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排序方式: 共有885条查询结果,搜索用时 15 毫秒
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The uptake of cholesterol from high-density lipoproteins (HDL) labeled with 125I and [3H]cholesterol was examined in cultured rat luteal cells. Luteal cells were incubated with labeled HDL, following which the metabolic fate of the apolipoproteins and cholesterol moieties of the receptor-bound HDL were examined. About 50% of the originally bound HDL apolipoproteins were released into the medium in 24 h by a temperature-dependent process while only 5% of the HDL cholesterol was released unmetabolized. Inclusion of unlabeled HDL in the chase incubation resulted in increased release of apolipoprotein-derived radioactive products without significant change in the release of unmetabolized cholesterol. 60% of the apolipoprotein-derived radioactivity could be precipitated with trichloroacetic acid; the remaining trichloroacetic acid-soluble radioactive fraction was identified as [125I]iodotyrosine. Gel filtration chromatography of the chase-released material showed that the trichloroacetic acid-precipitable products, which contained no detectable amounts of cholesterol, eluted over a range of molecular sizes (9-80 kDa). No intact HDL was retroendocytosed. About 80% of trichloroacetic acid-precipitable products could be immunoadsorbed on anti-apolipoprotein A-I antibody immobilized on CNBr-activated Sepharose, suggesting the presence of fragments containing apolipoprotein A-I. This material was also capable of reassociating with native HDL. Lysosomal inhibitors were partially effective in inhibiting the amount of trichloroacetic acid-soluble products formed. The lysosomal degradation appeared to have no role in the uptake of HDL-derived cholesterol. These studies demonstrate preferential and total uptake of HDL cholesterol by luteal cells, with concomitant degradation of the lipoprotein. 相似文献
4.
The mechanism of ‘down regulation’ of luteinizing hormone receptors was investigated in pseudopregnant rats using a modified
radioimmunoassay capable of measuring endogenous tissue-bound hormone. Treatment of pseudopregnant animals with a desensitizing
dose (desensitization treatment) of human chorionic gonadotropin resulted in a decrease in receptor concentration. This decrease
was prevented if the animals were treated prior to the desensitization treatment with indomethacin, an inhibitor of prostaglandin
biosynthesis, suggesting a role for prostaglandins in down regulation. The desensitization treatment resulted in a time-dependent
decrease in subsequent responsiveness of the tissue to luteinizing hormone. Basal progesterone production rate was also decreased
following desensitization. Total tissue cholesterol was found to be decreased following desensitization treatment, without
any change in the ratio of free to esterified cholesterol. Mitochondrial cholesterol was significantly reduced and pregnenolone
production by the mitochondria of desensitized corpora lutea was also markedly reduced. However, when cholesterol was added
to the mitochondria of desensitized corpora lutea, pregnenolone production was increased, reaching values almost equal to
that shown by the control mitochondria. These results show that decrease in the responsiveness following desensitization treatment
is due to, besides receptor loss, decrease in tissue cholesterol, in particular mitochondrial cholesterol. The cholesterol
side chain cleavage activity, although low, appears to be functionally intact; the low activity could be attributed to low
levels of mitochondrial cholesterol. 相似文献
5.
Stable transfection of calbindin-D28k into the GH3 cell line alters calcium currents and intracellular calcium homeostasis. 总被引:10,自引:0,他引:10
Previous work demonstrating the presence and differential distribution of Ca(2+)-binding proteins in the CNS has led to the proposal that cytosolic proteins, such as calbindin-D28k (CB), may play a pivotal role in neurons. We have used a retrovirus containing the full-length cDNA for CB to transfect the pituitary tumor cell line GH3, to generate CB-expressing GH3 cells and to investigate whether ionic channel activities as well as the concentration of intracellular free Ca2+ ([Ca2+]i) homeostasis could be altered by the presence of this Ca(2+)-binding protein. We show that CB-transfected GH3 cells exhibited lower Ca2+ entry through voltage-dependent Ca2+ channels and were better able to reduce [Ca2+]i transients evoked by voltage depolarizations than the wild-type parent cell line. These observations provide a mechanism by which CB may protect tissues against Ca(2+)-mediated excitotoxicity. 相似文献
6.
Molecular basis of organ-specific selection of viral variants during chronic infection. 总被引:19,自引:16,他引:3
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![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
R Ahmed C S Hahn T Somasundaram L Villarete M Matloubian J H Strauss 《Journal of virology》1991,65(8):4242-4247
Viral variants of different phenotypes are present in the central nervous system (CNS) and lymphoid tissues of carrier mice infected at birth with the Armstrong strain of lymphocytic choriomeningitis virus. The CNS isolates are similar to the parental virus and cause acute infections in adult mice, whereas the lymphoid isolates cause chronic infections associated with suppressed T-cell responses. In this study, we provide a molecular basis for this organ-specific selection and identify a single amino acid change in the viral glycoprotein that correlates with the tissue specific selection and the persistent and immunosuppressive phenotype of the variants. This phenylalanine (F)-to-leucine (L) change at position 260 of the viral glycoprotein was seen in the vast majority (43 of 47) of the lymphoid isolates, and variants with L at this residue were selected in spleens of persistently infected mice. In striking contrast, isolates with the parental sequence (F at residue 260) predominated (48 of 59 isolates) in the CNS of the same carrier mice. Complete nucleotide sequence analysis of the major structural genes of several independently derived (from different mice) spleen isolates showed that these variants were greater than 99.8% identical to the parental virus. In fact, the only common change among these spleen isolates was the F----L mutation at residue 260 of the glycoprotein. These results show that an RNA virus can exhibit minimal genetic drift during chronic infection in its natural host, and yet a single or few mutations can result in the organ-specific selection of variants that are markedly different from the parental virus. 相似文献
7.
Measurements were made of the water proton relaxation rate (T?12 = R2), electron spin resonance (ESR) six-line signal of ‘free’ Mn2+, and O2-evolution activity in thylakoid membranes from pea leaves. The main results are: (1) Aging of thylakoids at 35°C causes a parallel decrease in O2-evolution activity, in R2 and in the content of bound Mn, suggesting that R2 may be related to the loosely bound Mn involved in O2 evolution. (2) Treatment of thylakoids with tetraphenylboron (TPB) at [TPB] > 2 mM produces a 2-fold increase in R2, without release of Mn2+. The titration curve exhibits three sharp end points. The first end point occurs at a of 1.25, at which the O2 evolution is completely inhibited. (3) Treatment of thylakoids with NH2OH also increases R2 by nearly 2-fold, either by the reduction of the higher oxidation states of Mn to Mn2+ and / or by exposing the Mn to solvent protons. Also, progressive release of bound Mn occurs at [NH2OH] ≥ 1 mM as shown by an increase increase in the Mn2+ ESR signal and a decrease in R2. (4) Addition of H2O2 (0.1–1.0%) to thylakoids causes an enhancement of R2 similar to that by NH2OH, but without the release of Mn2+. (5) Heat treatment of thylakoids at 40–50°C releases Mn2+ and increases R2. Conversely, pH values of 7 to 4 release Mn2+ without changing R2 while pH values of 7–9 increase R2 without releasing Mn2+. Thus, both high and low pH values as well as the heat treatment cause structural changes enhancing the relaxivity of the bound Mn or of other paramagnetic species. 相似文献
8.
9.
Calcium ion binding to delta- and to beta-crystallins. The presence of the "EF-hand" motif in delta-crystallin that aids in calcium ion binding 总被引:1,自引:0,他引:1
Y Sharma C M Rao M L Narasu S C Rao T Somasundaram A Gopalakrishna D Balasubramanian 《The Journal of biological chemistry》1989,264(22):12794-12799
Abnormal levels of endogenous calcium ions are known to induce eye lens opacity, and a variety of causative factors has been proposed, including calcium-mediated aggregation and precipitation of the lens proteins crystallins. We have specifically looked in some detail at the interaction of Ca2+ with various crystallins and its consequences. Lenses incubated in solutions containing 10 mM Ca2+ or 5 mM Tb3+ opacified. Fluorescence titration of crystallins with TbCl3 revealed that this ion binds to delta- and beta-crystallins in solution. Equilibrium dialysis showed that four Ca2+ ions bind to one delta-crystallin tetramer with an affinity of 4.3 x 10(3) M-1. Analysis of the amino acid sequence of delta-crystallin reveals the presence of a calmodulin-type "helix-loop-helix" or "EF-hand" calcium ion binding conformational motif in the region comprising residues 300-350. This is a novel feature of the molecule not reported so far. No other crystallins appear to have this motif. beta-Crystallin also binds four Ca2+ ions/aggregate unit of mass 160 kDa, with an affinity of 2.6 x 10(3) M-1, presumably in the midregion of the molecule that is rich in anionic and polar residues. Circular dichroism spectroscopy shows that the binding of calcium ion leads to subtle conformational changes in the molecules, notably in the tertiary structure. 相似文献
10.
Reduction of Selenate and Selenite to Elemental Selenium by a Pseudomonas stutzeri Isolate 总被引:2,自引:1,他引:1
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![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
L. Lortie W. D. Gould S. Rajan R. G. L. McCready K.-J. Cheng 《Applied microbiology》1992,58(12):4042-4044
A Pseudomonas stutzeri isolate rapidly reduced both selenite and selenate ions to elemental selenium at initial concentrations of both anions of up to 48.1 mM. Optimal selenium reduction occurred under aerobic conditions between pH 7.0 and 9.0 and at temperatures of 25 to 35°C. Reduction of both selenite and selenate was unaffected by a number of anions except for sulfite, chromate, and tungstate ions, which inhibited both growth and reduction. 相似文献