Search PubMed⌕ Search

Biomedical subjects

K Zhang

Publications and source records attributed to K Zhang.

At least 379 records · Page 21Linked to original sources
PubMed1994

[Polarographic determination of N,N'-methylene-bis(2-amino-1,3,4-thiadiazole) in water].

The polarographic behavior of the product of N,N'-methylene-bis(2-amino-1,3,4-thiadiazole) (I) with nitrite and N-(1-naphthyl)-ethylendiamine is reported. The effects of acidity and reagents on the polarography was studied, and a polarographic method was established for the determination of (I). The relative standard deviation was 2.2-8.4%, and the recovery in the range of 80-103%. The proposed method was sensitive, accurate, reproducible and suitable for the determination of microamount of (I) in water.

Insecticides↗
PubMed1993

XAFS studies of carboxypeptidase A: detection of a structural alteration in the zinc coordination sphere coupled to the catalytically important alkaline pKa.

X-ray absorption fine structure (XAFS) spectra of carboxypeptidase A (ZnCPD) show progressive spectral changes particularly in the near edge region when the pH is changed from neutral to alkaline values. Both least square fitting and radial distribution function (RDF) data analysis yield two distributions of atoms in the first coordination shell of ZnCPD at all pH values and at both 150 and 297 K. Direct comparison of the first and higher coordination shells of ZnCPD reveals structural differences between pH 7.0 and pH 9.9. At pH 7.0, the zinc ion has four ligand atoms (N or O) at an average distance of 2.024 +/- 0.006 A, and a smaller distribution of 1.3 atoms (N or O) at 2.54 +/- 0.05 A from the zinc ion (from the least square fitting analysis). At pH 9.9, the larger distribution contains four atoms at a 0.022 A shorter distance (2.002 A) from the zinc, while the smaller distribution contains 0.7 atoms at 2.52 +/- 0.06 A. The smaller distribution can be attributed mainly to the contribution of the epsilon 2-oxygen of Glu 72 and to the atoms farther away for which the contribution cannot be fully separated from the first shell peak. The structural changes of ZnCPD at intermediate pHs are consistent with the changes observed at pH 7.0 and pH 9.9. The XAFS Debye-Waller factor shows an increased structural disorder for the four atom distribution at the alkaline pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Mutational analysis of a ribonuclease III processing signal.

A mutational approach was employed to identify sequence and structural elements in a ribonuclease III processing signal that are important for in vitro enzymatic cleavage reactivity and selectivity. The substrate analyzed was the bacteriophage T7 R1.1 processing signal, a 60 nucleotide irregular RNA hairpin exhibiting an upper and lower dsRNA stem, separated by an asymmetric internal loop which contains the scissile phosphodiester bond. Altering the length of either the upper or lower dsRNA segment in R1.1 RNA dose not change the site of RNase III cleavage. However, decreasing the size of either the upper or lower dsRNA segment causes a progressive inhibition of processing reactivity. Omitting monovalent salt from the reaction buffer promotes cleavage of otherwise unreactive R1.1 deletion mutants. Accurate processing is maintained with R1.1 variants containing specific point mutations, designed to disrupt Watson-Crick (WC) base-pairing in a conserved sequence element within the upper dsRNA stem. The internal loop is not required for processing reactivity, as RNase III can accurately and efficiently cleave R1.1 variants in which this structure is WC base-paired. Moreover, an additional cleavage site is utilized in these variants, which occurs opposite the canonical site, and is offset by two nucleotides. The fully base-paired R1.1 variants form a stable complex with RNase III in Mg(2+)-free buffer, which can be detected by a gel electrophoretic mobility shift assay. In contrast, the complex of wild-type R1.1 RNA with RNase III is unstable during nondenaturing gel electrophoresis. Thus, a functional role of the T7 R1.1 internal loop is to enforce single enzymatic cleavage, which occurs at the expense of RNase III binding affinity.

Bacteriophage T7↗
PubMed1993

Alpha 6 integrin is up-regulated in step increments accompanying neoplastic transformation and tumorigenic conversion of human fibroblasts.

Integrins are a family of transmembrane glycoproteins that serve as cell-cell and cell-substratum adhesion molecules and help regulate cellular differentiation and proliferation. In malignant cells, which exhibit abnormal differentiation and growth properties, the expression of an altered integrin repertoire could therefore be expected. From a tumorigenic human fibrosarcoma cell line we isolated a unique complementary DNA corresponding to the alpha 6 integrin subunit. Northern blot analysis using this complementary DNA as probe indicated that alpha 6 integrin mRNA was abundantly expressed in all neoplastically transformed fibroblast cell lines but not in normal diploid fibroblasts. In addition to its potential as a marker for the neoplastic transformation of human fibroblasts, the alpha 6 integrin mRNA was also found to be consistently expressed at higher levels in tumorigenic fibroblasts than in immortalized but nontumorigenic fibroblasts. This differential expression of alpha 6 integrin was reflected at the cell surface protein level using cytofluorometric analysis with specific monoclonal antibody. In contrast, the levels of cell surface expression of other integrins were unchanged (such as alpha 3 and beta 1) or down-regulated (such as alpha 5) when transformed cells were compared with normal fibroblasts. The incremental up-regulation of alpha 6 integrin was selective and paralleled the progression of normal cells to immortalized cells and finally to tumorigenic cells. This elevated alpha 6 subunit associated with the beta 1 subunit to form a heterodimer receptor for laminin. Since fibrosarcoma cell invasion of basement membrane has been shown to involve alpha 6 beta 1 integrin, then the induction or up-regulation of alpha 6 expression is an important step in tumor progression and evolution to the invasive phenotype in fibrosarcoma.

Animals↗
PubMed1993

Inhibition by the serotonin1A agonist, 8-hydroxy-2- (di-n-propylamino)tetralin, of antidromically identified paraventricular nucleus neurons in the rat.

Experiments were performed using urethane-anesthetized rats to delineate the role of serotonin (5-hydroxytryptamine, 5-HT) receptors of the 5-HT1A subtype in the neural regulation of hypothalamo-pituitary-adrenocortical (HPA) secretion. The activity of single paraventricular nucleus (PVN) neurons antidromically identified as projecting to the median eminence was recorded following injection of the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Intraperitoneal administration of increasing doses of 8-OH-DPAT caused dose-dependent decreases in the spontaneous activity of all neurons tested. The data yielded an ED50 value of 0.19 mumol/kg for this effect. The decreases in PVN neuronal activity were accompanied by sustained decreases in mean arterial blood pressure, with an ED50 of 0.18 mumol/kg. These results provide evidence for an inhibitory role of 5-HT1A receptors in the regulation of PVN neuronal activity and support the suggestion that 5-HT may inhibit HPA secretory activity.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed1993

Lectin-like inhibition of immune complex receptor-mediated stimulation of neutrophils. Effects on cytosolic calcium release and superoxide production.

We have tested the role of lectin-like interactions, with particular emphasis on CR3, in insoluble immune complex (IC)-mediated activation of human polymorphonuclear leukocytes (PMN). The ability of IC to trigger intracellular Ca2+ release and O2- production by normal PMN, saccharide-treated cells, and CR3-deficient PMN (leukocyte adhesion deficiency, LAD, patients) were tested. When indo-1-labeled normal PMN were stimulated with IC in Ca(2+)-free buffer, intracellular Ca2+ rose from approximately 100 nM to approximately 230 nM. However, when LAD PMN were tested, a small rise in intracellular Ca2+ was observed. Because previous studies have shown that certain saccharides inhibit CR3-Fc gamma RIII co-capping, we tested a panel of saccharides to determine their ability to influence IC-mediated intracellular Ca2+ release. When normal PMN were exposed to 0.15 M N-acetyl-D-glucosamine (NADG), D-mannose, or alpha-methyl-mannoside, the Ca2+ response to IC was significantly reduced. However, addition of 0.15 M glucose, raffinose, sucrose, galactose, fructose, or sorbitol did not significantly affect the Ca2+ response, suggesting that the response was specific for certain saccharides. To test the physiologic consequences of these Ca2+ signals, we have examined the ability of saccharides to influence O2- production by normal PMN and the ability of LAD PMN to produce O2- upon triggering by IC. Normal PMN stimulated with IC generated 4.3 +/- 1.7 nmol/10(6) cells/30 min of O2-. In contrast, O2- production was inhibited by 0 to 20% by glucose, galactose, sucrose, sorbitol, fructose, and raffinose and > or = 50% by NADG and mannose. LAD PMN, which display diminished Ca2+ signals, were found to produce O2- at 47 +/- 6% of control levels. NADG and mannose dose-response studies indicated that they cooperatively block O2-.

Adult↗
PubMed1993

Ribonuclease III cleavage of a bacteriophage T7 processing signal. Divalent cation specificity, and specific anion effects.

Escherichia coli ribonuclease III, purified to homogeneity from an overexpressing bacterial strain, exhibits a high catalytic efficiency and thermostable processing activity in vitro. The RNase III-catalyzed cleavage of a 47 nucleotide substrate (R1.1 RNA), based on the bacteriophage T7 R1.1 processing signal, follows substrate saturation kinetics, with a Km of 0.26 microM, and kcat of 7.7 min.-1 (37 degrees C, in buffer containing 250 mM potassium glutamate and 10 mM MgCl2). Mn2+ and Co2+ can support the enzymatic cleavage of the R1.1 RNA canonical site, and both metal ions exhibit concentration dependences similar to that of Mg2+. Mn2+ and Co2+ in addition promote enzymatic cleavage of a secondary site in R1.1 RNA, which is proposed to result from the altered hydrolytic activity of the metalloenzyme (RNase III 'star' activity), exhibiting a broadened cleavage specificity. Neither Ca2+ nor Zn2+ support RNase III processing, and Zn2+ moreover inhibits the Mg(2+)-dependent enzymatic reaction without blocking substrate binding. RNase III does not require monovalent salt for processing activity; however, the in vitro reactivity pattern is influenced by the monovalent salt concentration, as well as type of anion. First, R1.1 RNA secondary site cleavage increases as the salt concentration is lowered, perhaps reflecting enhanced enzyme binding to substrate. Second, the substitution of glutamate anion for chloride anion extends the salt concentration range within which efficient processing occurs. Third, fluoride anion inhibits RNase III-catalyzed cleavage, by a mechanism which does not involve inhibition of substrate binding.

Anions↗
PubMed1993

Cell transformation by ras and regulation of its protein product.

We are studying the biological activity and regulation of mammalian Ras protein in tumours and in physiological signalling. We have shown that GAP (the GTPase-activating protein) is a potent negative regulator of normal Ras in cells. Reduction or loss of the NF1 gene product neurofibromin, in association with genetic abnormalities of the NF1 locus, has been identified in schwannoma cell lines from patients with neurofibromatosis and in melanoma and neuroblastoma lines from patients without neurofibromatosis. Although loss of neurofibromin in the schwannoma lines was associated with a high proportion of normal Ras protein in the active GTP-bound state, Ras-GTP appeared to be appropriately regulated in the melanoma and neuroblastoma lines, which contain normal levels of GAP. Therefore the GTPase-activating activity of neurofibromin is not essential for negative regulation of Ras in some cell types and the putative tumour suppressor function of neurofibromin in such cell types is independent of its GTPase-activating activity. Mitogen activation of Ras in fibroblasts is mediated primarily by exchange factors, which probably interact with a region on the Ras protein distinct from the region required for interaction with GAP. Multiple full-length cDNAs have identified a mouse gene whose products are related to yeast CDC25 guanine nucleotide exchange factor.

Animals↗
PubMed1993

In vivo transport and excretion of corrosion products from accelerated anodic corrosion of porous coated F75 alloy.

Corrosion of porous coated cobalt chromium specimens surgically implanted subcutaneously in hamsters was accelerated by application of a fixed anodic potential. Corrosion current measurements were utilized for prediction of metal ion release based on Faraday's law. Daily urine samples were collected pre- and post-stimulation, and, at animal sacrifice, organs and blood samples were taken for chemical analysis. By using in vitro release data from previous studies, and efficiency correction factors, the amount of metal ion release was calculated. The results demonstrated rapid and complete excretion of nickel and molybdenum. Most of the cobalt was excreted, with elevation of cobalt levels in liver, kidney, and lung. Chromium excretion was much lower due to significant red cell binding and in vivo storage, especially in the kidney and spleen.

Animals↗
PubMed1993

Epidermal growth factor (EGF) promotes human keratinocyte locomotion on collagen by increasing the alpha 2 integrin subunit.

The migration of human keratinocytes across the would bed is an early and critical event in the reepithelialization of cutaneous wounds. Epidermal growth factor (EGF) has been shown to accelerate the healing of fresh, split-thickness cutaneous wounds when applied topically. The mechanism(s) by which this accelerated healing occurs remains unknown. Using an assay that directly evaluates human keratinocyte locomotion without confounding the possibility of cell proliferation, we examined the influence of EGF on human keratinocyte motility. Both recombinant epidermal growth factor (rEGF) and transforming growth factor-alpha (TGF-alpha) promoted human keratinocyte locomotion when the cells were apposed to connective tissue matrices of collagen or fibronectin, important components of the wound bed. Other growth factors studied did not enhance keratinocyte migration. Blocking the EGF/TGF-alpha receptor on the cell surface of keratinocytes with specific antibody inhibited the stimulation of keratinocyte locomotion by rEGF and TGF-alpha. Flow cytometry analysis of keratinocytes migrating on type I collagen in the presence of rEGF or TGF-alpha revealed increased expression of the alpha 2 integrin subunit on the keratinocyte surface. The alpha 2 beta 1 integrin mediates keratinocyte migration on collagens type I and IV, and inhibition of migration via antibody blockade of the alpha 2 beta 1 integrin can be partially overcome by increasing the concentration of rEGF present in the medium. Our study demonstrates that the growth-independent stimulation of keratinocyte locomotion via regulation of integrin expression may be one mechanism by which EGF accelerates the reepithelialization of human cutaneous wounds.

Basement Membrane↗
PubMed1993

Enhanced functional expression of transient outward current in hypertrophied feline myocytes.

Cardiac hypertrophy can decrease myocardial contractility and alter the electrophysiological activity of the heart. It is well documented that action potentials recorded from hypertrophied feline ventricular cells can exhibit depressed plateau voltages and prolonged durations. Similar findings have been made by others in rabbit, rat, guinea pig, and human heart. Whole-cell patch voltage-clamp studies designed to explain these changes in the action potential suggest that the only component of the membrane current recorded from feline right ventricular (RV) myocytes found to be substantially different from normal is the 4-amino-pyridine-sensitive transient outward current (I(to)). However, it was not clear if the change in I(to) could explain the changes in the action potential of hypertrophied cardiocytes, nor was it clear if these changes reflect an alteration in the electrophysiological character of the channels underlying I(to). A kinetic comparison of I(to) elicited by hypertrophied RV myocytes with that elicited by comparable normal RV myocytes previously revealed no differences, suggesting that the increased magnitude of the peak I(to) recorded from hypertrophied myocytes arises because the current density increases and not because of any alteration in the kinetic parameters governing the current. This finding suggests that in hypertrophy additional normal channels are expressed rather than a kinetically different channel subtype emerging. Investigations designed to determine if enhancement of I(to) could explain the hypertrophy-induced changes in plateau voltage and action potential duration suggest that a change in I(to) density can indeed explain the entire effect of hypertrophy on RV action potentials. If this notion is correct, the likelihood of "sudden death" in patients with myocardial hypertrophy might be decreased by a blocker selective for cardiac I(to).

Action Potentials↗