Endogenous human Ouabain: Another controversial fact of life

It's there whether you like it or not.

Endogenous Ouabain: Recent Advances and Controversies.  

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EO and Its Isomers

EO was first identified in human plasma 25 years ago.^1,2 Despite confirmation in humans and other mammals with mass spectrometry (MS; Figure; Figures S1–S6 in the online-only Data Supplement), nuclear magnetic resonance, and combined liquid chromatography (LC)–immunology methods,^3–6 human EO has remained controversial.⁷ New analytic studies and related findings should allay skepticism. For example, employment of multistage MS (MS–MS and MS–MS–MS) to examine the effects of pregnancy and of central angiotensin (Ang) II infusion on EO in rat plasma led to the discovery of 2 novel EO isomers.^8,9 Isomer 1 has MS–MS and MS–MS–MS product ion spectra indistinguishable from those of EO, but is slightly more polar than EO; it binds to the antibody used in our radioimmunoassay. Isomer 2 is slightly less polar than EO, has a distinct MS–MS–MS spectrum, and cross reacts weakly in our radioimmunoassay. The primary structural difference(s) between EO and these isomers may involve the steroid nucleus. Importantly, neither isomer was previously described or is detectable in commercial (plant) ouabain.^8,9"