Na/K Pump in Inside-Out Vesicles Utilizing ATP Synthesized at the Membrane

The Na/K pump in human red cells is fueled by ATP synthesized during glycolysis. In human red cells, it has been suggested that two glycolytic enzymes, glyceraldehydes-3-phosphate dehydrogenase (GAPDH) and phosphoglycerate kinase (PGK), bound to the red cell membrane synthesize ATP and deposit it in a membrane-associated compartment from which it is used by the Na/K pump. Active Na transport into inside-out vesicles (IOVs) is promoted by addition of the substrates for GAPDH and PGK, without added ATP. The GAPDH–PGK complex synthesizes ATP that remains associated with the red cell membrane. The membrane-associated ATP must be compartmentalized because it is inaccessible to degradation catalyzed by hexokinase. The membrane-associated ATP fuels the pump because its level is reduced by the addition of Na and this reduction is inhibited by strophanthidin.