Purification and characterization of digitalislike factors from human plasma

Increased levels of a humoral inhibitor of active sodium transport have been associated with the response to acute and chronic hypervolemia and various forms of experimental as well as human essential hypertension. In this report, we describe the purification of inhibitors of Na+, K+-adenosine triphosphatase (ATPase) from the plasma of volume-expanded individuals. Of the two amphipathic materials obtained, only one of the factors when present in high concentrations showed the slow time-dependent component of inactivation similar to that of the cardiac glycosides. Inhibition was reduced in the presence of plasma proteins and was freely reversible. Both factors inhibited potassium-dependent p-nitrophenylphosphatase activity and specific [3H]ouabain binding in a manner similar to the cardiac glycosides. In contrast to ouabain and vanadate, however, high concentrations of potassium or norepinephrine, respectively, did not affect the magnitude or kinetic characteristics of inhibition. Structural analysis by mass spectroscopy showed a mass of 444 for factor 1, whereas factor 2 was conclusively identified as lysophosphatidylcholine-gamma-palmitoyl. These factors probably inhibit Na+, K+-ATPase by a nonspecific mechanism involving reversible perturbation of lipid-enzyme interactions required for normal catalytic activity. The significance of these factors as modulators of sodium transport may be limited to pathological states associated with abnormalities in plasma protein binding or lipid metabolism. They do not appear to be directly related to the humorally mediated disturbance of cellular sodium transport in hypertension.