150
4. Nitrogenous Compounds
Tsuda (1966) also documents the seasonal variation of fugu toxicity, which
may well be related to the sexual cycle of the puffers. Toxicity is highest
during the winter months, which coincides with the prespawning period as
well as the time when the fish tastes best. But even during the time of greatest
toxicity only about half of the tested specimens were toxic. Of some thirtyseven species of puffers that are recognized around Japan only about six
are considered food fishes and among these torafugu, Fugu rubripes rubripes,
is held in highest esteem. Because of ever more effective control measures
intoxications and fatalities have been steadily decreasing in Japan.
Although an impure toxic fraction, then called tetrodotoxin, was first
obtained by Tawara (1909), pure crystalline toxin, then named spheroidine,
was isolated only in 1948 by Yokoo. Tsuda and Kawamura (1952) crystallized the toxin independently and resumed the earlier name tetrodotoxin.
This was the beginning of Tsuda's extensive researches on the structure of
tetrodotoxin. Other groups that carried out independent structural investigations were those of Professor Hirata at Nagoya, Professor Woodward at
Harvard, and Professor Mosher at Stanford. All four groups concluded the
structural research successfully and about at the same time. It was a nice and
fitting coincidence that the full details of the tetrodotoxin structure were
first disclosed in the spring of 1964 at the Third International Symposium on
the Chemistry of Natural Products held in Kyoto, Japan. All four groups
have published details of their researches (Tsuda et al, 1964; Tsuda, 1966;
Goto et al, 1965; Woodward, 1964; Mosher et al, 1964).
b. Structure
Tetrodotoxin is a colorless weakly basic optically active, crystalline substance that is virtually insoluble in all but acidic media. Its exact elemental
composition, CnH 1 7 N 3 0 8 , was in doubt for a long time because the toxin
retains moisture and solvents easily and effectively. Its composition shares
with saxitoxin the interesting property that the number of oxygen and nitrogen
atoms equals or exceeds the number of carbon atoms. The composition of
the two marine toxins differ from each other, however, in that saxitoxin is
heavily nitrogenous while tetrodotoxin is highly oxygenated.
Woodward (1964) demonstrated that the three nitrogen atoms of tetrodotoxin are present in the molecule as a guanidine moiety by isolating guanidine
picrate in yields exceeding 30% following vigorous oxidation of the toxin
with aqueous sodium permanganate at 75°.
A wide variety of drastic degradations (Woodward, 1964; Tsuda et al,
1962a,b,c)—warm aqueous sodium hydroxide, pyridine-acetic anhydride
followed by vacuum pyrolysis, phosphorus and hydrogen iodide followed by
potassium ferricyanide, and concentrated sulfuric acid—all yielded closely
related quinazoline derivatives of structure 108, where the nature of R and
4. Nitrogenous Compounds
Tsuda (1966) also documents the seasonal variation of fugu toxicity, which
may well be related to the sexual cycle of the puffers. Toxicity is highest
during the winter months, which coincides with the prespawning period as
well as the time when the fish tastes best. But even during the time of greatest
toxicity only about half of the tested specimens were toxic. Of some thirtyseven species of puffers that are recognized around Japan only about six
are considered food fishes and among these torafugu, Fugu rubripes rubripes,
is held in highest esteem. Because of ever more effective control measures
intoxications and fatalities have been steadily decreasing in Japan.
Although an impure toxic fraction, then called tetrodotoxin, was first
obtained by Tawara (1909), pure crystalline toxin, then named spheroidine,
was isolated only in 1948 by Yokoo. Tsuda and Kawamura (1952) crystallized the toxin independently and resumed the earlier name tetrodotoxin.
This was the beginning of Tsuda's extensive researches on the structure of
tetrodotoxin. Other groups that carried out independent structural investigations were those of Professor Hirata at Nagoya, Professor Woodward at
Harvard, and Professor Mosher at Stanford. All four groups concluded the
structural research successfully and about at the same time. It was a nice and
fitting coincidence that the full details of the tetrodotoxin structure were
first disclosed in the spring of 1964 at the Third International Symposium on
the Chemistry of Natural Products held in Kyoto, Japan. All four groups
have published details of their researches (Tsuda et al, 1964; Tsuda, 1966;
Goto et al, 1965; Woodward, 1964; Mosher et al, 1964).
b. Structure
Tetrodotoxin is a colorless weakly basic optically active, crystalline substance that is virtually insoluble in all but acidic media. Its exact elemental
composition, CnH 1 7 N 3 0 8 , was in doubt for a long time because the toxin
retains moisture and solvents easily and effectively. Its composition shares
with saxitoxin the interesting property that the number of oxygen and nitrogen
atoms equals or exceeds the number of carbon atoms. The composition of
the two marine toxins differ from each other, however, in that saxitoxin is
heavily nitrogenous while tetrodotoxin is highly oxygenated.
Woodward (1964) demonstrated that the three nitrogen atoms of tetrodotoxin are present in the molecule as a guanidine moiety by isolating guanidine
picrate in yields exceeding 30% following vigorous oxidation of the toxin
with aqueous sodium permanganate at 75°.
A wide variety of drastic degradations (Woodward, 1964; Tsuda et al,
1962a,b,c)—warm aqueous sodium hydroxide, pyridine-acetic anhydride
followed by vacuum pyrolysis, phosphorus and hydrogen iodide followed by
potassium ferricyanide, and concentrated sulfuric acid—all yielded closely
related quinazoline derivatives of structure 108, where the nature of R and