Active transport of Na* and

Figure 9.6 A model for the active transport of Na+ and K+ by the (Na+-K+)-ATPase. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

The key experiment was one which would not be carried out until Skou learned of a paper published in German four years earlier, which showed that the movement of cations across the red cell membrane was inhibited by cardiac glycosides such as ouabain, plant alkaloids used for some 200 years in the therapy of heart failure. In 1957, Skou was unaware of this important finding, and wrote later that he had not done the crucial experiment to show Na/K ATPase as the transport system. When done, the experiment was decisive ouabain inhibited the ATPase activity exactly as it did the cation fluxes. This led to a flurry of activity in many biochemical laboratories and allowed Skou (nine years after his original publication) to write a review in which he concluded that the enzyme fulfilled the requirements for a system responsible for active transport of Na+ and K+ across the cell membrane. Thus the Na/K ATPase had the following properties ... 

The enzyme was found in all cells that had coupled active transport of Na+ and K+. 

Alternatively, A and B may refer to the concentrations of an ion or molecule on the outside and inside of a cell, as in the active transport of a nutrient. The active transport of Na+ and K+ across membranes is driven by the phosphorylation of the sodium-potassium pump by ATP and its subsequent dephosphorylation (Section 13.2.1). 

In cholera, adenylate cyclase is permanently turned on. This in turn stimulates active transport of Na" and water from epithelial cells, leading to diarrhea. 

For each action potential that is generated in an axon, only a very fetv Na" ions enter and a very few ions depart the cell. Thus, in a poisoned nerve cell, many tens of thousands of impulses may be conducted before ionic equilibrium across the membrane is achieved. The active transport of Na" and across the membrane may be best viewed as necessary in the long run but not in the short run. 

Dilantin reversal of cardiac glycoside toxicity was well documented. Woodbury has shown that dilantin augments the active transport of Na and K" " in the brain, skeletal and cardiac muscle. Its ability to reverse digitalis toxicity has been attributed to a direct action on Na,K-ATPase . 

Another strong proponent of this Idea was E. J. Conway In Dublin, who had participated in a classic series of experiments just before World War II In Cambridge proving active transport of Na and K across cell membranes. Coincidentally, he was later responsible for developing an assay for urease and showed the presence of urease In the stomach. This urease was not recognized as the property of an infective organism, Helicobacter pylori, for another 40 years ... 

Skou, J. C., 1962, Preparation from mammalian brain and kidney of the enzyme system involved in active transport of Na and K, Biochim. Biophys. Acta 58 314. 

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