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41.
Sprague-Dawley albino rats or Long-Evans pigmented rats were exposed during the dark phase of the daily light:dark cycle to various intensities of a sunlight-stimulating white fluorescent light (0.022, 0.044, 0.110, 0.220, 0.440 or 2.200 μW/cm2) for 30 min; pineal glands and trunk blood samples were then collected and assayed for melatonin by radioimmunoassay. Albino rats exposed to irradiances of 0.110 μW/cm2 or less had pineal melatonin levels that were not significantly different from those of unexposed animals; higher irradiances significantly (P < 0.001) reduced melatonin levels. In contrast, as little as 0.022 μW/cm2 significantly (P < 0.02) reduced pineal and serum melatonin levels in the pigmented rats. These results suggest that something other than the simple presence or absence of eye pigmentation is the critical factor in determining the sensitivity of the rat's pineal to retinal-mediated photic suppression of melatonin synthesis. 相似文献
42.
Abnormal high density lipoproteins from patients with liver disease regulate cholesterol metabolism in cultured human skin fibroblasts 总被引:2,自引:0,他引:2
J S Owen H Goodall P Mistry D S Harry R C Day N McIntyre 《Journal of lipid research》1984,25(9):919-931
Apolipoprotein B (apoB) of plasma low density lipoproteins (LDL) binds to high affinity receptors on many cell types. A minor subclass of high density lipoproteins (HDL), termed HDL1, which contains apoE but lacks apoB, binds to the same receptor. Bound lipoproteins are engulfed, degraded, and regulate intracellular cholesterol metabolism and receptor activity. The HDL of many patients with liver disease is rich in apoE. We tested the hypothesis that such patient HDL would reduce LDL binding and would themselves regulate cellular cholesterol metabolism. Normal HDL had little effect on binding, uptake, and degradation of 125I-labeled LDL by cultured human skin fibroblasts. Patient HDL (d 1.063-1.21 g/ml) inhibited these processes, and in 15 of the 25 samples studied there was more than 50% inhibition at 125I-labeled LDL and HDL protein concentrations of 10 micrograms/ml and 25 micrograms/ml, respectively. There was a significant negative correlation between the percentage of 125I-labeled LDL bound and the apoE content of the competing HDL (r = -0.54, P less than 0.01). Patient 125I-labeled HDL was also taken up and degraded by the fibroblasts, apparently through the LDL-receptor pathway, stimulated cellular cholesterol esterification, increased cell cholesteryl ester content, and suppressed cholesterol synthesis and receptor activity. We conclude that LDL catabolism by the receptor-mediated pathway may be impaired in liver disease and that patient HDL may deliver cholesterol to cells. 相似文献
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Cor Dijkema Robert Pels Rijcken Harry C.M. Kester Jaap Visser 《FEMS microbiology letters》1986,33(1):125-131
Abstract The composition of the polyol pools in Aspergillus nidulans mycelium during active growth on sucrose depends strongly on pH. At pH 2.5, only mannitol is present. A comparison between nitrate- and ammonium-grown cultures shows stimulation of the arabitol content with nitrate a former nitrogen source. When starved mycelium is incubated either with natural-abundance or 13 C-enriched glucose, label appears rapidly in mannitol and arabitol, regardless of the nitrogen source or the pH used. 相似文献
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In an attempt to probe the topography of 5 S, 16 S and 23 S RNAs in a functionally engaged ribosome, polysomes were probed using the structure-sensitive, guanine-specifie reagent kethoxal. Reactivities of guanine residues at 38 specific ribosomal RNA sites in polysomes were compared with their corresponding reactivities in vacant 70 S ribosomes. No polysome-specific protection was seen for 5 S RNA. In 16 S RNA, positions 530, 693 or 1079, 966, 1338 and 1517 showed protection in polysomes; all of these sites have highly conserved primary and secondary structures, and include several methylated nucleotides. In 23 S RNA, polysome protection is seen at positions 277, 1071, 1475 or 2112, 2116 and 2751. We attribute polysome-specific protection either to direct contact of transfer RNA and/or messenger RNA with the protected sites or to tRNA and/or mRNA-induced changes in ribosome conformation involving the protected sites. 相似文献
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Cholesterol biosynthesis in transplantable hepatomas: evidence for impairment of uptake and storage of dietary cholesterol 总被引:3,自引:0,他引:3
Cholesterol feeding inhibits cholesterol biosynthesis in normal but not in malignant liver tissue. It has been postulated that hepatomas have suffered a specific intracellular deletion of the cholesterol feedback control mechanism, but there is little direct evidence to support this hypothesis. Rats bearing Morris transplantable hepatomas were fed high cholesterol diets for periods of up to 21 days. Cholesterol biosynthesis, as expected, was suppressed in the normal liver but not in hepatomas. The livers accumulated large amounts of cholesteryl ester but the hepatomas showed little or no increase in ester content. Cholesterol-1alpha-(3)H was administered intragastrically to other tumor-bearing rats. Uptake of radioactivity by the tumors was much slower than by normal liver. Comparison of the specific activities of liver and tumor cholesterol with that of the plasma suggested that the liver took up dietary cholesterol selectively from the blood, while the appearance of radioactivity in the tumors could be explained by slow equilibration with plasma cholesterol. Our results suggest that the insensitivity of cholesterol biosynthesis to dietary cholesterol in hepatomas could be explained by an impairment in the uptake and storage of dietary cholesterol and that the concept of an intracellular deletion of the feedback mechanism requires further evidence. 相似文献
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Pamela J. Letts Les Pinteric Harry Schachter 《Biochimica et Biophysica Acta (BBA)/General Subjects》1974,372(2):304-320
Golgi fractions prepared from rat testis have been shown to be enriched in the following glycoprotein glycosyltransferases: N-acetylglucosaminyltransferase, 47-fold, galactosyltransferase, 33-fold, and N-acetylglucosaminide fucosyltransferase, 15-fold. Appreciably lower transferase levels were obtained in other subcellular fractions. In the mouse, Golgi fractions were prepared from testis homogenates, testis cell suspensions and partially purified testis germinal cells; these fractions were also enriched in the above glycoprotein glycosyltransferases. Electron microscopic analysis indicated that a major portion of the total transferase activity was located in the Golgi apparatus of both rat and mouse testis although these experiments could not rule out the possible presence of some transferase activity in other organelles. 相似文献