ton species Lioluphura japonica incorporates 14C_
acetate not into cholesterol but into cholest-7-en3~-01; the il7 -sterol is also produced from labelled
cholesterol. In the same way, the 24-alkylated il7_
sterols, present as subsidiary components, arise
from il 5 -sterols in the diet [117]. As in other invertebrates, the molluscs also have steroids of the
vertebrate hormone type. The production of testosterone from cholesterol has been demonstrated in tracer experiments on the gonads of the
cephalopod Sepia ofiicinalis [37, 95]. Progesterone, testosterone and oestrone have been identified in the free-living nematode Turbatrix aceti
[69].
16.2.4 Sterols of the Echinoderms
The principal sterols of the echinoderms are CZ7
sterols; il 5 -sterols, such as cholesterol, occur
in the echinoids, ophiurids and crinoids,
and il 7 -sterols, such as 5a-cholest-7-en-3~-01
(Fig. 16.6b), are present in the asteroids and
holothurians. The holothurians also have 5acholestan derivatives. In particular, the fraction
of saturated and il 5 -unsaturated Cz7 sterols is
often more than 50 % sulphate esterified, and
there are large differences between the spectra of
the free and the sulphated sterols. For example,
in the sea cucumber Psolus fabricii 14a-methyl5a-cholest-9(11)-en-3~-01 (Fig. 16.6 c) makes up
60-70 % of the free sterols but only 10 % of the
sulphate esters; the latter are mainly 5acholestan-3B-ol [38]. Sterol sulphate esters have
also been found in trace amounts in other invertebrates and the mammals; only in the echinoderms are they found in large amounts. Their
biological significance is, however, still something
of a puzzle. In addition to the C27 sterols,
the echinoderms also have CZ6 sterols, such as
24-norcholesta-5,22-dien-3B-ol and asterosterol
(Fig. 16.6 d-e) , as well as more conventional CZ8
and CZ9 sterols. As these animals can synthesize
C27 but not CZ8 or CZ9 sterols, the latter undoubtedly originate from the diet. The sterol spectrum
of the echinoderms, like that of the other marine
invertebrates, can be extremely complex. For
example, 29 sterols have been identified in the
holothurian Stichopus japonicus; these include
68 % sulphate esters and also ~-xylosides and sterol ethers [60].
The ability to synthesize sterols has been
shown by tracer experiments in representatives of
all five echinoderm classes. Echinus esculentus
incorporates mevalonic acid into squalene, lanos16.2.4 Sterols of the Echinoderms
633
terol and desmosterol; in experiments with Asterias rubens and radioactive mevalonate or cholesterol, 5a-cholest-7-en-3B-ol was most heavily
labelled. The conversion of il 5 _ to il 7 -sterols can
occur by two different pathways; the pathway via
the 4-en-3-one compound and 5a-stanol has been
demonstrated in tracer experiments; the pathway
via a il5,7-sterol is, as yet, just a probability [37].
Steroids corresponding to the vertebrate steroid
hormones have also been detected in the echinoderms. These substances are apparently endogenous in origin; for example, extracts from the starfish Asterias rub ens converted cholesterol and
other sterols in vitro to progesterone, testosterone and 11-deoxycorticosterone. Both of the
pathways for steroid biosynthesis recognized in
the vertebrates, the il4 and il 5 pathways (p. 639),
are also active in Asterias [124] . Asterias and
other echinoderms contain estrogen but, as for
other invertebrates, they are not able to synthesize this steroid [37].
Numerous pharmacologically active steroid
glycosides and steroid saponins are known from
the echinoderms. The steroid glycosides of the
starfish are termed (astero-)saponins because
they produce soap-like solutions at very low
concentrations; they have haemolytic and
membrane-toxic detergent activity. Polyhydroxy
sterols in the starfish are mainly found in the
saponins but have also been reported as free sterols, e.g. 5a-cholestan-hexol, -heptol and -octol
neurotoxins (Fig. 16.6 f) in the Pacific asteroid
Protoreaster nodosus, and 5~-cholestan-pentol
bissulphate (Fig. 16.6 g) in the ophiurid Ophioderma longicaudum [97, 98]. The asterosaponins
are always mixtures which vary in composition
between the individual organs of an animal
[36]. Usually they are .L\9(1l)-unsaturated, 3B,6ahydroxylated 23-oxosteroids with a carbohydrate
chain on the 6-hydroxyl and a sulphate residue on
the 3-hydroxyl group. 24-0-glycosylated asterosaponins are also found [121]. Hydrolysis of the
asterosaponins almost always yields asterone
(Fig. 16.7a), which is now accepted as an isolation
artefact. The neutral aglycone is thornasterol A
(Fig. 16.7b), and there are apparently others [14].
The carbohydrate chain on the 6-hydroxyl group
consists of five to six sugars, in particular Dquinovose as well as D-fucose, D-xylose and Dgalactose [81]. Species-specific mixtures of highly
toxic steroid saponins are found in almost all
holothurians; they are stored in the skin glands
and the so-called Cuvier organs, sticky tubes that
at the approach of danger can be ejected from the
body cavity. To date, at least 19 different aglyco-
acetate not into cholesterol but into cholest-7-en3~-01; the il7 -sterol is also produced from labelled
cholesterol. In the same way, the 24-alkylated il7_
sterols, present as subsidiary components, arise
from il 5 -sterols in the diet [117]. As in other invertebrates, the molluscs also have steroids of the
vertebrate hormone type. The production of testosterone from cholesterol has been demonstrated in tracer experiments on the gonads of the
cephalopod Sepia ofiicinalis [37, 95]. Progesterone, testosterone and oestrone have been identified in the free-living nematode Turbatrix aceti
[69].
16.2.4 Sterols of the Echinoderms
The principal sterols of the echinoderms are CZ7
sterols; il 5 -sterols, such as cholesterol, occur
in the echinoids, ophiurids and crinoids,
and il 7 -sterols, such as 5a-cholest-7-en-3~-01
(Fig. 16.6b), are present in the asteroids and
holothurians. The holothurians also have 5acholestan derivatives. In particular, the fraction
of saturated and il 5 -unsaturated Cz7 sterols is
often more than 50 % sulphate esterified, and
there are large differences between the spectra of
the free and the sulphated sterols. For example,
in the sea cucumber Psolus fabricii 14a-methyl5a-cholest-9(11)-en-3~-01 (Fig. 16.6 c) makes up
60-70 % of the free sterols but only 10 % of the
sulphate esters; the latter are mainly 5acholestan-3B-ol [38]. Sterol sulphate esters have
also been found in trace amounts in other invertebrates and the mammals; only in the echinoderms are they found in large amounts. Their
biological significance is, however, still something
of a puzzle. In addition to the C27 sterols,
the echinoderms also have CZ6 sterols, such as
24-norcholesta-5,22-dien-3B-ol and asterosterol
(Fig. 16.6 d-e) , as well as more conventional CZ8
and CZ9 sterols. As these animals can synthesize
C27 but not CZ8 or CZ9 sterols, the latter undoubtedly originate from the diet. The sterol spectrum
of the echinoderms, like that of the other marine
invertebrates, can be extremely complex. For
example, 29 sterols have been identified in the
holothurian Stichopus japonicus; these include
68 % sulphate esters and also ~-xylosides and sterol ethers [60].
The ability to synthesize sterols has been
shown by tracer experiments in representatives of
all five echinoderm classes. Echinus esculentus
incorporates mevalonic acid into squalene, lanos16.2.4 Sterols of the Echinoderms
633
terol and desmosterol; in experiments with Asterias rubens and radioactive mevalonate or cholesterol, 5a-cholest-7-en-3B-ol was most heavily
labelled. The conversion of il 5 _ to il 7 -sterols can
occur by two different pathways; the pathway via
the 4-en-3-one compound and 5a-stanol has been
demonstrated in tracer experiments; the pathway
via a il5,7-sterol is, as yet, just a probability [37].
Steroids corresponding to the vertebrate steroid
hormones have also been detected in the echinoderms. These substances are apparently endogenous in origin; for example, extracts from the starfish Asterias rub ens converted cholesterol and
other sterols in vitro to progesterone, testosterone and 11-deoxycorticosterone. Both of the
pathways for steroid biosynthesis recognized in
the vertebrates, the il4 and il 5 pathways (p. 639),
are also active in Asterias [124] . Asterias and
other echinoderms contain estrogen but, as for
other invertebrates, they are not able to synthesize this steroid [37].
Numerous pharmacologically active steroid
glycosides and steroid saponins are known from
the echinoderms. The steroid glycosides of the
starfish are termed (astero-)saponins because
they produce soap-like solutions at very low
concentrations; they have haemolytic and
membrane-toxic detergent activity. Polyhydroxy
sterols in the starfish are mainly found in the
saponins but have also been reported as free sterols, e.g. 5a-cholestan-hexol, -heptol and -octol
neurotoxins (Fig. 16.6 f) in the Pacific asteroid
Protoreaster nodosus, and 5~-cholestan-pentol
bissulphate (Fig. 16.6 g) in the ophiurid Ophioderma longicaudum [97, 98]. The asterosaponins
are always mixtures which vary in composition
between the individual organs of an animal
[36]. Usually they are .L\9(1l)-unsaturated, 3B,6ahydroxylated 23-oxosteroids with a carbohydrate
chain on the 6-hydroxyl and a sulphate residue on
the 3-hydroxyl group. 24-0-glycosylated asterosaponins are also found [121]. Hydrolysis of the
asterosaponins almost always yields asterone
(Fig. 16.7a), which is now accepted as an isolation
artefact. The neutral aglycone is thornasterol A
(Fig. 16.7b), and there are apparently others [14].
The carbohydrate chain on the 6-hydroxyl group
consists of five to six sugars, in particular Dquinovose as well as D-fucose, D-xylose and Dgalactose [81]. Species-specific mixtures of highly
toxic steroid saponins are found in almost all
holothurians; they are stored in the skin glands
and the so-called Cuvier organs, sticky tubes that
at the approach of danger can be ejected from the
body cavity. To date, at least 19 different aglyco-