[Active transport of Na+ by modified Na,K-ATPase]

The ability of ATP, CTP, ITP, GTP and UTP to induce ouabain-sensitive accumulation of Na+ by proteoliposomes with a reconstituted Na/K-pump was studied. At low Na+/K+ ratio (20 mM/50 mM), a correlation was observed between the proton-accepting capacity of the nucleotide and its efficiency as an active transport substrate. In order to test the hypothesis on the role of the negative charge in position 1 of the purine (3-pyrimidine) base of the nucleotide in the reversible transitions from the Na- to the K-conformations of Na,K-ATPase, two ATP analogs (N1-hydroxy-ATP possessing a proton-accepting ability and N1-methoxy-ATP whose molecule carries a negative charge quenched by a methyl group) were used. The first substrate provides for active accumulation of Na+ by proteoliposomes at a rate similar to that of ATP, whereas the second substrate is fairly ineffective.