16 Sterols and Steroids
16.1
16.2
16.2.1
16.2.2
16.2.3
16.2.4
16.2.5
16.2.6
16.2.7
16.3
Biosynthesis of Cholesterol
Sterols of Different Animal Groups
Sterols of the Vertebrates
Sterols of the Insects
Sterols ofTunicates, Annelids, Crustaceans
and Molluscs
Sterols of the Echinoderms
Sterols of the Cnidaria
Sterols of the Porifera
Sterols of the Protozoa
Steroid Hormones of the Vertebrates
Sterols and steroids, like the terpenes and carotinoids, are isoprene derivatives. The sterols are
components of the membranes of all eukaryotic
cells and they bind and condense the phospholipid bilayer. The outer cell membrane is particularly rich in sterols, with a molar ratio of sterols to
phospholipids of 0.8-0.1, compared with the usual
value of 0.1-0.3 for intracellular membranes.
The parent substance of all sterols is cholesterol
(Fig. 16.1). The sterols are classified with the steroid hormones and bile salts as steroids because
they also have the gonan (formerly steran) fourring system and have their biosynthetic origin in
cholesterol. Steroids with hormonal functions are
known from the vertebrates (sex hormones and
corticosteroids) and arthropods (ecdysteroids).
However, the same molecular species are also
found in other metazoans, protozoans, plants,
and even in primitive eukaryotes such as yeast
and lower fungi; they are clearly phylogenetically
very old. The critical event for the evolution of
hormone systems based on steroids in the vertebrates and arthropods was not the occurrence of
new steroids but the "invention" of steroid receptors [23]. The calciferols or D vitamins
(Fig. 16.18) are usually discussed in the context of
the steroids, although, strictly speaking, they do
not belong together because the B ring of the
gonan is in this case open; they arise by UV
effects on sterols. Finally, several different animal
groups possess pharmacologically active glycosi16.3.1
16.3.2
16.3.3
16.3.4
16.4
16.5
16.5.1
16.5.2
16.6
Biosynthesis of Steroid Hormones
Steroid-Binding Proteins
Corticosteroids
Sex Hormones
Ecdysteroids
Bile Acids and Bile Alcohols
Biosynthesis of Bile Acids and Bile Alcohols
Bile Salts of Individual Animal Groups
Calciferols
References
des, saponins and alkaloids derived from cholesterol.
The common structural element of all steroids,
the gonan, consists of four rings, A-D, with a
total of 17 C atoms, of which the six linking the
rings are asymmetrical (Fig. 16.1). The ring system is more-or-Iess flat: the H atoms or substituents of the asymmetrical C atoms which lie
beneath the ring are indicated with an interrupted
line and the Greek letter "a", whilst those above
the plane of the ring are indicated with a continuous line and "W'. The position of the H atom on
C-5 also indicates the configuration of the AlB
ring linkage at C-5: 5a steroids have trans-linked
A and B rings, and in 5~ steroids they are cis
linked. The nature of the link between rings A
and B determines the shape of the molecule; the
ring system of the 5a steroids is flatter than that
of the 5~ steroids (Fig. 16.16a). Of the 64 theoretically possible gonan variants, only two are found
in nature; the ring connections BC and CD always
have the trans configuration. Methyl groups at C10 and C-13 (both always in the ~ position) and
different side-chains on C-17 (also always ~) lead
to the formation of other structural elements,
from which an almost unlimited variety of molecular species is derived by the introduction of
double bonds into the ring system or the sidechains and substitutions at different C atoms.
Several hundred natural steroids are known to
date. As can be easily imagined, there are consi-
16.1
16.2
16.2.1
16.2.2
16.2.3
16.2.4
16.2.5
16.2.6
16.2.7
16.3
Biosynthesis of Cholesterol
Sterols of Different Animal Groups
Sterols of the Vertebrates
Sterols of the Insects
Sterols ofTunicates, Annelids, Crustaceans
and Molluscs
Sterols of the Echinoderms
Sterols of the Cnidaria
Sterols of the Porifera
Sterols of the Protozoa
Steroid Hormones of the Vertebrates
Sterols and steroids, like the terpenes and carotinoids, are isoprene derivatives. The sterols are
components of the membranes of all eukaryotic
cells and they bind and condense the phospholipid bilayer. The outer cell membrane is particularly rich in sterols, with a molar ratio of sterols to
phospholipids of 0.8-0.1, compared with the usual
value of 0.1-0.3 for intracellular membranes.
The parent substance of all sterols is cholesterol
(Fig. 16.1). The sterols are classified with the steroid hormones and bile salts as steroids because
they also have the gonan (formerly steran) fourring system and have their biosynthetic origin in
cholesterol. Steroids with hormonal functions are
known from the vertebrates (sex hormones and
corticosteroids) and arthropods (ecdysteroids).
However, the same molecular species are also
found in other metazoans, protozoans, plants,
and even in primitive eukaryotes such as yeast
and lower fungi; they are clearly phylogenetically
very old. The critical event for the evolution of
hormone systems based on steroids in the vertebrates and arthropods was not the occurrence of
new steroids but the "invention" of steroid receptors [23]. The calciferols or D vitamins
(Fig. 16.18) are usually discussed in the context of
the steroids, although, strictly speaking, they do
not belong together because the B ring of the
gonan is in this case open; they arise by UV
effects on sterols. Finally, several different animal
groups possess pharmacologically active glycosi16.3.1
16.3.2
16.3.3
16.3.4
16.4
16.5
16.5.1
16.5.2
16.6
Biosynthesis of Steroid Hormones
Steroid-Binding Proteins
Corticosteroids
Sex Hormones
Ecdysteroids
Bile Acids and Bile Alcohols
Biosynthesis of Bile Acids and Bile Alcohols
Bile Salts of Individual Animal Groups
Calciferols
References
des, saponins and alkaloids derived from cholesterol.
The common structural element of all steroids,
the gonan, consists of four rings, A-D, with a
total of 17 C atoms, of which the six linking the
rings are asymmetrical (Fig. 16.1). The ring system is more-or-Iess flat: the H atoms or substituents of the asymmetrical C atoms which lie
beneath the ring are indicated with an interrupted
line and the Greek letter "a", whilst those above
the plane of the ring are indicated with a continuous line and "W'. The position of the H atom on
C-5 also indicates the configuration of the AlB
ring linkage at C-5: 5a steroids have trans-linked
A and B rings, and in 5~ steroids they are cis
linked. The nature of the link between rings A
and B determines the shape of the molecule; the
ring system of the 5a steroids is flatter than that
of the 5~ steroids (Fig. 16.16a). Of the 64 theoretically possible gonan variants, only two are found
in nature; the ring connections BC and CD always
have the trans configuration. Methyl groups at C10 and C-13 (both always in the ~ position) and
different side-chains on C-17 (also always ~) lead
to the formation of other structural elements,
from which an almost unlimited variety of molecular species is derived by the introduction of
double bonds into the ring system or the sidechains and substitutions at different C atoms.
Several hundred natural steroids are known to
date. As can be easily imagined, there are consi-