The structural characterization of the ouabain-like sodium pump inhibitor in mammals (so-called `endogenous ouabain') has long been hindered by the paucity of the sample material. While many microscale structural analyses, including LC/MS and sugar analysis, could not differentiate `endogenous ouabain' from ouabain, our past nanogram-scale `pentanaphthoylation' followed by HPLC and circular dic...