Automatic activity in depolarized guinea pig ventricular myocardium. Characteristics and mechanisms.
Membrane potential was changed uniformly in segments, 0.7-1.0 mm long, of guinea pig papillary muscles excised from the right ventricle by using extracellular polarizing current pulses applied across two electrically insulated cf preparations superfused with Tyrode's solution at maximum diastolic membrane potentials ranging from-35.2+/-7.5 (threshold) to +4.0+/-9.2 mV. The average maximum dV/dt of RAD ranged from 17.1 to 18.0 V/sec within a membrane potential range of -40 to +20 mV. Raising extracellular Ca2+ concentration [Ca2+]0 from 1.8 to 6.8 mM, or application of isoproterenol (10(-6)g/ml) enhanced the rate of RAD, but lowering [Ca2+]0 to 0.4 mM or exposure to MnCl2 (6 mM) abolished RAD. RAD were enhanced by lowering extracellular K+ concentration [K+]0 from 5.4 to 1.5 mM. RAD were suppressed in 40% of fibers by raising [K+]0 to 15.4 mM, and in all fibers by raising [K+]0 to 40.4 mM. This suppression was due to increased [K+]0 and not to K-induced depolarization because it persisted when membrane potential was held by means of a conditioning hyperpolarizing puled gradually after maximum repolarization. These observations suggest that the development of RAD in depolarized myocardium is associated with a time-dependent decrease in outward current (probably K current) and with increase in the background inward current, presumably flowing through the slow cha-nel carrying Ca or Na ions, or both. (+info)
Synechocystis sp. slr0787 protein is a novel bifunctional enzyme endowed with both nicotinamide mononucleotide adenylyltransferase and 'Nudix' hydrolase activities.
Synechocystis sp. slr0787 open reading frame encodes a 339 residue polypeptide with a predicted molecular mass of 38.5 kDa. Its deduced amino acid sequence shows extensive homology with known separate sequences of proteins from the thermophilic archaeon Methanococcus jannaschii. The N-terminal domain is highly homologous to the archaeal NMN adenylyltransferase, which catalyzes NAD synthesis from NMN and ATP. The C-terminal domain shares homology with the archaeal ADP-ribose pyrophosphatase, a member of the 'Nudix' hydrolase family. The slr0787 gene has been cloned into a T7-based vector for expression in Escherichia coli cells. The recombinant protein has been purified to homogeneity and demonstrated to possess both NMN adenylyltransferase and ADP-ribose pyrophosphatase activities. Both activities have been characterized and compared to their archaeal counterparts. (+info)
A functional model for O-O bond formation by the O2-evolving complex in photosystem II.
The formation of molecular oxygen from water in photosynthesis is catalyzed by photosystem II at an active site containing four manganese ions that are arranged in di-mu-oxo dimanganese units (where mu is a bridging mode). The complex [H2O(terpy)Mn(O)2Mn(terpy)OH2](NO3)3 (terpy is 2,2':6', 2"-terpyridine), which was synthesized and structurally characterized, contains a di-mu-oxo manganese dimer and catalyzes the conversion of sodium hypochlorite to molecular oxygen. Oxygen-18 isotope labeling showed that water is the source of the oxygen atoms in the molecular oxygen evolved, and so this system is a functional model for photosynthetic water oxidation. (+info)
Increased calcium entry into dystrophin-deficient muscle fibres of MDX and ADR-MDX mice is reduced by ion channel blockers.
1. Single fibres were enzymatically isolated from interosseus muscles of dystrophic MDX mice, myotonic-dystrophic double mutant ADR-MDX mice and C57BL/10 controls. The fibres were kept in cell culture for up to 2 weeks for the study of Ca2+ homeostasis and sarcolemmal Ca2+ permeability. 2. Resting levels of intracellular free Ca2+, determined with the fluorescent Ca2+ indicator fura-2, were slightly higher in MDX (63 +/- 20 nM; means +/- s.d.; n = 454 analysed fibres) and ADR-MDX (65 +/- 12 nM; n = 87) fibres than in controls (51 +/- 20 nM; n = 265). 3. The amplitudes of electrically induced Ca2+ transients did not differ between MDX fibres and controls. Decay time constants of Ca2+ transients ranged between 10 and 55 ms in both genotypes. In 50 % of MDX fibres (n = 68), but in only 20 % of controls (n = 54), the decay time constants were > 35 ms. 4. Bath application of Mn2+ resulted in a progressive quench of fura-2 fluorescence emitted from the fibres. The quench rate was about 2 times higher in MDX fibres (3.98 +/- 1.9 % min-1; n = 275) than in controls (2.03 +/- 1.4 % min-1; n = 204). The quench rate in ADR-MDX fibres (2.49 +/- 1.4 % min-1; n = 87) was closer to that of controls. 5. The Mn2+ influx into MDX fibres was reduced to 10 % by Gd3+, to 19 % by La3+ and to 47 % by Ni2+ (all at 50 microM). Bath application of 50 microM amiloride inhibited the Mn2+ influx to 37 %. 6. We conclude that in isolated, resting MDX muscle fibres the membrane permeability for divalent cations is increased. The presumed additional influx of Ca2+ occurs through ion channels, but is well compensated for by effective cellular Ca2+ transport systems. The milder dystrophic phenotype of ADR-MDX mice is correlated with a smaller increase of their sarcolemmal Ca2+ permeability. (+info)
SodA and manganese are essential for resistance to oxidative stress in growing and sporulating cells of Bacillus subtilis.
We constructed a sodA-disrupted mutant of Bacillus subtilis 168, BK1, by homologous recombination. The mutant was not able to grow in minimal medium without Mn(II). The spore-forming ability of strain BK1 was significantly lower in Mn(II)-depleted medium than that of the wild-type strain. These deleterious effects caused by the sodA mutation were reversed when an excess of Mn(II) was used to supplement the medium. Moreover, the growth inhibition by superoxide generators in strain BK1 and its parent strain was also reversed by the supplementation with excess Mn(II). We therefore estimated the Mn-dependent superoxide-scavenging activity in BK1 cells. Whereas BK1 cells have no detectable superoxide dismutase (Sod) on native gel, the superoxide-scavenging activity in crude extracts of BK1 cells grown in Mn(II)-supplemented LB medium (10 g of tryptone, 5 g of yeast extract, and 5 g of NaCl per liter) was significantly detected by the modified Sod assay method without using EDTA. The results obtained suggest that Mn, as a free ion or a complex with some cellular component, can catalyze the elimination of superoxide and that both SodA and Mn(II) are involved not only in the superoxide resistance of vegetative cells but also in sporulation. (+info)
RNA metabolism, manganese, and RNA polymerases of zinc-sufficient and zinc-deficient Euglena gracilis.
The three major RNA classes from zinc-sufficient [(+Zn)] and zinc-deficient [(=Zn)] Euglena gracilis have been separated by affinity chromatography on oligo(dT)- and N-[N'-[m-(dihydroxyboryl)phenyl]succinamoyl]aminoethyl (DBAE)-celluloses. The total RNA content and the ribosomal and transfer RNA fractions are the same in (+Zn) and (=Zn) cells. IN (-Zn) cells, the messenger RNA fraction increases, and its altered base composition reveals additional bases and a 2-fold increase in the (G+C)/(A+U) ratio. Since the intracellular content of manganese increases in (-Zn) cells, we have examined its role in determining these changes in RNA composition. An increase in the Mn2+ content from 1 to 10 mM in assays with RNA polymerases I and II from (+Zn) cells and those with the single RNA polymerase from (-Zn) cells decreases the ratio of UMP to CMP incorporated from 1.7 to 1.0, 2.1 to 0.8 and 3.5 to 0.4, respectively. Thus, Mn2+ concentration can significantly alter the products of the enzymatic action of RNA polymerases from both (+Zn) and (-Zn) E. gracilis cells. (+info)
Sequential activation of different Ca2+ entry pathways upon cholinergic stimulation in mouse pancreatic acinar cells.
1. We have studied capacitative calcium entry (CCE) under different experimental conditions in fura-2-loaded mouse pancreatic acinar cells by digital microscopic fluorimetry. CCE was investigated during [Ca2+]i decay after cell stimulation with a supramaximal concentration of ACh (10 microM) or during Ca2+ readmission in Ca2+-depleted cells (pretreated with thapsigargin or ACh). 2. La3+ and Zn2+ (100 microM) inhibited CCE during Ca2+ readmission but had negligible effects during ACh decay. In contrast flufenamic acid (100 microM), an inhibitor of non-selective cation channels, genistein (10 microM), a broad-range tyrosine kinase inhibitor, and piceatannol (10 microM), an inhibitor specific for non-receptor Syk tyrosine kinase, inhibited CCE during ACh decay but not during Ca2+ reintroduction. 3. Simultaneous detection of Mn2+ entry and [Ca2+]i measurement showed that, in the presence of extracellular calcium, application of 100 microM Mn2+ during ACh decay resulted in manganese influx without alteration of calcium influx, whilst when applied during Ca2+ readmission, Mn2+ entry was significantly smaller and induced a clear inhibition of CCE. 4. Application of the specific protein kinase C inhibitor GF109293X (3 microM) reduced CCE in Ca2+-depleted cells, whereas the activator phorbol 12-myristate, 13-acetate (3 microM) increased Ca2+ entry. 5. Based on these results we propose that cholinergic stimulation of mouse pancreatic acinar cells induces Ca2+ influx with an initial phase operated by a non-specific cation channel, sensitive to flufenamic acid and tyrosine kinase inhibitors but insensitive to lanthanum and divalent cations, followed by a moderately Ca2+-selective conductance inhibited by lanthanum and divalent cations. (+info)
Arginase from human full-term placenta.
Arginase was purified about 1800-fold from extracts of human full-term placenta; the enzyme appeared to be homogenous by disc electrophoresis and molecular-sieve chromatography. The mol. wt. determination by gel filtration and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis yielded a value of 70000 for the most pure and the partially purified enzyme. The human placenta arginase is a metalloenzyme with an optimum pH of 9.1. The Km for L-arginine is 27 mM. L-Ornithine and L-lysine show competitive inhibition with Ki values of 6.3 and 14 mM respectively. (+info)