A voltage-activated cation transport pathway associated with the sodium pump

Mechanism responsible for oligomycin-induced occlusion of Na+ within Na/K-ATPase

The mechanism whereby oligomycin occludes Na+ within Na/K-ATPase was investigated to study Na+ and K+ transport mechanisms. Oligomycin stimulated Na+ binding to Na/K-ATPase but inhibited Na-K and Na-Na exchange. The oligomycin concentration required to stimulate Na+ binding to half-maximal was 4.5 microM, which was close to the concentration that reduced Na-Na and Na-K exchange and ATPase activity to half-maximal, suggesting that Na/K-ATPase possesses an oligomycin binding site responsible for stimulating Na+ binding and reducing ion exchange and ATPase activity. In contrast, neither K+ binding nor K+ transport was affected by oligomycin. Limited tryptic digestion of Na/K-ATPase showed that, unlike Na+, K+, and ouabain, oligomycin treatment did not result in a specific digestion pattern. Oligomycin appeared to inhibit ouabain binding in a noncompetitive manner, whereas it did not affect ATP binding. Na/K-ATPase isoforms with low and high sensitivities to ouabain were equally sensitive to oligomycin. These results suggest that the oligomycin binding site is located on the extracellular side of Na/K-ATPase, at a different position from the ouabain binding site, and this antibiotic did not induce a conformational change of Na/K-ATPase. We propose that oligomycin interacts with the Na+ occlusion site from the extracellular side of Na/K-ATPase, which delays Na+ release to the extracellular side without inducing a conformational change, suggesting that the pathways responsible for Na+ and K+ transport differ.

The membrane defect in hereditary stomatocytosis

Hereditary stomatocytosis and allied conditions represent a series of diseases in which abnormal movements of univalent cations across the plasma membrane play an important part in cellular disease. The primary problem lies not in the active transporters but in the basal permeability of the membrane, which is always increased, and the extent of the increase correlates with the cellular dysfunction. A number of structural abnormalities have been described in these membranes, but the most consistent and convincing is the deficiency of a hitherto uncharacterized integral membrane protein of molecular weight 31 kDa in the severe, 'overhydrated' form of the disease. The true function of this protein remains enigmatic, but its deficiency in this condition indicates that it may have a role in the regulation of cation transport.