules are quite similar except in the orientation of the methoxy group. The cyclopropane ring shows the expected bond-length asymmetry with C(2)-C(3) as the longest bond. The two cis-arranged phenyl rings adopt similar conformations as observed in diaryl-cyclopropanes, with one ring in the bisecting position and the other near the perpendicular position. The conformation of the third aryl ring is also near the perpendicular position. The overall conformation of the three aryl rings is different from the helical propeller conformation consistently observed in tamoxifen and all other known tri(tetra)aryl-vinyl systems.

Chemical Phenomena↗
PubMed1992

Structure of asperketal B.

3,7,11-Trimethyl-13-oxabicyclo[8.3.0]trideca-2,6-diene-12-spiro-2' -(5',5'-dimethyltetrahydrofuran), asperketal B, C20H30O2, M(r) = 302.46, orthorhombic, P2(1)2(1)2(1), a = 10.064(1), b = 22.214(2), c = 8.330 (1) A, V = 1862.3 A3, Z = 4, Dx = 1.08 g cm-3, Cu K alpha, lambda = 1.54178 A, mu = 4.55 cm-1, F(000) = 664, T = 294 (1) K, R = 0.031, wR = 0.027 for 953 data. The backbone of the molecule is composed of a cyclodecadiene ring which is cis fused to a bicyclic ketal system. The relative configurations at C(1), C(10), C(11) and C(12) are 1S*, 10R*, 11R* and 12R*.

Chemical Phenomena↗
PubMed1992

Contribution of tyrosine 6 to the catalytic mechanism of isoenzyme 3-3 of glutathione S-transferase.

The role of the hydroxyl group of tyrosine 6 in the catalytic mechanism of isoenzyme 3-3 of rat glutathione S-transferase has been examined by x-ray crystallography and site-specific replacement of the residue with phenylalanine and evaluation of the catalytic properties of the mutant enzyme. This particuar tyrosine residue is conserved in the sequences of all of the cytosolic enzymes and is found, in crystal structures of both isoenzyme 3-3 from the mu-gene class and an isoenzyme from the pi-gene class, to be proximal to the sulfur of glutathione (GSH) or glutathione sulfonate bound at the active site. The 2.2-A structure of the binary complex of isoenzyme 3-3 and GSH indicates that the hydroxyl group of Tyr6 is located 3.2-3.5 A from the sulfur of GSH, well within hydrogen bonding distance. Removal of the hydroxyl group of Tyr6 has essentially no effect on the dissociation constant (22 +/- 3 microM) for GSH. Nevertheless the Y6F mutant exhibits a turnover number which is only about 1% that of the native enzyme when assayed at pH 6.5 with either 1-chloro-2,4-dinitrobenzene (CDNB) or 4-phenyl-3-buten-2-one. UV difference spectra of the binary enzyme-GSH complexes suggest that the predominant ionization state of GSH in the active site of the Y6F mutant is the neutral thiol (e.g. EY6F.GSH) which is in contrast to the native enzyme in which the thiol is substantially deprotonated (e.g. E.GS-). Spectrophotometric titration suggests that the pKa of the thiol is 6.9 +/- 0.3 in the E.GSH complex and greater than or equal to 8 in the EY6F.GSH binary complex. In addition, the pH dependence of kcat/KmCDNB reveals that the reactions catalyzed by the native enzyme and the Y6F mutant are dependent on a single ionization in the E.GSH and EY6F.GSH complexes with pKa = 6.2 +/- 0.1 and 7.8 +/- 0.3, respectively. The results suggest that the hydrogen bond between Tyr6 and the enzyme-bound nucleophile helps to lower the pKa of GSH in the binary enzyme-substrate complex.

Animals↗
PubMed1992

Pulmonary artery diastolic pressure: a simultaneous Doppler echocardiography and catheterization study.

Pulmonary hypertension is an important determinant of the clinical presentation of and surgical approach to patients with heart disease. To confirm the utility of continuous wave Doppler echocardiography in assessing the pulmonary artery diastolic pressure in patients with pulmonary regurgitation, 51 patients representing the wide hemodynamic spectrum of pulmonary artery pressure underwent simultaneous determination of pulmonary artery diastolic pressure by continuous wave Doppler echocardiography and cardiac catheterization. Pulmonary artery diastolic pressure was estimated from the Doppler recordings by the end-diastolic pressure gradient obtained by the modified Bernoulli equation plus the estimated right atrial pressure. A correlation was observed (r = 0.935, SEE = 7.4 mmHg) between Doppler and catheterization pulmonary artery diastolic pressure. In addition, comparison between the mean diastolic pressure gradient across the pulmonary valve by Doppler and pulmonary artery diastolic pressure at catheterization yielded a high correlation (r = 0.947, SEE = 5.1 mmHg). These data demonstrate that continuous wave Doppler echocardiography is a useful noninvasive technique for evaluating the pulmonary artery diastolic pressure in patients with pulmonary regurgitation.

Adolescent↗
PubMed1991

Changes of nerve excitability and urinary deltamethrin in sprayers.

The nerve excitability of median nerve as well as the urinary deltamethrin and its metabolite dibromovinyl-dimethyl-cyclopropane carboxylic acid (Br2A) were detected in 24 deltamethrin sprayers in an assessment of the exposure and effect of deltamethrin. Twenty-nine male, unexposed referents of the same age range were also selected. The urinary deltamethrin and its metabolite Br2A were detectable by GC and HPLC in the sprayers after exposure. The nerve excitability detected with an electromyograph and pairs of stimuli at variable intervals showed that there was a prolongation of supernormal period in median nerve of the exposed group after a 3-d spraying compared with that before spraying which became more significant 2 d after cessation of exposure. In the mean time, no change of supernormal period in the median nerve of reference group was found at the 3-d interval. Nearly half of the sprayers had a supernormal period prolonged by more than 4 ms after spraying, whereas nearly none of the reference group showed similar changes after repeated examinations. Although there was neither correlation between the nerve excitability changes and the urinary deltamethrin or Br2A excretion, nor was a case of acute deltamethrin poisoning diagnosed, the non invasive technique used for nerve excitability detection in this study seems to be valuable in studying deltamethrin toxicity on human.

Adolescent↗
PubMed1991

[Significance of electron microscopy in the diagnosis of undifferentiated neoplasms of the nose and nasal sinuses].

Undifferentiated neoplasms of nose and nasal sinuses are very rare. They are very difficult to diagnose by both light and electron microscopies. Twelve cases of undifferentiated neoplasms of nose and nasal sinuses were collected and the morphological features under light and electron microscopes compared histologically. The results showed that correct diagnoses were only obtained in six cases by light microscopy. The other six cases were diagnosed by electron microscopy as malignant melanomas in two cases, leiomyosarcoma in one case, olfactory neuroblastomas in two cases and malignant fibrous histiocytoma in one case. It showed that a correct diagnosis for undifferentiated neoplasm of nose and nasal sinuses was impossible to obtain by light microscopy only. Poorly differentiated olfactory neuroblastoma was also difficult to diagnose under electron microscope because the neurosecretory cytoplasmic granules were not easy to find and several hours would be required to search for them under electron microscope.

Adolescent↗