A Search for Endogenous Na+,K+-ATPase Inhibitor in Acutely Volume-Expanded Hog Plasma led to Lysophosphatidylcholine ??-Stearoyl

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Abstract
An Na+,K+—ATPase inhibitor possessing inhibitory activity against the specific binding of ouabain to Na+,K+—ATPase has been purified from the plasma of acutely salineinfused hogs. The purification was performed by a combination of Amberlite XAD—2 adsorption chromatography and five steps of high-pressure liquid chromatography (HPLC). Fast atom bombardment mass and proton nuclear magnetic resonance (NMR) spectrometric studies identified the purified substance as lysophosphatidylcholine γ— stearoyl (LPCS). The ouabain-displacing activity in plasma, due to this compound, increased with time during saline infusion. The maximal level reached was approximately 12 times higher than that in the pre-infusion plasma sample. Lysophosphatidylcholines (LPCs) containing myristoyl, palmitoyl and oleoyl groups were also inhibitory to Na+,K+—ATPase and ouabain—binding to the enzyme. These LPCs were effective at 100 µmol/l concentrations in attaining 50% inhibition of the enzyme activity and ouabain—binding activity of Na+,K+—ATPase. These results suggest that LPCs containing long chain fatty acids could play an important role as a Na+,K+— ATPase inhibitors under volume—expanded conditions.

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