Similarities and differences between organic cation inhibition of the Na,K-ATPase and PMCA

The effects of three classes of organic cations on the inhibition of the plasma membrane Ca pump (PMCA) were determined and compared to inhibition of the Na pump. Quaternary amines (tetramethylammonium, tetraethylammonium, and tetrapropylammonium, TMA, TEA, and TPA, respectively) did not inhibit PMCA. This is not to imply that PMCA is inherently selective against monovalent cations because guanidine and tetramethylguanidine inhibited PMCA by competing with Ca(2+). The divalent organic cation, ethyl diamine, inhibited PMCA but was not competitive with Ca(2+). In contrast, propyl diamine did compete with Ca(2+) and was about 10-fold more potent than butyl diamine in inhibiting PMCA. For the Na pump, both TEA and TPA inhibited, but TMA did not. TEA, guanidine, and tetramethylguanidine inhibition was competitive with Na(+) for ATPase activation and with K(+) for pNPPase activation, both of which are cytoplasmic substrate cation effects. Thus, these findings are consistent with TEA, guanidine, and tetramethylguanidine inhibiting from the cytoplasmic side of the Na pump; in contrast, we have previously shown that TPA did not inhibit from the cytoplasmic side. The divalent alkane diamines ethyl, propyl, and butyl diamine all inhibited the Na pump and all competed at the intracellular surface. The order of potency was ED > PD > BD consistent with an optimal size for binding; similarly, for the quaternary amines TMA is apparently too small to make appropriate contacts, and TPA is too large. Homology models based upon the high-resolution SERCA structure are included to contextualize the kinetic observations.