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Phosphate inhibition of the human red cell sodium pump: simultaneous binding of adenosine triphosphate and phosphate.
Author(s) -
Sachs J R
Publication year - 1988
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1988.sp017136
Subject(s) - chemistry , adenosine triphosphate , sodium , atpase , phosphate , adenosine , efflux , adenosine diphosphate , enzyme , nucleotide , biophysics , non competitive inhibition , biochemistry , biology , platelet , organic chemistry , platelet aggregation , gene , immunology
1. The Na+‐K+ exchange carried out by the Na+ pump of human red cell ghosts and the Na+ + K+‐dependent adenosine triphosphatase (Na+,K+‐ATPase) activity of human red cell membranes are inhibited by MgPO4 rather than by free phosphate; similarly, the substrate for the K+‐K+ exchange carried out by the pump is MgPO4 rather than free phosphate. 2. Inhibition of the Na+, K+‐ATPase activity by MgPO4 is only partially competitive (mixed type) with ATP, and MgPO4 inhibition of the Na+‐K+ exchange measured in Na+‐free solutions and in K+‐free ghosts which contain ATP at relatively high concentration is partially uncompetitive (mixed type) with external K+. 3. When measurements were made in K+‐free ghosts and Na+‐free solutions, or when Na+,K+‐ATPase activity was measured at high ATP concentrations, inhibition by MgPO4 was non‐competitive with cell Na+. This observation is not consistent with the Albers‐Post reaction mechanism of the Na+ pump, and suggests the presence of an alternative reaction pathway in which ATP combines with the enzyme before phosphate is released. 4. MgPO4 monotonically inhibited the uncoupled Na+ efflux which occurs in solutions free of both Na+ and K+. The uncoupled efflux seemed to be more sensitive to MgPO4 inhibition than the Na+‐K+ exchange. 5. Trinitrophenyladenosine‐5'‐tetraphosphate stimulated the K+‐K+ exchange in the presence of MgPO4, and the characteristics of stimulation by TNP adenosine tetraphosphate were little different from the characteristics of stimulation by trinitrophenyladenosine‐5'‐triphosphate or ‐5'‐diphosphate. The nucleotide binding site at which K+‐K+ exchange is stimulated must be able to accommodate a nucleotide with a linear array of four phosphate groups.