In plasma membranes of intact cells an enzymatic pump actively transports sodium ions inward and potassium ions outward. In preparations of broken membranes it appears as an adenosine triphosphatase dependent on magnesium, sodium, and potassium ions together. In this adenosine triphosphatase a phosphorylated intermediate is formed from adenosine triphosphate in the presence of sodium ions and is hydrolyzed with the addition of potassium ions. The normal intermediate was not split by adenosine diphosphate. However, selective poisoning by N-ethylmaleimide or partial inhibition by a low magnesium ion concentration yielded an intermediate split by adenosine diphosphate and insensitive to potassium ions. Pulse experiments on the native enzyme supported further a hypothesis of a sequence of phosphorylated forms, the first being made reversibly from adenosine triphosphate in the presence of sodium ion and the second being made irreversiblyfrom the first and hydrolyzed in the presence of potassium ion. The cardioactive steriod inhibitor, ouabain, appeared to combine preferentially with the second form. Phosphorylation was at the same active site according to electrophoretic patterns of proteolytic phosphorylated fragments of both reactive forms, It is concluded that there is a conformational change in the active center for phosphorylation during the normal reaction sequence. This change may be linked to one required theoretically for active translocation of ions across the cell membrane.The basic idea in this paper is a change in the shape of the sodium and potassium pump during its reaction cycle. This change is not necessarily that assumed for translocation (1), but rather a change in the conformation of an active center. The evidence is indirect. It is assumed that the reactivity of a phosphoryl group attached covalently to an active site in an active center depends on the conformation of the active center surrounding the site. Thus a change in reactivity implies a change in conformation. It should be enough to show that this group at a single active site changes its reactivity in the course of the normal reaction sequence. The phosphoryl group is attached to sodiumpotassium-dependent adenosine triphosphatase, an aspect of the pump.

1 Diabetes mellitus is known to produce alterations in vascular reactivity. The present study examined the effects of endothelium-dependent and endothelium-independent relaxing substances on thoracic aorta from control and spontaneously diabetic rats. 2 Endothelium-dependent relaxation produced by acetylcholine or the calcium ionophore, A23817, in aortic rings precontracted with phenylephrine was significantly attenuated in diabetic vessels. 3 Relaxations produced by sodium nitroprusside or adenosine in diabetic preparations were comparable to those in control vessels. 4 The results show that diabetes leads to a specific impairment of endothelial-dependent relaxation.

No abstract

No abstract