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21.
D. DiFrancesco F. Porciatti I. S. Cohen 《Cellular and molecular life sciences : CMLS》1991,47(5):449-452
Summary The isolation of ionic fluxes contributing to electric currents through cell membranes often requires block of other undesired components which can be achieved, among others, by divalent cations. Mn2+ and Ba2+ are often used, for example, to block Ca and K currents. Here we have investigated the effects of these two cations on the properties of the hyperpolarization-activated pacemaker current if, in rabbit sino-atrial node myocytes, as obtained by voltage clamp analysis. We find that 2 mM Mn2+ shifts the if activation curve by 3.2±0.3 mV towards more positive values. However, when 1 mM Ba2+ is also added, the positive shift is more than halved (1.3±0.2 mV). We find, too, that in the absence of blocking cations the ACh-induced if inhibition is slightly higher than in their presence. These results indicate that the alteration of if kinetic properties by Ba2+ plus Mn2+-containing solutions is minimal. 相似文献
22.
Ribonuclease S-protein exhibits a pH-dependent conformational transition between folded and unfolded states, and some unfolded S-protein persists up to pH 8. The histidine C2 proton resonance of the unfolded species was erroneously assigned by Bradbury et al. to histidine residue 119 of the folded species. 相似文献
23.
Cooperative tandem binding of met repressor of Escherichia coli 总被引:10,自引:0,他引:10
S E Phillips I Manfield I Parsons B E Davidson J B Rafferty W S Somers D Margarita G N Cohen I Saint-Girons P G Stockley 《Nature》1989,341(6244):711-715
We present biochemical and genetic data to support the hypothesis that the Escherichia coli met repressor, MetJ, binds to synthetic and natural operator sequences in tandem arrays such that repression depends not only on the affinity of the DNA-protein interaction, but also on protein-protein contacts along the tandem array. This represents a novel form of regulatory switch. Furthermore, there seems to be homology between the organization of the met and trp operators. 相似文献
24.
The molecular mechanism by which insulin stimulates glycogen synthesis in mammalian skeletal muscle 总被引:37,自引:0,他引:37
The ability of insulin to promote the phosphorylation of some proteins and the dephosphorylation of others is paradoxical. An insulin-stimulated protein kinase is shown to activate the type-1 protein phosphatase that controls glycogen metabolism, by phosphorylating its regulatory subunit at a specific serine. Furthermore, the phosphorylation of this residue is stimulated by insulin in vivo. Increased and decreased phosphorylation of proteins by insulin can therefore be explained through the same basic underlying mechanism. 相似文献
25.
E. Cohen 《Cellular and molecular life sciences : CMLS》1985,41(4):470-472
Summary To facilitate massive screening and for structure-activity relationship studies of chitin synthesis inhibitors, methods to obtain the chitin synthetase (CS) containing microsomal fraction from the postmitochondrial supernatant were examined. Compared with fractionation by differential centrifugation, the CaCl2 precipitate yielded the most active CS preparation. Acidification (pH 5.6) and polyethylene glycol 8000 (5%) treatments resulted in relatively low CS activity. Inhibitory effects were detected with polyoxin-D and 1-geranyl-2-methyl benzimidazole, a novel CS inhibitor, but not with benzoylphenyl ureas. 相似文献
26.
提出了随机分配的质点之间距离分布的概念,给出了最近距离分布公式,得到了最近距离和k次距离分布的众数,矩及质点空间密度D的充分统计量和极大似然估计. 相似文献
27.
Enhancement of the high-magnetic-field critical current density of superconducting MgB2 by proton irradiation 总被引:2,自引:0,他引:2
Bugoslavsky Y Cohen LF Perkins GK Polichetti M Tate TJ Gwilliam R Caplin AD 《Nature》2001,411(6837):561-563
Magnesium diboride, MgB2, has a relatively high superconducting transition temperature, placing it between the families of low- and high-temperature (copper oxide based) superconductors. Supercurrent flow in MgB2 is unhindered by grain boundaries, making it potentially attractive for technological applications in the temperature range 20-30 K. But in the bulk material, the critical current density (Jc) drops rapidly with increasing magnetic field strength. The magnitude and field dependence of the critical current are related to the presence of structural defects that can 'pin' the quantized magnetic vortices that permeate the material, and a lack of natural defects in MgB2 may be responsible for the rapid decline of Jc with increasing field strength. Here we show that modest levels of atomic disorder induced by proton irradiation enhance the pinning of vortices, thereby significantly increasing Jc at high field strengths. We anticipate that either chemical doping or mechanical processing should generate similar levels of disorder, and so achieve performance that is technologically attractive in an economically viable way. 相似文献
28.
Dopamine has a central role in motivation and reward. Dopaminergic neurons in the ventral tegmental area (VTA) signal the discrepancy between expected and actual rewards (that is, reward prediction error), but how they compute such signals is unknown. We recorded the activity of VTA neurons while mice associated different odour cues with appetitive and aversive outcomes. We found three types of neuron based on responses to odours and outcomes: approximately half of the neurons (type I, 52%) showed phasic excitation after reward-predicting odours and rewards in a manner consistent with reward prediction error coding; the other half of neurons showed persistent activity during the delay between odour and outcome that was modulated positively (type II, 31%) or negatively (type III, 18%) by the value of outcomes. Whereas the activity of type I neurons was sensitive to actual outcomes (that is, when the reward was delivered as expected compared to when it was unexpectedly omitted), the activity of type II and type III neurons was determined predominantly by reward-predicting odours. We 'tagged' dopaminergic and GABAergic neurons with the light-sensitive protein channelrhodopsin-2 and identified them based on their responses to optical stimulation while recording. All identified dopaminergic neurons were of type I and all GABAergic neurons were of type II. These results show that VTA GABAergic neurons signal expected reward, a key variable for dopaminergic neurons to calculate reward prediction error. 相似文献
29.
Ageing, fitness and neurocognitive function. 总被引:18,自引:0,他引:18
A F Kramer S Hahn N J Cohen M T Banich E McAuley C R Harrison J Chason E Vakil L Bardell R A Boileau A Colcombe 《Nature》1999,400(6743):418-419
30.