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PubMed1974

Photoaffinity labeling of the ouabain-binding site on (Na+ plus K+) adenosinetriphosphatase.

An ethyl diazomalonyl derivative of cymarin was synthesized in order to photoaffinity label the cardiac glycoside-binding site on (Na(+) + K(+)) adenosinetriphosphate (EC 3.6.1.3). When a noncovalent complex of the enzyme and this cardiac glycoside derivative was photolyzed, a covalent bond was formed between the ligand and the larger of the two polypeptide subunits of the enzyme. Several control experiments demonstrate that this photochemical reaction occured while the ligand was bound to the site at which it inhibits the enzyme activity. Another specific inhibitor, tentatively identified as the ethyl chloromalonyl derivative of cymarin, produced similar photoaffinity labeling of the larger subunit, demonstrating that the photolytic dissociation of the diazo group may not be responsible for the photochemical reaction. Since the cardiac glycoside-binding site, which is accessible from the outside surface of the plasma membrane, and the phosphorylation site, which is accessible from the inside surface, are both on the larger polypeptide subunit of (Na(+) + K(+)) adenosinetriphosphatase, this polypeptide has sequences exposed to both sides of the membrane.

Adenosine Triphosphatases↗
PubMed1967

Sodium extrusion by internally dialyzed squid axons.

A method has been developed which allows a length of electrically excitable squid axon to be internally dialyzed against a continuously flowing solution of defined composition. Tests showed that diffusional exchange of small molecules in the axoplasm surrounding the dialysis tube occurred with a half-time of 2-5 min, and that protein does not cross the wall of the dialysis tube. The composition of the dialysis medium was (mM): K isethionate 151, K aspartate 151, taurine 275, MgCI(2) 4-10, NaCl 80, KCN 2, EDTA 0.1, ATP 5-10, and phosphoarginine 0-10. The following measurements were made: resting Na influx 57 pmole/cm(2)sec (n = 8); resting potassium efflux 59 pmole/ cm(2)sec (n = 4); stimulated Na efflux 3.1 pmole/cm(2)imp (n = 9); stimulated K efflux 2.9 pmole/cm(2)imp (n = 3); resting Na efflux 48 pmole/cm(2)sec (n = 18); Q(10) Na efflux 2.2 (n = 5). Removal of ATP and phosphoarginine from the dialysis medium (n = 4) or external application of strophanthidin (n = 1) reversibly reduced Na efflux to 10-13 pmole/cm(2)sec. A general conclusion from the study is that dialyzed squid axons have relatively normal passive permeability properties and that a substantial fraction of the Na efflux is under metabolic control although the Na extrusion mechanism may not be working perfectly.

Adenosine Triphosphate↗