Masaki Hashimoto

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.

Maitotoxin-photoactive probe binds to membrane proteins in blood cells

- The photoactive and biotinylating ligand was prepared from MTX and maleimide-conjugated Hatanaka reagent with use of Diels-Alder reaction. Blood cells were subjected to affinity labelling experiments using the ligand thus obtained. The labelled band on SDS-PAGE was replaced not by MTX but by brevetoxin B (PbTx2), which suggested the presence of binding proteins in blood cells. Screening of polyether compounds for MTX inhibitory activity using Ca 2+ flux assays in C6 cells disclosed that a synthetic fragment of the hydrophilic portion of MTX inhibited the MTX activity.

In situ infrared spectroscopic investigation of 12-tungstphosphoric acid and its cesium hydrogen salts in relation to catalytic activity

The nature of hydroxyl groups of H 3 PW 12 O 40 and Cs x H 3- x PW 12 O 40 has been investigated by means of IR spectroscopy. Novel behavior of protons in HPW·nH 2 O was observed with the change of the amount of water molecules, n, and the acidic OH band was identified. A good correlation between the surface acidity and the catalytic activity was confirmed for the Cs hydrogen salts by low temperature CO adsorpton.

Selective Oxidation of Hydrocarbons with Molecular Oxygen Catalyzed by Transition-Metal-Substituted Silicotungstates

A review on the synthesis and liq.-phase selective oxidn. catalysis by ﾎｳ-SiW10{Fe(OH2)}2O386- with O2. This Keggin-type di-Fe-substituted silicotungstate was prepd. by the reaction of the lacunary [ﾎｳ-SiW10O36]8- with Fe(NO3)3 in an acidic aq. soln. and isolated as the Bu4N salt. The synthesized compd. showed the highest turnover no. for the oxidn. of cyclohexane with 1 atm O among Fe- and tri-transition-metal-substituted silicotungstates tested. [on SciFinder(R)]