Horizons in biochemistry and biophysics最新文献_第10页

Practical applications and philosophy of optical spectroscopic probes. 光学光谱探针的实际应用和原理。

Horizons in biochemistry and biophysics Pub Date : 1976-01-01

G Weber

引用次数: 0

Molecular aspects of electrical excitation in lipid bilayers and cell membranes. 脂质双分子层和细胞膜电激发的分子方面。

Horizons in biochemistry and biophysics Pub Date : 1976-01-01

P Mueller

{"title":"Molecular aspects of electrical excitation in lipid bilayers and cell membranes.","authors":"P Mueller","doi":"","DOIUrl":"","url":null,"abstract":"Several compounds of fungal or bacterial origin (EIM, alamethicin, monazomycin, DJ400B) can be incorporated into planar lipid bilayers where they form molecular channels and generate voltage-dependent ion conductances. When studied by voltage clamp, the kinetic and steady-state characteristics of these conductance changes are in every respect identical to those found in excitable cell membranes, and their major aspects can be quantitatively described by the Hodgkin-Huxley equations. Thus, the steady-state conductance is an expotential function of the membrane potential, the conductance rises with a sigmoid time course and decays exponentially, and the time constants of the conductance changes go through a maximum as a function of the potential. The conductances also show inactivation as seen in the sodium channels of nerve and the potassium channels of muscle. In addition, there appear for particular pulsing sequences certain kinetic transients that cannot be accounted for by the Hodgkin-Huxley equations but are also seen in identical form in nerve. Because the kinetics are identical in all excitable cell membranes and in these bilayers, it is likely that, in spite of the diverse chemical nature of the channel-forming molecules in the bilayers and the widely differing ion selectivities in the cellular systems, the mechanism by which the membrane opens and closes for the flow of ions is essentially the same in all cases. The kinetic data imply that a cooperative process is involved in the gating action. In principle, two different concepts could account for the kinetics--one involving an intramolecular configurational change within a complex permanent channel, the other, the assembly of a channel through the voltage-dependent aggregation of monomeric channel precursors. In the bilayers the high-order dependence of the steady-state conductance and of the gating time constants on the concentration of the channel formers suggests an aggregation mechanism in which the gating involves the voltage-induced insertion of all or part of the channel-forming molecules from the membrane surface into the hydrocarbon region and their subsequent aggregation into open channels by lateral diffusion. The mathematical description of this two-step insertion-aggregation mechanism accounts quantitatively for the entire conductancb-voltage kinetics including inactivation and other kinetic features which deviate from the Hodgkin-Huxley kinetics in the sense that the rate constants of the changes are dependent not only on the membrane potential but also on the value of the conductance and on time. The proposed mechanism is also in agreement with single-channel data for alamethicin which suggest that both the insertion and the aggregation rate constants are voltage-dependent...","PeriodicalId":75908,"journal":{"name":"Horizons in biochemistry and biophysics","volume":"2 ","pages":"230-84"},"PeriodicalIF":0.0,"publicationDate":"1976-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"11971886","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

引用次数: 0

Sugar and amino acid transport in animal cells. 糖和氨基酸在动物细胞中的运输。

Horizons in biochemistry and biophysics Pub Date : 1976-01-01

U Hopfer

{"title":"Sugar and amino acid transport in animal cells.","authors":"U Hopfer","doi":"","DOIUrl":"","url":null,"abstract":"The molecular basis of intracellular metabolism of nutrients and its control is quite well understood in animal cells. Comparable knowledge about solute entry into cells is still lacking, as, in contrast to metabolism, no chemical reactions seem to be directly associated with the known nutrient transport. Nevertheless, translocations of sugars and amino acids across the plasma membrane are specific and controlled processes, biologically as well as chemically. Recent advances in techniques for isolation of plasma membranes have made it feasible to study transport properties of animal cells without the complications encoutered in viable cells. This approach has been applied to sugar and amino acid transport in plasma membranes of several tissues, and intact transport systems for D-glucose, D-fructose, neutral L-amino acids, and dipeptides have been demonstrated. This demonstration of intact transport systems in an in vitro setting accomplishes the first step in the direction of molecular isolation of transport systems. Furthermore, the information obtained about the transport mechanism catalyzed by some systems has settled controversies on active nutrient transport. For example, electrogenic cotransport of sodium and D-glucose or of sodium and neutral L-amino acids has been shown to form the basis for active, sodium-dependent absorption of these nutrients. A consequence of this type of mechanism is interaction between sugar and amino acid transport via the common charged cosubstrate sodium. Moreover, different types of transport systems for the same substrate have been demonstrated in the luminal and contraluminal regions of the plasma membrane of epithelial cells, which explains unidirectional transepithelial transport. The luminal membrane contains sodium-dependent, active transport systems, and the contraluminal membrane passive, facilitated diffusion systems. In vivo, the lower intracellular sodium potential would result in concentrative nutrient uptake from the lumen, but would not influence exit on the contraluminal side. Variations in the electrical components of the sodium potential, which have not been measured, may explain apparently contradicting results on active sugar and amino acid transport with various tissue preparations.","PeriodicalId":75908,"journal":{"name":"Horizons in biochemistry and biophysics","volume":"2 ","pages":"106-33"},"PeriodicalIF":0.0,"publicationDate":"1976-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"11230807","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

引用次数: 0

Ferredoxins and photosynthesis. 铁氧化还原蛋白和光合作用。

Horizons in biochemistry and biophysics Pub Date : 1974-01-01

D I Arnon, B B Buchanan

引用次数: 0

The quest for coupling site 1. 寻找耦合点1。

Horizons in biochemistry and biophysics Pub Date : 1974-01-01

T P Singer, M Gutman

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Metabolic regulatory functions of oxalacetate. 草酸酯的代谢调节功能。

Horizons in biochemistry and biophysics Pub Date : 1974-01-01

B A Ackrell