Structure of 'endogenous ouabain'

The structural characterization of the ouabain-like sodium pump inhibitor i n mammals (so-called 'endogenous ouabain') has long been hindered by the pa ucity of the sample material. While many microscale structural analyses, in cluding LC/MS and sugar analysis, could not differentiate 'endogenous ouaba in' from ouabain, our past nanogram-scale 'pentanaphthoylation' followed by HPLC and circular dichroic spectroscopy (CD) had thought to have distingui shed the two. 'Endogenous ouabain' has since been considered as a subtle st ructural isomer of ouabain. However, further search for endogenous ouabain has now shown that this is not the case. The factors responsible for the un successful characterization for several decades were a series of unexpected reactions, including formation of mixtures of ouabain berates and glycerol naphthoates. The in vivo species giving rise to the reported biological ac tivities of hypothalamic inhibitory factor (HIF) remains to be clarified in full.