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51.
P Nambi J R Peters D R Sibley R J Lefkowitz 《The Journal of biological chemistry》1985,260(4):2165-2171
We have used a recently developed cell-free system (cell lysate) derived from turkey erythrocytes to explore the potential role of cAMP-activated and other protein kinase systems in desensitizing the adenylate cyclase-coupled beta-adrenergic receptor. Desensitization by the agonist isoproterenol required more than simple occupancy of the receptor by the agonist since under conditions where adenylate cyclase was not activated, no desensitization occurred. As in whole cells, addition of cyclic nucleotides to the cell lysate produced only approximately 50% of the maximal isoproterenol-induced desensitization obtainable. Addition of the purified cAMP-dependent protein kinase holoenzyme plus isoproterenol to isolated turkey erythrocyte plasma membranes mimicked the submaximal desensitization induced in lysates by cAMP. This effect was entirely blocked by the specific inhibitor of the cAMP-dependent protein kinase. By contrast, maximal desensitization induced in lysates by isoproterenol was only approximately 50% attenuated by the protein kinase inhibitor. In the lysate preparations, isoproterenol was also shown to induce, in a stereospecific fashion, phosphorylation of the beta-adrenergic receptor. Phosphorylation promoted by isoproterenol was attenuated by cAMP-dependent protein kinase inhibitor to the same extent as desensitization (i.e. approximately 50%). Phorbol diesters also promoted receptor desensitization and phosphorylation in cell lysates. The desensitization was mimicked by incubation of isolated turkey erythrocyte membranes with partially purified preparations of protein kinase C plus phorbol diesters. In the cell lysate, calmodulin also promoted receptor phosphorylation and desensitization which was blocked by EGTA. Desensitization of adenylate cyclase by isoproterenol, phorbol diesters, and calmodulin was not observed to be additive. These findings suggest that: (a) multiple protein kinase systems, including cAMP-dependent, protein kinase C-dependent, and Ca2+/calmodulin-dependent kinases, are capable of regulating beta-adrenergic receptor function via phosphorylation reactions and that (b) cAMP may not be the sole mediator of isoproterenol-induced phosphorylation and desensitization in these cells. 相似文献
52.
53.
Dictyosiphon foeniculaceus from Sweden and Newfoundland was studied in laboratory culture. Zoids from unilocular sporangia developed into dioecious microscopic filamentous gametophytes which produced uniseriate plurilocular gametangia in low temperatures (0 to 8 °C). Zygotes and unfused isogametes gave rise to filamentous protonemata on which parenchymatous macroscopic sporophytes were formed. Isolates from Sweden and Newfoundland were interfertile. Although formed in culture, genetically unisexual sporophytes were not detected in nature. Female gametes ofD. foeniculaceus produced a sexual pheromone. It was identified as finavarrene, which is also known as the sperm attractant inAscophyllum nodosum. 相似文献
54.
Summary The ultrastructure of the midgut epithelium of Phalangium opilio was examined. In the anterior part of the midgut the epithelium consists of three different types of cells, called resorption, digestion, and excretion cells according to their presumed functions. Excretion cells may represent old digestion cells. The relation between resorption and digestion cells needs further investigation. The epithelium of the posterior part of the midgut consists of two types, transport and secretion cells, which seem to serve mainly for the resorption of water and the secretion of peritrophic membranes, respectively.Peritrophic membranes are secreted by the anterior midgut epithelium mainly in a period between 2 and 4 h after feeding. Chitin or chitin precursors could be localized in vesicles and in the brush border of midgut cells, and in the peritrophic membranes, using colloidal gold labelled with wheat germ agglutinin. Two different textures of chitin-containing microfibrils were found in the peritrophic membranes, either a random or a hexagonal texture. The latter results if the microfibrils polymerize between the basal parts of the microvilli. Irregularities of the hexagonal texture can be correlated with an irregular pattern of the microvilli. In the posterior midgut peritrophic membranes with a random texture, chitin-containing microfibrils are continuously secreted in the form of patches. 相似文献
55.
The gizzard of earthworms is definitely of endodermal origin. Contrary to the opinion that tissue of endodermal origin is unable to synthesize chitin, the gizzard epithelium secretes large amounts of chitin-containing material which has special properties. 28.7 +/- 5.7% of the dry weight proved to be chitin; the microfibres form a random felt-like texture. They are not arranged in layers comparable to those found in arthropod cuticle. The protein content amounts to 44.9 +/- 7.1% of the dry weight; the remaining material contains at least uronic acids. In the protein matrix large amounts of the enzymes amylase and protease have been demonstrated. As the so-called cuticle is sloughed off at the lumen side, these enzymes are mingled with the gut contents. It might be called a gastric shield, as it closely resembles this structure, widespread among Mollusca. The thickness of the gizzard cuticle varies between 10 and 90 mum in Lumbricus terrestris and 10-50 mum in L. rubellus. Autoradiography has shown that it is replaced at a high rate. In Lumbricus terrestris it is renewed completely in less than 60 hr, and in smaller Lumbricus rubellus, this takes place in about 48 hr. These results are in agreement with the biochemical findings. 相似文献
56.
Formation of hydroxyl radicals in the presence of ferritin and haemosiderin. Is haemosiderin formation a biological protective mechanism? 总被引:4,自引:0,他引:4 下载免费PDF全文
M O''Connell B Halliwell C P Moorhouse O I Aruoma H Baum T J Peters 《The Biochemical journal》1986,234(3):727-731
Horse spleen and human spleen ferritins increase the formation of hydroxyl radicals (OH) at both pH 4.5 and pH 7.4 in reaction mixtures containing ascorbic acid and H2O2. The generation of OH is inhibited by the chelator desferrioxamine. Human spleen haemosiderin also accelerates OH generation in identical reaction mixtures, but is far less effective (on a unit iron basis) than ferritin under all reaction conditions. It is proposed that conversion of ferritin into haemosiderin in iron overload is biologically advantageous in that it decreases the ability of iron to promote oxygen-radical reactions. 相似文献
57.
High-performance liquid chromatography of uroporphyrinogen and coproporphyrinogen isomers with amperometric detection. 总被引:1,自引:0,他引:1 下载免费PDF全文
A reversed-phase h.p.l.c. system, with an ODS-Hypersil column with acetonitrile or methanol in ammonium acetate buffer as mobile phase, is described for the separation of uro-and copro-porphyrinogen isomers. The porphyrinogens are detected amperometrically with sensitivity comparable with that of the fluorescent detection of porphyrins. The effects of pH, buffer concentration and organic modifiers on retention and resolution were studied. The method is suitable for both analytical and preparative separation of porphyrinogens. 相似文献
58.
59.
R A Jackson P M Blix J A Matthews N Peters T R Pilkington A H Rubenstein J D Nabarro 《Hormones et métabolisme》1983,15(12):585-588
Forearm glucose uptake (FGU) was studied during 100 g oral glucose tolerance tests (GTT) in nonobese, nondiabetic men who had suffered a myocardial infarction (MI) at or before the age of 40, and the results compared with the response in age-matched normal men. In the MI group the rise in both glucose and insulin concentrations after glucose loading was similar to that in normal subjects, although in the former, peak levels tended to be slightly higher. Concomitant FGU, however, was significantly greater in the MI group than in control subjects in the period 0-90 min and in the test as a whole (0-180 min). The results show that at least in some nondiabetics suffering MI at an early age hyperinsulinism is not a feature and peripheral tissue sensitivity is increased. 相似文献
60.
The use of drugs to dissect the pathway for secretion of the glycoprotein hormone chorionic gonadotropin by cultured human trophoblastic cells 总被引:4,自引:0,他引:4
B P Peters M Brooks R J Hartle R F Krzesicki F Perini R W Ruddon 《The Journal of biological chemistry》1983,258(23):14505-14515
Agents that affect intracellular cation and pH gradients and inhibit energy production have been tested for their ability to modulate the processing and secretion of the free alpha subunit and the alpha beta dimer of human chorionic gonadotropin (hCG) by cultured human trophoblastic cells (JAR). Incubation of JAR cells with monensin or nigericin, monovalent cation ionophores that produce equilibration of Na+ and K+ across cellular membranes, dicyclohexylcarbodiimide, an agent that inhibits intracellular membrane ATPases, and methylamine, which neutralizes intracellular pH gradients, produced similar effects on hCG processing and secretion. All these agents inhibited the processing of the asparagine-linked oligosaccharide chains of free alpha subunit and the alpha and beta subunits contained in the hCG dimer. Moreover, after treatment of JAR cells with these agents, there was an intracellular accumulation of precursor forms and an inhibition of secretion of "mature" forms of hCG. Monensin affected the processing and secretion of hCG subunits differently at different concentrations. At 5 X 10(-7) M, monensin inhibited the processing of the asparagine-linked oligosaccharides of hCG without altering the rate-limiting step in the secretory pathway or blocking hCG secretion. The intracellular hCG subunit precursors in both control and monensin-treated cells contained a similar array of high mannose oligosaccharides, predominantly of the Man8GlcNAc2 and Man9GlcNAc2 types. However, monensin-treated cells secreted hCG subunits that contained endo H-sensitive oligosaccharides of the high mannose (mostly Man5GlcNAc2) and hybrid types rather than the endo H-resistant complex chains synthesized by control cells. Nevertheless, a full complement of serine-linked oligosaccharides was added to the hCG-beta subunit in monensin-treated cells. These results indicate that the intracellular movement of hCG from the rough endoplasmic reticulum to the cell surface was not inhibited by monensin at a concentration that impaired Golgi-localized steps in the processing of asparagine-linked oligosaccharides. At 5 X 10(-6) M, monensin significantly inhibited secretion of hCG and created a new rate-limiting step in the processing pathway. hCG subunits bearing Man5GlcNAc2 units accumulated intracellularly, suggesting that the equilibration of intracellular Na+/K+ pools blocked oligosaccharide processing at an intra-Golgi point, perhaps by inhibiting movement of the glycoprotein hormone from the "cis" to the "trans" Golgi compartment. Since the other drugs mentioned above produced similar effects on hCG processing and secretion, it appears that maintenance of intracellular cation and pH gradients is necessary for the intra-Golgi transport of glycoprotein hormones.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献