A novel endogenous digitalis, telocinobufagin, exhibits elevated plasma levels in patients with terminal renal failure

Backround and objectives: There are several potential endogenous digitalis-like factors (EDLF) in mammalian body fluids, and marinobufagenin (MBG) may be the most potent EDLF. Improved assays are needed to confirm the potency of these metabolites. In the present study, we have identified MBG and telocinobufagin (TCB) in human plasma by high-resolution mass spectrometry (MS) and nuclear magnetic resonance (NMR). Methods and results: The high-resolution MS analysis revealed the molecular masses of TCB and MBG to be the same as their respective theoretical values. Using a tandem mass spectrometer, the mass–charge ratio for TCB was determined to be 403.2 for the parent ion and 349.2 for the daughter ion. The mass–charge ratio for MBG was m/z 383.2 and m/z 401.2. The NMR study revealed that the signals for MBG and TCB were the same as those obtained by MS analysis. In human blood, MBG and TCB were also identified by liquid chromatography (LC) as well as MS. In the LC/MS assay, proscillaridin A was used as an internal standard. The plasma was pretreated with Sep-Pak C18, and then 50 μL was applied to the C8 high-performance liquid chromatography (HPLC) column. The mean plasma concentration of MBG in healthy volunteers (0.94 ± 0.28 ng/mL) was significantly lower than that in patients undergoing regular hemodialysis (3.81 ± 1.92 ng/mL). The concentration of TCB in the healthy volunteers (1.80 ± 0.55 ng/mL) was also significantly lower than that in patients with terminal renal failure (6.86 ± 4.30 ng/mL). Conclusion: These results indicate that the major EDLF is TCB because its plasma concentration is the highest among the reported endogenous digitalis candidates.