Kazuo Tachibana

Melittin-Like Peptides from the Shark-Repelling Defense Secretion of the Sole Pardachirus pavoninus

Three ichthyotoxic peptides, pardaxins P-I to P-3, have been isolated from the defense secretion of the sole Pardachirus pavoninus. Pavoninins, the steroid glycosides with shark-repelling ability, had previously been isolated therefrom. Each pardaxin consists of 33 amino acid residues having a distinctly hydrophilic carboxyl terminal region and a predominantly hydrophobic remainder; the pardaxin is thus strongly surfactant. These peptides show marked physical and pharmacological similarities to melittin, the major active constituent of bee venom, yet they lack sequence homology. They are probably also responsible for the predator-repelling property of the sole.

Isolation and chemical structure of amphidinol 2, a potent hemolytic compound from marine dinoflagellate Amphidinium klebsii

Abstract A new polyene-polyhydroxy compound, amphidinol 2 ( 2 ), was isolated as a potent hemolytic and antifungal agent from a cultured strain of dinoflagellate Amphidinium klebsii . The structure elucidated by spectroscopic methods turned out to be partly analogous to amphidinol 1 ( 1 ), which had been reported from the different strain of A. klebsii .

Regioselective Carbon–Carbon Bond Formation in Proteins with Palladium Catalysis; New Protein Chemistry by Organometallic Chemistry

Palladium‐catalyzed reactions have contributed to the advancement of many areas of organic chemistry, in particular, the synthesis of organic compounds such as natural products and polymeric materials. In this study, we have used a Mizoroki–Heck reaction for site‐specific carbon–carbon bond formation in the Ras protein. This was performed by the following two steps: 1) the His6‐fused Ras protein containing 4‐iodo‐L‐phenylalanine at position 32 (iF32‐Ras‐His) was prepared by genetic engineering and 2) the aryl iodide group on the iF32‐Ras‐His was coupled with vinylated biotin in the presence of a palladium catalyst. The biotinylation was confirmed by Western blotting and liquid chromatography–mass spectrometry (LC‐MS). The regioselectivity of the Mizoroki–Heck reaction was furthermore confirmed by LC‐MS/MS analysis. However, in addition to the biotinylated product (bF32‐Ras‐His), a dehalogenated product (F32‐Ras‐His) was detected by LC‐MS/MS. This dehalogenation resulted from the undesired termination of the Mizoroki–Heck reaction due to steric and electrostatic hindrance around residue 32. The biotinylated Ras showed binding activity for the Ras‐binding domain as its downstream target, Raf‐1, with no sign of decomposition. This study is the first report of an application of organometallic chemistry in protein chemistry.

New strategy for convergent synthesis of trans-fused polyether frameworks based on palladium-catalyzed suzuki cross-coupling reaction

Abstract A new strategy for convergent synthesis of trans -fused polyethers based on palladium(0)-catalyzed Suzuki cross-coupling reaction of alkylboranes with cyclic enol triflates has been developed. The present method allows to construct polyether frameworks rapidly and efficiently.

Site‐Specific Functionalization of Proteins by Organopalladium Reactions

A new carbon–carbon bond has been regioselectively introduced into a target position (position 32 or 174) of the Ras protein by two types of organopalladium reactions (Mizoroki–Heck and Sonogashira reactions). Reaction conditions were screened by using a model peptide, and the stability of the Ras protein under the reaction conditions was examined by using the wild‐type Ras protein. Finally, the iF–Ras proteins containing a 4‐iodo‐L‐phenylalanine residue were subjected to organopalladium reactions with vinylated or propargylated biotin. Site‐specific biotinylations of the Ras protein were confirmed by Western blot and LC‐MS/MS.

Synthetic studies on a marine polyether toxin, gambierol: stereoselective synthesis of the EFGH ring system via B-alkyl Suzuki coupling

Abstract A synthetic route to the EFGH ring system ( 3 ) of gambierol ( 1 ), a marine polyether toxin isolated from the dinoflagellate Gambierdiscus toxicus , has been developed. The present synthesis features convergent coupling of the F and H rings followed by ring-closure of the G ring based on the B -alkyl Suzuki reaction of lactone-derived enol phosphates. An angular methyl group at C23 was stereoselectively introduced by treatment of sulfone 32 with trimethylaluminum. Installation of a tertiary alcohol at C21 was accomplished through stereoselcetive dihydroxylation of exo -methylene 36 followed by selective formation of the primary p -toluensulfonate and treatment of the resultant monotosylate 40 with lithium aluminum hydride. Finally, formation of the E ring as a lactone form completed the synthesis of 3 .

A general strategy for the convergent synthesis of fused polycyclic ethers via B-alkyl Suzuki coupling: synthesis of the ABCD ring fragment of ciguatoxins

Abstract A new method for convergent coupling of fused polycyclic ethers has been developed, which relies on B -alkyl Suzuki cross-coupling of lactone-derived enol triflates or phosphates. The strategy was successfully applied to a convergent synthesis of the ABCD ring fragment 4 of ciguatoxins, the causative toxin for ciguatera fish poisoning. The synthetic route includes a convergent union of the B and D rings ( 18 and 29c , respectively) by the B -alkyl Suzuki coupling, introduction of a double bond into the D ring followed by reductive closure of the tetrahydropyran C ring to afford the BCD ring system 51 , and, finally, ring-closing metathesis reaction to construct the oxepene A ring.

Synthesis of the FGH ring fragment of ciguatoxin

Abstract Stereoselective synthesis of the FGH ring fragment of ciguatoxin is described. The key steps in the present synthesis are an intramolecular radical cyclization to construct oxepane ring G and ring-closing metathesis reaction to construct hexahydrooxonin ring F.

Cholesterol-independent membrane disruption caused by triterpenoid saponins

The membrane-disrupting activity of 15 triterpenoid saponins, obtained from Chinese plants of the genus Aralia, was investigated using phosphatidylcholine liposomes with and without cholesterol. The permeability of the membrane was examined by monitoring the induced fluorescent dye release from the liposome. On the basis of the obtained results, the structure-activity relationship among glucuronides of oleanolic acid was discussed. This takes into account particularly the variation in the carboxyl function. Namely, the saponins could induce a permeability change on liposomal membrane without cholesterol when they are glycosylated at both C-3 and C-28 of the oleanolic acid. There also exists a great similarity in the time-course curves for dye-release within such saponins, reflecting their similar action with the lipid bilayer membrane. The saponins glycosylated only at C-3 could also exhibit the same activity with somewhat different action profiles when the glucuronic acid is esterified, while those with the free glucuronic acid required cholesterol in the liposomes to induce permeability change thereof.

Pavoninins: shark-repelling ichthyotoxins from the defense secretion of the pacific sole.

A series of ichthyotoxic and hemolytic steroid aminoglycosides, pavoninins-1, to -6, has been isolated from the defense secretion of the sole Pardachirus pavoninus, and their respective chemical structures have been established by spectroscopic studies and chemical conversions. The pavoninins exert repellent activity against sharks and are considered to be the factors responsible for the predator-repelling property of the sole.