Analyses of biologically active steroids: antitumor active OSW-1 and cardiotonic marinobufotoxin, by matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight tandem mass spectrometry.

Naturally occurring constituents of biological or pharmaceutical interest often exist in the form of glycosides or conjugates. Mass spectral investigations of these compounds require soft ionization techniques if information on molecular mass, sugar sequence, or conjugate content is desired. In this study, matrix-assisted laser desorption/ionization (MALDI) quadrupole ion trap (QIT) time-of-flight tandem mass spectrometry (TOF-MS(n)) was used to identify both OSW-1, an acetylated cholestane diglycoside showing antitumor activity, and the cardiotonic steroid, bufotoxin. Each molecular-related ion was identified, and subsequent collision-induced dissociation experiments in which a molecular-related ion was selected as a precursor ion produced the characteristic product ions that are essential for structural elucidation. OSW-1 and its analogue with a modified side chain, thienyl OSW-1, were synthesized, and bufotoxins, i.e., marinobufotoxin and its homologue, marinobufagin 3-pimeloylarginine ester, were isolated from toad venom. On MALDI-TOF-MS, sodium-adduct M+Na ions were observed in the steroid glycosides, although protonated M+H ions were relatively more abundant than sodium-adduct M+Na ions in the bufotoxins. On the basis of tandem MS results, we propose key fragmentation pathways. The sugar moiety or side chain from the precursor ion was eliminated in OSW-1. However, characteristic product ions originating from the cleavage of the side chain with an ester formation were observed in the bufotoxins. Post-source decay (PSD) on MALDI-TOF-MS is also described when evaluating alpha-cyano-4-hydroxycinnamic acid or 2,5-dihydroxybenzoic acid as a matrix to obtain useful ions required for the identification of compound.