Structural Analysis of Diastereomeric Cardiac Glycosides and Their Genins Using Ultraperformance Liquid Chromatography-Tandem Mass Spectrometry

Ultraperformance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS) is an economical and indispensable tool in natural product research to investigate novel metabolites, biomarker discovery, chemical diversity exploration, and structure elucidation. In this study, the structural analysis of 38 naturally occurring cardiac glycosides (CGs) in various tissues of Nerium oleander was achieved by the extensive use of mass spectrometry. The chemical diversity of CGs was described on the basis of characteristic MS/MS fragmentation patterns, accurate mass measurement, and published scientific information on CGs from Nerium oleander. It was observed that only six genins, viz., Δ¹⁶anhydrogitoxigenin, Δ¹⁶adynerigenin, gitoxigenin, oleandrigenin, digitoxigenin, and adynerigenine, produce 38 diverse chemical structures of CGs. Among them, 20 were identified as diastereomers having a difference in a sugar (l-oleandrose, β-d-diginose, and β-d-sarmentose) unit. However, the differentiation of diastereomeric CGs was not possible by only MS/MS fragments. Thus, the diastereomer's chromatographic elution order was assigned on the basis of the relative retention time (RRt) of two reference standards (odoroside A and oleandrin) among their diastereomers. Besides this, the in-source fragmentation of CGs and the MS/MS of m/z 325 and 323 disaccharide daughter ions also exposed the intrinsic structure information on the sugar units. The daughter ions m/z 162, 145, 113, 95, and 85 in MS/MS spectra indicated the abundance of l-oleandrose, β-d-diginose, and β-d-sarmentose sugars. At the same time, m/z 161, 143, 129, and 87 product ions confirmed the presence of a β-d-digitalose unit. As a result, the UPLC-ESI/TQD system was successfully utilized for the structure characterization of CGs in Nerium oleander tissues.

Ultra-specific immunoassays for small molecules: roles of wash steps and multiple binding formats

New immunometric forms of immunoassay are much more flexible to use than competitive-format immunoassays for small molecular analytes. An example of the utility of this flexibility is the ability to wash the capture antibody after it has been exposed to analyte but before addition of the labeled reagent. This simple maneuver has a large impact on the specificity obtained from already highly specific assays. We also show that specificity can be further increased by means of our multiple binding assay approach, in which the final reading reflects analyte binding to two different primary capture monoclonal antibodies.

Identification of two cardiac glycosides as Na(+)-pump inhibitors in rat urine and diet

Endogenous Na(+)-pump specific inhibitors are present in the plasma, urine, and tissues of humans and animals. To date, the source of these inhibitors has not been rigorously defined. In the present study, large amounts of several Na(+)-pump specific inhibitors have been demonstrated to exist in the urine of rats raised on a regular chow diet and tap water. All of the inhibitor levels have been found to increase 1.5-8-fold by the surgical preparation of reduced renal mass (RRM) and one-kidney, one-clip (IK, IC) hypertension. These urinary inhibitors, however, except for the ouabain-like inhibitor which eluted from a high performance liquid chromatography C18 column at the same retention time as [3H]ouabain, disappeared within a week after switching the diet from regular diet (number 5001, PMI Feeds, Inc.) to pure synthetic diet (number 5755). The urinary level of the ouabain-like inhibitor decreased to only one-half of the level in the control rat raised on a regular diet. Two of these inhibitors were purified from both urine and diet by a combination of Amberlite XAD-2 adsorption chromatography, reverse phase low pressure liquid chromatography, and several high performance liquid chromatographies. Reverse phase high performance liquid chromatography, liquid secondary ion and gas-liquid mass spectrometries, and proton nuclear magnetic resonance spectroscopy identified these inhibitors as a stereoisomer of convalloside, probably neoconvalloside, and a mono-rhamnoside of periplogenin or its stereoisomer. These cardiac glycosides exhibited inhibitory potencies comparable to ouabain against ouabain-displacement from Na+,K(+)-ATPase and against 86Rb uptake into human erythrocytes, and they also exhibited cross-reactivity to anti-ouabain antibodies and anti-digoxin antibodies. These results clearly demonstrate that the principal source of most of the inhibitors in rat urine is the diet. The results suggest that the ouabain-like inhibitor may be derived from an endogenous origin.

Retention behavior of cardiac steroids using cyclodextrin in the mobile phase in high-performance liquid chromatography

The retention behaviour of twenty cardiac steroids and four fluorescent derivatives was examined by the addition of cyclodextrin to the mobile phase in reversed-phase high-performance liquid chromatography. The addition of a suitable cyclodextrin improved the separation of isomeric cardiac steroids. The steroid A/B ring junction is the most important factor in the choice of the optimum cyclodextrin to be added; the C/D ring junction is less important. The hydroxyl group at the 3- or 12-position of the steroid enhanced the changes in retention times of these compounds. The effect of an unsaturated lactone ring at the 17 beta-position on the retention in the presence of cyclodextrin was also determined with cardenolide (five-membered ring) and bufadienolide (six-membered ring) but little difference was observed. Isomeric cardiac steroids, whose separation has not been done by the conventional method, were clearly separated by this method. The fluorescence intensity of 3-(1-anthroyl)-cardiac steroid was enhanced by the addition of cyclodextrin to the mobile phase.

Comparison of digitoxin and its major glucuronidated metabolite: inhibition of Na-K-ATPase and reactivity with the radioimmunoassay

The metabolism of digitoxin is known to occur through several major pathways including oxidation and glucuronidation. Several studies have compared the behavior of the various unconjugated metabolites with respect to reactivity toward Na-K-ATPase and toward the radioimmunoassay (RIA). Other studies with conjugated products synthesized chemically have also been performed. However, the activity of the conjugated products produced in vivo has not been reported and these metabolites are generally assumed to be inactive. In the present studies we have prepared and purified the glucuronide conjugate of digitoxigenin monodigitoxoside formed by rat liver microsomes and determined the activity of this metabolite as measured by inhibition of Na-K-ATPase and by the RIA. The concentration of the conjugated product needed to cause a fifty per cent inhibition of Na-K-ATPase was 5.40 microM compared to 0.68 for digitoxin and 0.08 for digitoxigenin monodigitoxoside. However, the conjugated product had a two-fold greater affinity for the antibody in the RIA procedure than did either digitoxin or digitoxigenin monodigitoxoside. Although these data cannot be extrapolated to the effects of these drugs on cardiac contractility, the results suggest that the contribution of the glucuronide conjugate to the therapeutic or toxic effect of digitoxin may be minimal. This metabolite may, however, lead to inaccurate estimation of blood levels by the RIA.

Gas-liquid chromatography of digitoxigenin and digoxigenin as acetates of their epoxygeninic acid methyl esters

Gas chromatographic analysis of ditigoxigenin and digoxigenin, the genins of the cardenolide glycosides digitoxin and digoxin, cannot be done without derivatization. However, during the derivatization, side-reactions often present serious problems. A procedure has been found for transforming digitoxigenin and digoxigenin into the corresponding acetates of their epoxygeninic acid methyl esters, which are stable compounds and suitable for gas chromatographic analysis.

Ring-D fragmentation of cardenolides

The major fragmentation paths typical for the cardenolides system have been elucidated by labelling studies.

Detection of lysergic acid diethylamide, delta-9-tetrahydrocannabinol and related compounds by plasma chromatography

Plasma chromatography as a method for ultratrace qualitative and quantitative detection of organic compounds is especially well suited for detection of gas chromatographic effluents. The optimum range of sample quantity is 10-6 to 10-12 g for detection and identification of a compound by use of its characteristic positive and negative mobility spectra. The type of reference mobility spectra produced by alkanes, aromatics, esters, halogenated compounds, nitrogenated compounds and organic acids have been previously reported. This study presents the reference mobility spectra produced for lysergic acid diethylamide (LSD), delta-9-tetrahydrocannabinol (delta-9-THC), digitoxigenin and several biochemical compounds of research significance. LSD and delat-9-THC in a mixture can be detected and identified by plasma chromatography positive mobility spectra in quantities of 10-7 g or less. All the compounds investigated in this study display strong MH-+ ions along with other ions primarily of the type (M)NO-+, (M)2H-+. None of these compounds exhibits negative mobility spectra.