Analyses of Biologically Active Steroids: Antitumor Active OSW-1 andCardiotonic Marinobufotoxin, by Matrix-Assisted Laser Desorption/Ionization Quadrupole Ion Trap Time-of-Flight Tandem MassSpectrometry

摘要

Naturally occurring constituents of biological or pharmaceutical interest often exist in the form of glyco-sides or conjugates. Mass spectral investigations of these compounds require soft ionization techniques if infor-mation on molecular mass, sugar sequence, or conjugate content is desired. In this study, matrix-assisted laserdesorption/ionization (MALDI) quadrupole ion trap (QIT) time-of-flight tandem mass spectrometry (TOF-MS")was used to identify both OSW-1, an acetylated cholestane diglycoside showing antitumor activity, and the car-diotonic steroid, bufotoxin. Each molecular-related ion was identified, and subsequent collision-induced dissocia-tion experiments in which a molecular-related ion was selected as a precursor ion produced the characteristicproduct ions that are essential for structural elucidation. OSW-1 and its analogue with a modified side chain,thienyl OSW-1, were synthesized, and bufotoxins, marinobufotoxin and its homologue, marinobufagin 3-pimeloylarginine ester, were isolated from toad venom. On MALDI-TOF-MS, sodium-adduct [M+Nar ionswere observed in the steroid glycosides, although protonated IM+Hr ions were relatively more abundant thansodium-adduct [M+Na]+ ions in the bufotoxins. On the basis of tandem MS results, we propose key fragmenta-tion pathways. The sugar moiety or side chain from the precursor ion was eliminated in OSW-1. However, char-acteristic product ions originating from the cleavage of the side chain with an ester formation were observed inthe bufotoxins. Post-source decay (PSD) on MALDI-TOF-MS is also described when evaluating a-cyano-4-hy-droxycinnamic acid or 2,5-dihydroxybenzoic acid as a matrix to obtain useful ions required for the identificationof compound.