Taro Nonomura

Synthesis and Evaluation of Lysophosphatidylserine Analogues as Inducers of Mast Cell Degranulation. Potent Activities of Lysophosphatidylthreonine and Its 2-Deoxy Derivative

In response to various exogenous stimuli, mast cells (MCs) release a wide variety of inflammatory mediators stored in their cytoplasmic granules and this release initiates subsequent allergic reactions. Lysophosphatidylserine (lysoPS) has been known as an exogenous inducer to potentiate histamine release from MCs, though even at submicromolar concentrations. In this study, through SAR studies on lysoPS against MC degranulation, we identified lysoPT, a threonine-containing lysophospholipid and its 2-deoxy derivative as novel strong agonists. LysoPT and its 2-deoxy derivative induced histamine release from MCs both in vitro and in vivo at a concentration less than one-tenth that of lysoPS. Notably, lysoPT did not activate a recently proposed lysoPS receptor on MCs, GPR34, demonstrating the presence of another undefined receptor reactive to both lysoPS and lysoPT that is involved in MC degranulation. Thus, the present strong agonists, lysoPT and its 2-deoxy derivative, will be useful tools to understand the mechanisms of lysoPS-induced activation of degranulation of MCs.

Long-range carbon-proton coupling constants for stereochemical assignment of acyclic structures in natural products: Configuration of the C5C9 portion of maitotoxin

Abstract Long-range carbon-proton coupling constants (2,3JC,H) were measured for maitotoxin (MTX), one of the largest natural non-biopolymers, by hetero-half filter experiments and phase-sensitive HMBC with use of 9 mg of a 4% 13C-enriched sample. The necessary coupling constants within the terminal acyclic portions of MTX, where NOE analysis was not successful owing to the presumed coexistence of multiple conformers, were thus obtained for the resultant elucidation of relative configurations for the acyclic stereogenic centers to be 5R ∗ , 7R ∗ , 8R ∗ , and 9S ∗ .

Synthesis and stereochemical confirmation of the cis-fused L/M and N/O ring systems of maitotoxin

Stereocontrolled synthesis of cis-fused 1,6-dioxadecalin system 1, which corresponds to the L/M and N/O rings of maitotoxin, was accomplished. Comparison of its 1H and 13C NMR data with those of the natural toxin established the earlier stereochemical assignments.

Inhibition of maitotoxin-induced Ca2+ influx in rat glioma C6 cells by brevetoxins and synthetic fragments of maitotoxin

Abstract: 45Ca2+ influx in rat glioma C6 cells induced by 0.3 nM maitotoxin (MTX) was markedly inhibited by brevetoxin A (PbTx1) and brevetoxin B (PbTx2), with EC50 values of 16 and 13 µM, respectively. This inhibition was observed immediately after addition of MTX when monitored with fura‐2, which suggests that PbTx2 directly blocks the action of MTX. This blockade by PbTx2 was not affected by tetrodotoxin, which excludes the involvement of voltage‐sensitive sodium channels. The depolarizing effects of these brevetoxins were also not a likely cause of this inhibition, because melittin, a channel‐forming peptide, did not significantly block MTX‐induced 45Ca2+ influx. Instead, this inhibition was thought to be mediated by blockade of an MTX‐binding site by the brevetoxins, based on the fact that these toxins, particularly PbTx2, closely mimic the partial structure of MTX. Synthetic fragments of MTX corresponding to the hydrophilic EF‐GH rings (200 µM) and LM‐NO rings (500 µM) of MTX significantly reduced MTX‐elicited Ca2+ influx. The observation that the effects of MTX were inhibited by structural mimics of both its hydrophobic and hydrophilic portions implies that both portions of the MTX molecule recognize its target.

Synthetic approach toward complete structure determination of maitotoxin. stereochemical assignment of the C63-C68 acyclic linkage

Abstract The relative configuration of carbons within the C63–C68 acyclic linkage of maitotoxin (MTX) was assigned by synthesis of stereodefined model compounds ( 1a - 1d ) and their comparison with MTX in 1 H and 13 C NMR spectra.