Letter to the editor: Comments on Blaustein (2018): “The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na⁺ pump endocrine system”

Abstract

In a recent article (2), Dr. Blaustein reiterates his hypothesis that a compound found in some plantsouabain-is identically present also in human circulation, originating from putative endogenous sources. In Table 2A of his article, Dr. Blaustein cites previous articles on several spectroscopic studies. In these studies, various mammalian materials were submitted to complex sample preparation procedures, in part including chemical derivatization to obtain mass or NMR spectra. These spectra were reported compatible with the presence of ouabain in the sample materials. While acceptable for generating hypotheses, these data are far from unequivocal identification of a compound. The main hypothesis generated from this previous work was that ouabain circulates in human blood in concentrations that make a biological function likely. At this point, a hypothesis generated based on qualitative spectroscopic data had to be confirmed by highly accurate and thoroughly validated quantitative spectrometric data.Atmospheric pressure ionization coupled to triple-stage mass spectrometry (API-MS/MS) is for more than two decades now the technology of choice for the sensitive and specific quantification of well-defined target analytes in complex biological matrices. The technology is widely available in research institutions worldwide now and state-of-the-art in quantitative biomedical analytics. Ouabain pure compound preparation is commercially available. It is an ideal target for API-MS/MS analyses with good ionization properties.Based on the reports by Blaustein and others, we decided in 2013 to develop a stable-isotope dilution API-MS/MS method for the quantification of ouabain in human plasma. We were able to develop a method of extremely high sensitivity detecting spiked ouabain in human plasma. The method was fully validated according to FDA guidelines and published in the official journal of the International Federation of Clinical Chemistry after peer review (1). Using this method, we did not detect ouabain within the calibration range of the method (1.7-172 pmol/l) in any of the analyzed unspiked human plasma samples.In his recent review, Dr. Blaustein now declares (2) a very minor region of increased background noise in a MS/MSchromatogram from our work (4) with a clearly distinct retention time compared with ouabain to represent a putative isomer of ouabain (footnote to Table 2).

Na+, K+-ATPase Signaling and Bipolar Disorder

“…Although this steroid was found in human plasma and urine more than 25 years ago, its exact structure is still under debate. Some claim that the endogenous ouabain is indistinguishable from the plant steroid [ 51 , 59 ], others maintain that the mass spectrum data relating to the endogenous steroid do not support this conclusion [ 60 , 61 , 62 ]. Clearly, additional analytical studies are required to solve this dispute.…”