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On the Structure of Endogenous Ouabain Proc. Natl. Acad. Sci. USA Vol. 96, pp. 6654–6659, June 1999 Biochemistry On the structure of endogenous ouabain AKIRA KAWAMURA*, JINSONG GUO*, YASUHIRO ITAGAKI*, CHARLES BELL†,YI WANG†,GARNER T. HAUPERT,JR.†, SHEILA MAGIL‡,REX T. GALLAGHER‡,NINA BEROVA*, AND KOJI NAKANISHI*§ *Department of Chemistry, Columbia University, New York, NY 10027; †Renal Unit, Medical Services, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129; and ‡BION, Inc., 840 Memorial Drive, Riverside Technology Center, Cambridge, MA 02139 Communicated by Gilbert Stork, Columbia University, New York, NY, April 19, 1999 (received for review February 15, 1999) ABSTRACT The ouabain-like sodium pump inhibitor in ATPase, and inhibition of ‘‘backdoor’’ phosphorylation of mammals (so-called ‘‘endogenous ouabain’’) has been consid- purified Na1,K1-ATPase (9), binding kinetics to kidney epi- ered a subtle structural isomer of ouabain. Its structural thelial cells and purified enzyme (10, 11), lipid bilayer perme- investigation, however, has long been hindered by the paucity ability (11), and reversible inotropic effect on cardiac myocytes of sample material. Our recent purification of endogenous (12). For the structural characterization of HIF, further puri- ouabain (3 mg) from bovine hypothalamus allowed the mea- fication was carried out by using affinity chromatography surement of its 1H-NMR. The obtained spectrum as well as combined with RP-HPLC (13). It turned out that the purified reexamination of past microscale structural studies on en- HIF was identical to ouabain by LCyMS. Furthermore, gly- dogenous ouabain led us to identify the purified material as cosidase treatment and acid hydrolysis showed HIF to be an ouabain in an unusual manner. It turned out that the struc- a-L-rhamnoside, as is ouabain; also, des-rhamnosyl HIF, the tural analysis had been complicated by a facile ouabain– aglycone, was indistinguishable from ouabagenin by LCyMS. borate complexation in borosilicate glassware. In retrospect, These findings led to attempts to differentiate HIF and plant it is not surprising that the polyhydroxylated ouabain mole- ouabain by using nanogram-scale chemical derivatization. cule serves as a polydentate ligand to inorganic species. In its Naphthoylation of HIF (300 ng) followed by RP-HPLC physiological environment, ouabain may exist as some un- showed that the major derivatization product from HIF eluted known complex. The chemical species giving rise to the slightly earlier than ouabain pentanaphthoate, the major prod- reported biological activities of hypothalamic inhibitory factor uct from ouabain naphthoylation; moreover, CD spectroscopy preparations remain to be clarified. revealed that this HIF derivative showed no distinct CD, whereas ouabain pentanaphthoate showed a clear positive CD 1 1 couplet. Although the molecular ion peak of this HIF deriv- Search for the endogenous ligands of Na ,K -ATPase has ative was not clear, the product was described as HIF pentan- been an enticing yet puzzling research subject for the last aphthoate (13). Subsequent microscale derivatization of OLC several decades (1–4). Putative endogenous sodium pump (400 ng, gift from Upjohn Laboratories) also gave the same inhibitors have been detected many times in various mamma- RP-HPLC and CD (‘‘zero-CD’’) profiles as those of HIF lian tissues and plasma. However, purification, structural pentanaphthoate (14). This led to the conclusion that the analysis, and physiological characterization of such compounds ouabain-like sodium pump inhibitor is an ouabain isomer, have been fraught with difficulties. Part of the reason for this often referred to as endogenous ouabain (2, 4, 5), even though problem is that the enzyme assays are susceptible to many the physiological discrepancies between HIF and other com- nonspecific interferences, which sometimes lead to false- pounds still remained to be clarified (2): the term endogenous positive results (1). Recognition of this problem led to the ouabain has been used to distinguish this compound from plant employment of multiple assay systems, which greatly reduced 1 1 ouabain (2, 4, 5), although its true origin has not yet been the risk of falsely detecting putative physiologic Na ,K - clarified. ATPase inhibitors. Nevertheless, progress is still hindered by Based on the chromatographic and spectroscopic informa- the extreme paucity of material available from tissues. 1 1 tion from the naphthoylation studies on endogenous ouabain, Among the putative endogenous inhibitors of Na ,K - a search for the isomer of ouabain pentanaphthoate with the ATPase are a group of compounds that are considered to be same HPLC and CD (zero-CD) profile was initiated. Two lines related to ouabain, a plant-origin cardiac glycoside (5). These of structural evidence, the fact that the HIF aglycone was compounds have been extracted from various animal tissues, indistinguishable from ouabagenin by LCyMS and that the and in several cases sufficient material, albeit submicrogram- sugar moiety of HIF was a-L-rhamnoside, suggested that HIF to-low microgram quantity, was purified to allow further could be an a-L-rhamnoside position isomer of ouabain. This chemical and physiological characterizations. The molecular led to the computation of CD spectra of all possible sugar mass appeared to be identical to ouabain by MS, and the position isomers of ouabain pentanaphthoate and determina- retention time of reversed-phase HPLC (RP-HPLC) was the tion of the validity of calculations by preparing as many same as ouabain. Moreover, two compounds, ouabain-like synthetic isomers as possible (15). The series of comparison compound (OLC) from human plasma (6) and ‘‘adrexin C’’ between theoretical and experimental data demonstrated that from bovine adrenal (7), were indistinguishable from plant theoretical CD calculation can fairly accurately reproduce ouabain by multiple biochemical and physiological criteria. experimental CD couplets of ouabain pentanaphthoate iso- On the other hand, a factor from bovine hypothalamus mers despite their substantial conformational flexibility. How- (hypothalamic inhibitory factor, HIF) showed different phys- ever, the theoretically calculated 18 isomers and experimen- iological profiles from ouabain in various assays, such as tally prepared 13 isomers showed that all searched isomers inhibition of ouabain-insensitive isoform (rat a1) of Na1,K1- 21 ATPase (8), inhibition of sarcoplasmic reticulum Ca - Abbreviations: HIF, hypothalamic inhibitory factor from bovine hy- pothalamus; OLC, ouabain-like compound from human plasma; RP- The publication costs of this article were defrayed in part by page charge HPLC, reversed-phase high performance liquid chromatography; ESI, electrospray (ionization); ROESY, rotating-frame nuclear Over- payment. This article must therefore be hereby marked ‘‘advertisement’’ in hauser effect spectroscopy; TFA, trifluoroacetic acid. accordance with 18 U.S.C. §1734 solely to indicate this fact. §To whom reprint requests should be addressed. e-mail: kn5@ PNAS is available online at www.pnas.org. columbia.edu. 6654 Downloaded by guest on October 1, 2021 Biochemistry: Kawamura et al. Proc. Natl. Acad. Sci. USA 96 (1999) 6655 have intense positive couplets (15). With no conceivable tion at 220 nm, was submitted to the 1H-NMR measurement, candidate among sugar position isomers of ouabain, our study in which signal was averaged by overnight accumulation. came to the point of reexamining the results obtained from the Preparation of Ouabain Tetrahedral-Borate. Method A. previous nanogram-scale naphthoylation of HIF and OLC Ouabain aqueous solution (1 mgorlessin100ml of water) was (13, 14). placed in a new borosilicate vial (a disposable vial, 1 dram), More recently, we succeeded in the purification of 3 mgof kept at ambient temperature overnight, and lyophilized to give HIF (‘‘3-mg HIF’’) with a modified HIF purification protocol a white powder. ESIMS showed distinct peaks at myz 633.24 that omitted cation and anion exchangers and substituted an (ouabain 1 11BO-H 1 Na)1 and at 632.26 (ouabain 1 10BO-H affinity column by using anti-digoxin antibody 26-10 (16, 17) 1 Na)1. ESIMSyMS of the peak at myz 633.24 gave a for immobilized Na1,K1-ATPase to improve the recovery of dominant fragment peak at myz 487.24 (ouabagenin 1 11BO-H HIF. The 1H-NMR of the purified HIF as well as the reex- 1 Na)1. It should be noted that a new disposable glass vial (1 amination of the past nanogram-scale naphthoylation study led dram) could contain soluble borate enough to convert as much to identify this 3-mg HIF as ouabain in an unusual manner. The as '20 mg of ouabain into its borate complex, although this past microscale structural analyses on endogenous ouabain depends on the kind of borosilicate glass. Rinsing a new vial (13, 14) are reevaluated in light of our current findings, and the removes most of the borate and other ionic species; however, implications of the current findings with respect to endogenous plastic vials should be used for long-term storage of low ouabain research are discussed. microgram or submicrogram quantities of ouabain. Small amounts of ouabain that have already been dried in borosili- MATERIALS AND METHODS cate glassware can be efficiently dissolved in 10% acetonitrile in water, and ouabain can be recovered by RP-HPLC using Materials. Ouabain octahydrate and sodium tetraborate 0.1% trifluoroacetic acid (TFA)-containing eluents. decahydrate were from Acros Organics (Pittsburgh), and all Method B. Sodium tetraborate (2.6 mg, 6.8 mmol) was added other chemicals and HPLC solvents were from Aldrich. Anti- to ouabain octahydrate (5 mg, 6.9 mmol) in water (1 ml), and digoxin antibody 26-10 (16, 17) was a gift from M. N. Margolies the solution was lyophilized. 1H- and 13C-NMR assignments at Harvard Medical School (Boston). Electrospray ionization are shown in Table 1. For the 1H-NMR measurement in acidic (ESI) MS was measured by using Micromass Q-Tof at Micro- pH (Fig. 4A), the obtained material (0.2 mg) was dissolved in mass (Manchester, U.K.) and the Suntory Institute for Bioor- 10% acetonitrile-d3 in deuterium oxide containing 0.1% TFA. ganic Research (Osaka).
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