On the structure of endogenous ouabain

The ouabain-like sodium pump inhibitor in mammals (so-called "endogenous ou abain") has been considered a subtle structural isomer of ouabain. Its stru ctural investigation, however, has long been hindered by the paucity of sam ple material, Our recent purification of endogenous ouabain (3 mu g) from b ovine hypothalamus allowed the measurement of its H-1-NMR, The obtained spe ctrum as well as reexamination of past microscale structural studies on end ogenous ouabain led us to identify the purified material as ouabain in an u nusual manner. It turned out that the structural analysis had been complica ted by a facile ouabain-borate complexation in borosilicate glassware. In r etrospect, it is not surprising that the polyhydroxylated ouabain molecule serves as a polydentate ligand to inorganic species. In its physiological e nvironment, ouabain may exist as some unknown complex, The chemical species giving rise to the reported biological activities of hypothalamic inhibito ry factor preparations remain to be clarified.