The ouabain-like sodium pump inhibitor in mammals (so-called "endogenous ouabain") has been consid- ered a subtle structural isomer of ouabain. Its structural investigation, however, has long been hindered by the paucity of sample material. Our recent purification of endogenous ouabain (3 #mu#g) from bovine hypothalamus allowed the mea- surement of its ~1H-NMR. The obtained spectrum as well as reexamination of past microscale structural studies on en- dogenous ouabain led us to identify the purified material as ouabain in an unusual manner. It turned out that the struc- tural analysis had been complicated by a facile ouabain- borate complexation in borosilicate glassware. In retrospect, it is not surprising that the polyhydroxylated ouabain mole- cule serves as a polydentate ligand to inorganic species. In its physiological environment, ouabain may exist as some un- known complex. The chemical species giving rise to the reported biological activities of hypothalamic inhibitory factor preparations remain to be clarified.
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