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21.
Summary Sodium-calcium exchange has been suggested to play a pivotal role in the regulation of cytosolic free calcium (Ca
f
) by epithelial cells. Using isolated epithelial cells from the toad urinary bladder, Ca
f
has been measured using the intracellular Casensitive fluorescent dyes Fura 2 and Quin. 2. Dye loading did not alter cell viability as assessed by measurements of ATP and ADP content or cell oxygen consumption. When basal Ca
f
was examined over a wide range of cell dye content (from 0.04 to 180 nmol dye/mg protein) an inverse relationship was observed. At low dye content, Ca
f
was 300–380 nM and, as dye content was increased, Ca
f
progressively fell to 60 nM. Using low dye content cells, in which minimal alteration in Ca steady state would be expected, the role for plasma membrane Na–Ca exchange was examined using either medium sodium substitution or ouabain. While medium sodium substitution increased Ca
f
, prolonged treatment with ouabain had no effect on Ca
f
despite a clear increase in cell sodium content. The lack of effect of ouabain suggests that Na–Ca exchange-mediated Ca efflux plays a minimal role in the regulation of basal Ca
f
. However, exchange-mediated Ca efflux may play a role in Ca
f
regulation when cytosolic calcium is elevated. 相似文献
22.
d-Aspartate in Human Brain 总被引:3,自引:3,他引:0
Eugene H. Man George H. Fisher Iris L. Payan Rodolfo Cadilla-Perezrios Nancy M. Garcia Radhika Chemburkar Georgine Arends William H. Frey II 《Journal of neurochemistry》1987,48(2):510-515
The presence of the biologically uncommon D-aspartic acid (D-aspartate) in human brain white matter has been previously reported. The earlier study has now been expanded to include D/L-aspartate ratios from 67 normal brains. The data show that the D-aspartate content increases rapidly from 1 year to approximately 35 years of age, levels off in middle age, and then appears to decrease somewhat. The D-aspartate content in gray matter remains at a consistently low level (half of that found in white matter) throughout the human life span. Within the limitations of current analytical methods, there was no detectable difference in D/L-aspartate ratios in white and gray matter of brains with Alzheimer's disease and several other pathologies when compared with brains of normal subjects. However, the presence of a significant D-aspartate level in white matter during the adult life span may lead to changes in protein configuration related to dysfunctions associated with the aging brain. 相似文献
23.
An antibody to the receptor for insulin-like growth factor I inhibits the growth of MCF-7 cells in tissue culture 总被引:12,自引:0,他引:12
Q T Rohlik D Adams F C Kull S Jacobs 《Biochemical and biophysical research communications》1987,149(1):276-281
Alpha IR-3, a monoclonal antibody to the insulin-like growth factor I receptor which blocks insulin-like growth factor I binding and inhibits its activity, inhibits the binding of 125I-insulin-like growth factor I to MCF-7 cells (an estrogen dependent human breast carcinoma cell line) with an IC-50 of approximately 100 ng/ml. It also inhibits the growth of MCF-7 cells cultured in 5% calf serum with approximately the same IC-50. Inhibition of growth occurs both when cells are cultured in the presence and absence of estrogen and is more pronounced when cells are grown at a low density. These findings demonstrate a requirement for insulin-like growth factor I for optimal growth of MCF-7 cells and suggest that it is an autocrine growth factor in these cells. 相似文献
24.
Water proton relaxation rates of various complexes of cholesterol side chain cleavage cytochrome P-450 (-450scc) were investigated to gain information about the structure and dynamics of the steroid binding site. In all cases bulk water protons were found to be in rapid exchange with protons near the paramagnetic Fe3+ center, and the long electron spin relaxation time of the heme iron, tau s approximately 0.3 ns, resulted in fast relaxation rates. For the steroid-free enzyme, the closest approach of exchangeable protons is approximately 2.5 A, a distance consistent with a water molecule binding directly to the heme iron or rapidly exchanging with a coordinated ligand. When cholesterol was bound, the distance increased to approximately 4 A, indicative of displacement of water from the immediate coordination sphere of the heme but still in close proximity to the active site. For the complex with (22R)-22-hydroxycholesterol, a distance of approximately 2.7 A is observed, suggesting a reorganization of the active site when this intermediate is formed from cholesterol. Complexes of P-450scc with the competitive inhibitors (22R)-22-aminocholesterol, 22-amino-23,24-bisnor-5-cholen-3 beta-ol, or (20R)-20-phenyl-5-pregnene-3 beta,20-diol, also yielded distances of approximately 2.5 A and reveal no effect of side chain size on access of protons to the heme. In the nitrogen-coordinated amino-steroid complexes, the distances observed indicate solvent proton exchange with the heme-bound nitrogen ligand.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
25.
26.
Oxidation of protoporphyrinogen to protoporphyrin, a step in chlorophyll and haem biosynthesis. Purification and partial characterization of the enzyme from barley organelles. 总被引:5,自引:1,他引:4 下载免费PDF全文
The protoporphyrinogen-oxidizing enzyme from Triton X-100 extracts of the mitochondrial and etioplast fractions of etiolated barley was purified by using ion-exchange and hydroxyapatite chromatography. The purified enzyme from both organelle fractions exhibited a Km of 5 microM and was labile to mild heat and acidification. The pH optimum (5-6) and the substrate-specificity (mesoporphyrinogen was oxidized as rapidly as protoporphyrinogen) revealed properties very different from the protoporphyrinogen-oxidizing enzyme of rat liver or yeast mitochondria, which is specific for protoporphyrinogen as substrate. The purest fractions showed a polypeptide band corresponding to an Mr of approx. 36,000 on SDS/polyacrylamide-gel electrophoresis. This is the first purification and characterization of the enzyme from a plant, and indicates no readily detectable differences between the enzyme isolated from mitochondrial or etioplast fractions, although only the latter organelle has the capacity for both haem and chlorophyll synthesis. 相似文献
27.
Several nodulins of soybean share structural domains but differ in their subcellular locations. 总被引:10,自引:2,他引:8 下载免费PDF全文
Four soybean cDNA nodule-specific clones encoding nodulin-23, -26b, -27 and -44 were observed to cross-hybridize under low stringency conditions. Nucleotide sequence analysis revealed that the cDNAs contain three distinct domains: two domains with 70 to 95% homology separated by a third domain unique to each cDNA. Despite a number of nucleotide insertions and deletions, the protein sequences are conserved in the two domains which correlate with the homologous nucleotide domains. The amino terminal domain of each nodulin contains putative signal sequences for membrane translocation, although only two (nodulin-23 and -44) meet all the criteria for a functional signal. Immuno-precipitation of hybrid-release translation products of the four cDNAs revealed that nodulin-23 is associated with the peribacteroid membrane while nodulin-27 is in the cytoplasmic fraction of the nodule. These four nodulins are members of a diverse family with conserved structural features and the genes encoding them appear to have recently evolved from a common ancestor. 相似文献
28.
Drosophila neuroglian: a member of the immunoglobulin superfamily with extensive homology to the vertebrate neural adhesion molecule L1 总被引:30,自引:0,他引:30
A J Bieber P M Snow M Hortsch N H Patel J R Jacobs Z R Traquina J Schilling C S Goodman 《Cell》1989,59(3):447-460
Drosophila neuroglian is an integral membrane glycoprotein that is expressed on a variety of cell types in the Drosophila embryo, including expression on a large subset of glial and neuronal cell bodies in the central and peripheral nervous systems and on the fasciculating axons that extend along them. Neuroglian cDNA clones were isolated by expression cloning. cDNA sequence analysis reveals that neuroglian is a member of the immunoglobulin superfamily. The extracellular portion of the protein consists of six immunoglobulin C2-type domains followed by five fibronectin type III domains. Neuroglian is closely related to the immunoglobulin-like vertebrate neural adhesion molecules and, among them, shows most extensive homology to mouse L1. Its homology to L1 and its embryonic localization suggest that neuroglian may play a role in neural and glial cell adhesion in the developing Drosophila embryo. We report here on the identification of a lethal mutation in the neuroglian gene. 相似文献
29.
R M McMillan D J Masters V C Vickers M P Dicken V N Jacobs 《Biochimica et biophysica acta》1989,1005(2):170-176
Several alternative fatty acid substrates have been employed to characterise the kinetics of rat basophilic leukaemia cell (RBL-1) 5-lipoxygenase. Using arachidonic acid (AA) as substrate, enzymes rates declined at high substrate concentrations (greater than 25 microM) and were associated with pronounced lag phases. The concentrations of AA at which apparent substrate inhibition and lag phases were observed were comparable with those at which AA induced emulsion formation in aqueous media. No evidence for substrate inhibition or lag phases was observed using eicosapentaenoic acid (EPA), a more soluble substrate which did not induce emulsion formation at concentrations up to 100 microM. Reactions catalysed by RBL-1 5-lipoxygenase terminated before exhaustion of substrate. AA and EPA induced time-dependent enzyme inactivation at concentrations 100-fold lower than their apparent Km values for the enzyme. The ability of several fatty acids to induce time-dependent inactivation was directly proportional to their substrate potency. We conclude that apparent substrate inhibition is a consequence of a change from monomeric to micellar substrate which has a lower affinity for the enzyme and that premature termination of the enzyme reactions is a consequence of product-induced enzyme inactivation. 相似文献
30.
Porcine D-amino acid oxidase: production of the biologically active enzyme in Escherichia coli 总被引:4,自引:0,他引:4
E Ciccarelli M Massaer J P Guillaume A Herzog R Loriau A Cravador P Jacobs A Bollen 《Biochemical and biophysical research communications》1989,161(2):865-872
DNA molecules coding either for mature porcine D-amino acid oxidase or for truncated forms of the enzyme have been obtained by stepwise addition of synthetic oligonucleotides to a partial cDNA. Under the control of the lambda PL thermoregulatable promoter, these DNAs were respectively expressed in Escherichia coli as 36, 28 and 25 kilodalton polypeptides, specifically recognised by antibodies raised against the natural enzyme. None of the truncated proteins were biologically active whereas the mature recombinant species was able to hydrolyze D-alanine in vitro as efficiently as the natural product. 相似文献