Masayuki Kuniyoshi

Toxins associated with medicinal and edible seaweeds

Toxins associated with medicinal and edible seaweeds are reviewed with an emphasis on chemistry. The red alga Digenea simplex has been used for the treatment of roundworm disease for centuries. Its active principle is kainic acid . The related domoic acid is a constituent of another red alga, Chondria armata, used for the same purpose. These compounds known as kainoids are potent neurotoxins and excitatory amino acids. Kainoids are important tools in neurophysiological research. Domoic acids are also produced by diatoms and were responsible for the shellfish poisonings known as amnesic shellfish poisonings which occurred in Canada in 1987. Caulerpin and Caulerpicin have been described as toxic constituents of edible species of the green algal genus Caulerpa, but evidences in later studies indicate that they have no acute toxicity. Caulerpin, which has a structure related to auxin, promotes plant growth. Caulerpenyne, a toxic constituent of Caulerpa taxifolia and other inedible species, has been evaluated for its ecotoxicological effect in the Mediterranean where C. taxifolia bloomed explosively. Three different classes of compounds have been identified in the poisonings with species in the genus Gracilaria. They are prostaglandin E2 from G. verrucosa in Japan, aplysiatoxins and related compounds from G. coronopifolia in Hawaii, and polycavernosides from G. tsudai in Guam.

Bioactive phloroglucinols from the brown alga Zonaria diesingiana

Three phloroglucinols with a C-20 acyl side chain were isolated from marine brown alga Zonaria diesingiana. The structures were determined on the basis of NMR spectral analyses and comparison with data in the literature. They all showed antibacterial activity against Bacillus subtilis and Staphylococcus aureus and cytotoxic activity by inhibiting cell division in fertilized sea urchin eggs (Echinometra mathaei). These activities suggest their possible for pharmacological purposes. These phloroglucinols also showed toxicity against brine shrimp (Artemia salina), guppy fish (Poecilia reticulata), rice-land shrimp (Macrobrachium lanchesteri) and the diatom Chaetoceros gracilis. They probably act as chemical defenses against herbivores and also reduce surface fouling by epiphytic algae and larvae, suggesting their important roles in the marine ecosystem.

Diverse metabolites of coral reef organisms

A detailed mechanism for the spontaneous transformation of 2-chloro-1-hydroxyoctoda-3(8),5-dien-4-one, a metabolite of Portieria hornemanni, into 4,5-dimethylbenzo[b]furan was presented. Five new terpenoids have been isolated from the red alga Laurencia luzonensis. Five species of sponges yielded various metabolites: furanosesterterpenes from Ircinia sp., scalarane derivatives from Phyllospongia sp., polyketides from Theonella cf. swinhoei, a polyacetylene from Callyspongia sp., and cyclic depsipeptides from Suberites japonicus. Other new metabolites were sponge-derived secomanoalide derivative and imidazole alkaloids from nudibranchs, briarane class diterpenes from octocorals, and cyclofarnesylated hydroquinones, floresolides A-C, from an ascidian, Aplidium sp. Many of these compounds showed cytotoxicity.

Biotransformation of bromosesquiterpenes by marine fungi

Biotransformation of bromosesquiterpenes was investigated with two types of fungi, Rhinocladiella atrovirens NRBC 32362 and also Rhinocladiella sp. K-001, isolated from the Okinawan brown alga Stypopodium zonale. R. atrovirens NRBC 32362 converted aplysistatin 1 into three compounds 5alpha-hydroxyaplysistatin 4, 5alpha-hydroxyisoaplysistatin 5 and 9beta-hydroxyaplysistatin 6. Transformation of 1, palisadin A 2 and 12-hydroxypalisadin B 3 by Rhinocladiella sp. K-001 gave two compounds, 3,4-dihydroaplysistatin 7 and 9,10-dehydrobromopalisadin A 8.

3-Bromobarekoxide, an unusual diterpene from Laurencia luzonensis

A new, rare diterpene characterized by a cycloheptane ring trans-fused to a trans-decalin moiety was isolated from Laurencia luzonensis and its absolute structure determined by X-ray crystallography.