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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
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cDNA Clones for Corn Leaf NADH:Nitrate Reductase and Chloroplast NAD(P):Glyceraldehyde-3-Phosphate Dehydrogenase : Characterization of the Clones and Analysis of the Expression of the Genes in Leaves as Influenced by Nitrate in the Light and Dark 总被引:6,自引:3,他引:3 下载免费PDF全文
cDNA clones were selected from a corn (Zea mays L.) leaf lambda gt11 expression library using polyclonal antibodies for corn leaf NADH:nitrate reductase. One clone, Zmnrl, had a 2.1 kilobase insert, which hybridized to a 3.2 kilobase mRNA. The deduced amino acid sequence of Zmnrl was nearly identical to peptide sequences of corn leaf NADH:nitrate reductase. Another clone, Zm6, had an insert of 1.4 kilobase, which hybridized to a 1.4 kilobase mRNA, and its sequence coded for chloroplastic NAD(P)+:glyceraldehyde-3-phosphate dehydrogenase based on comparisons to sequences of this enzyme from tobacco and corn. When nitrate was supplied to N-starved, etiolated corn plants, nitrate reductase, and glyceraldehyde-3-phosphate dehydrogenase mRNA levels in leaves increased in parallel. When green leaves were treated with nitrate, only nitrate reductase mRNA levels were increased. Nitrate is a specific inducer of nitrate reductase in green leaves, but appears to have a more general effect in etiolated leaves. In the dark, nitrate induced nitrate reductase expression in both etiolated and green leaves, indicating light and functional chloroplast were not required for enzyme expression. 相似文献
5.
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
6.
Phytochrome-mediated light regulation of nitrate reductase expression in squash cotyledons 总被引:12,自引:3,他引:9 下载免费PDF全文
In etiolated squash (Cucurbita maxima L.) cotyledons, nitrate-inducible NADH:nitrate reductase activity and protein were increased in darkness by red light pulses with red/far-red photoreversibility. Continuous far-red light also led to increased levels of nitrate reductase activity and protein. Poly(A)+RNA, which hybridizes to squash nitrate reductase cDNA, was also increased by light treatments. Thus, we found that after nitrate triggering, nitrate reductase expression appears to be regulated by light via phytochrome. 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
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Codon usage in higher plants, green algae, and cyanobacteria 总被引:3,自引:1,他引:2
Codon usage is the selective and nonrandom use of synonymous codons by an organism to encode the amino acids in the genes for its proteins. During the last few years, a large number of plant genes have been cloned and sequenced, which now permits a meaningful comparison of codon usage in higher plants, algae, and cyanobacteria. For the nuclear and organellar genes of these organisms, a small set of preferred codons are used for encoding proteins. Codon usage is different for each genome type with the variation mainly occurring in choices between codons ending in cytidine (C) or guanosine (G) versus those ending in adenosine (A) or uridine (U). For organellar genomes, chloroplastic and mitochrondrial proteins are encoded mainly with codons ending in A or U. In most cyanobacteria and the nuclei of green algae, proteins are encoded preferentially with codons ending in C or G. Although only a few nuclear genes of higher plants have been sequenced, a clear distinction between Magnoliopsida (dicot) and Liliopsida (monocot) codon usage is evident. Dicot genes use a set of 44 preferred codons with a slight preference for codons ending in A or U. Monocot codon usage is more restricted with an average of 38 codons preferred, which are predominantly those ending in C or G. But two classes of genes can be recognized in monocots. One set of monocot genes uses codons similar to those in dicots, while the other genes are highly biased toward codons ending in C or G with a pattern similar to nuclear genes of green algae. Codon usage is discussed in relation to evolution of plants and prospects for intergenic transfer of particular genes. 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 下载免费PDF全文
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