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191.
192.
Nicotine Enhances the Cyclic AMP-Dependent Protein Kinase-Mediated Phosphorylation of α4 Subunits of Neuronal Nicotinic Receptors 总被引:2,自引:2,他引:0
Abstract: Studies determined whether α4β2 or α3β2 neuronal nicotinic receptors expressed in Xenopus oocytes are substrates for cyclic AMP-dependent protein kinase (PKA) and whether nicotine affects receptor phosphorylation. The cRNAs for the subunits were coinjected into oocytes, and cells were incubated for 24 h in the absence or presence of nicotine (50 n M for α4β2 and 500 n M for α3β2 receptors). Nicotine did not interfere with the isolation of the receptors. When receptors isolated from oocytes expressing α4β2 receptors were incubated with [γ-32 P]ATP and the catalytic subunit of PKA, separated by electrophoresis, and visualized by autoradiography, a labeled phosphoprotein with the predicted molecular size of the α4 subunit was present. Phosphorylation of α4 subunits of α4β2 receptors increased within the first 5 min of incubation with nicotine and persisted for 24 h. In contrast, receptors isolated from oocytes expressing α3β2 receptors did not exhibit a labeled phosphoprotein corresponding to the size of the α3 subunit. Results suggest that the PKA-mediated phosphorylation of α4 and not α3 subunits may explain the differential inactivation by nicotine of these receptors subtypes expressed in oocytes. 相似文献
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Inhibition of 3-Phosphoglycerate-Dependent O(2) Evolution by Phosphoenolpyruvate in C(4) Mesophyll Chloroplasts of Digitaria sanguinalis (L.) Scop 总被引:1,自引:1,他引:0
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The effects of phosphoenolpyruvate (PEP), inorganic phosphate (Pi), and ATP on 3-phosphoglycerate (PGA)-dependent O2 evolution by chloroplasts of Digitaria sanguinalis (L.) Scop. (crabgrass) were evaluated relative to possible mechanisms of PEP transport by the C4 mesophyll chloroplast. Crude and Percoll purified chloroplast preparations exhibited rates of PGA-dependent O2 evolution in the range of 90 to 135 micromoles O2 per milligram chlorophyll per hour, and up to 180 micromoles O2 per milligram chlorophyll per hour at optimal Pi concentrations (approximately 0.2 millimolar at 9 millimolar PGA). Higher concentrations of Pi were inhibitory. PEP inhibited O2 evolution (up to 70%) in both chloroplast preparations when the PEP to PGA ratio was high (i.e. 9 millimolar PEP to 0.36 millimolar PGA). Usually no inhibition was seen when the PEP to PGA ratio was less than 2. PEP acted as a competitive inhibitor and, at a concentration of 9 millimolar, increased the apparent Km (PGA) from 0.15 to 0.53 millimolar in Percoll purified chloroplasts. A low concentration of PGA and high ratio of PEP to PGA, which are considered unphysiological, were required to detect any inhibition of O2 evolution by PEP. Similar results were obtained from crude versus Percoll purified preparations. Neither the addition of Pi nor ATP could overcome PEP inhibition. As PEP inhibition was competitive with respect to PGA concentration, and as addition of ATP or Pi could not prevent PEP inhibition of PGA-dependent O2 evolution, the inhibition was not due to PEP exchange of adenylates or Pi out of the chloroplast. Analysis of the effect of Pi and PEP, separately and in combination, on PGA-dependent O2 evolution suggests interactions between PEP, Pi, and PGA on the same translocator in the C4 mesophyll chloroplast. C3 spinach chloroplasts were also found to be sensitive to PEP, but to a lesser extent than crabgrass chloroplasts. The apparent Ki values (PEP) were 3 and 21 millimolar for crabgrass and spinach, respectively. 相似文献
197.
D. Przybyl E. Fritzsche K. Edwards H. Kössel H. Falk J. A. Thompson G. Link 《Plant molecular biology》1984,3(3):147-158
Summary The genes coding for rRNAs from mustard chloroplasts were mapped within the inverted repeat regions of intact ctDNA and on ctDNA fragments cloned in pBR322. R-loop analysis and restriction endonuclease mapping show that the genes for 16S rRNA map at distances of 17 kb from the junctions of the repeat regions with the large unique region. The genes for 23S rRNA are located at distances of 2.8 kb from the junctions with the small unique region. Genes for 4.5S and 5S rRNA are located in close proximity to the 23S rRNA genes towards the small unique region. DNA sequencing of portions of the 5 terminal third from the mustard 16S rRNA gene shows 96–99% homology with the corresponding regions of the maize, tobacco and spinach chloroplast genes. Sequencing of the region proximal to the 16S rRNA gene reveals the presence of a tRNAVal gene in nearly the same position and with identical sequence as in maize, tobacco and spinach. Somewhat less but still strong homology is also observed for the tDNA Val/16S rDNA intercistronic regions and for the regions upstream of the tRNAVal gene. However, due to many small and also a few larger deletions and insertions in the leader region, common reading frames coding for homologous peptides larger than 44 amino acids can not be detected; it is therefore unlikely that this region contains a protein coding gene. 相似文献
198.
Purified pyrophosphate: fructose 6-phosphate 1-phosphotransferase (EC 2.7.1.90) was used to measure the inorganic pyrophosphate in unfractionated extracts of tissues of Pisum sativum L. The fructose 1,6-bisphosphate produced by the above enzyme was measured by coupling to NADH oxidation via aldolase (EC 4.1.2.13), triosephosphate isomerase (EC 5.3.1.1) and glycerol-3-phosphate dehydrogenase (EC 1.1.1.8). Amounts of pyrophosphate as low as 1 nmol could be measured. The contents of pyrophosphate in the developing embryo of pea, and in the apical 2 cm of the roots, were appreciable; 9.4 and 8.9 nmol g-1 fresh weight, respectively. The possibility that pyrophosphate acts in vivo as an energy source for pyrophosphate: fructose 6-phosphate 1-phosphotransferase and for UDPglucose pyrophosphorylase (EC 2.7.7.9) is considered. 相似文献
199.
The incorporation of [-32P]ATP into proteins of rat brain polyribosomes was studied in vitro. The effects of cyclic nucleotides, calcium, hemin, ACTH, GTP, and spermine were examined. The incorporation of phosphate into proteins increased with time and phosphatase activity was very low; thus, the extent of phosphorylation was predominantly a reflection of protein kinase activity. Phosphorylation of proteins was not sensitive to Ca2+ in the presence or absence of either calmodulin or phosphatidylserine. Phosphorylation was also unaffected by cyclic nucleotides in the absence of exogenous enzymes. However, addition of a cMAP-dependent protein kinase together with cAMP resulted in a stimulation of the incorporation of phosphate into 4 phosphoproteins (pp70, pp58, pp43, and pp32); phosphorylation of pp32 was completely dependent on the addition of the kinase. ACTH (1–24), (11–24), and spermine inhibited the endogenous phosphorylation of one protein band (pp30). The phosphorylation of this 30 kD band was also selectively increased by hemin (5 M). Higher concentrations of hemin exerted an inhibitory effect on the majority of the phosphoproteins. Protein phosphatase activity was not influenced by ACTH or spermine. The specific inhibition of pp30 phosphorylation by ACTH or spermine is most probably explained by an interaction with a cyclic nucleotide- and Ca2+-independent protein kinase. 相似文献
200.
C. Edwards 《FEMS microbiology letters》1984,21(3):319-322
Abstract Washed cell suspensions of Crithidia oncopelti oxidizing a variety of substrates gave complex plots for the inhibition of respiration by potassium cyanide or azide. The data indicated the presence of at least two and possibly three terminal oxidases on the basis of their differential sensitivity to these inhibitors. The oxidase most sensitive to cyanide, azide and CO accounted for approx. 65–70% of whole cell respiration and is probably cytochrome oxidase a/a3 . A second oxidase exhibiting low affinity for CO required high concentrations of KCN or azide for inhibition. This haemoprotein had the spectral characteristics of cytochrome o and accounted for 15–20% of cell respiration. Incomplete inhibition of respiration by high concentrations of KCN or azide suggested the presence of a third oxidase which was CO-unreactive. 相似文献