water, is reported to have steroid receptors that
bind more firmly to chromatin than do those of
the mammals, with a tissue osmolality of about
300 mOsmoUkg water. The oestrogen receptor of
the dogfish is eluted from DNA-cellulose with
0.55 moUl NaCl, compared with 0.22 moUI
required for the freshwater ray Potamotrygon sp.
and the mammals [16]. The sequence similarity
between members of the family is 2::42 % in the
DNA-binding domain C, usually 2::15 % in the
hormone-binding domain E, and negligible in the
other domains [28, 127]. Sequence comparisons
between receptors of the same type show, for
example, 81-82 % agreement between, the
mouse progesterone receptor and those of
humans and rabbits, and 50 % agreement with
that of the chicken [104]; the thyroid receptor of
the frog Xenopus laevis agrees 92 % with that of
the chicken [11]. The human and chicken oestrogen receptors have 80 % of their 589 amino acids
in common, and the corresponding receptor from
the rainbow trout shows 92-95 % similarity to
other species in domain E, but lacks domain A
[66,88].
16.3.1 Biosynthesis of Steroid Hormones
All vertebrate steroid hormones share a common
synthesis pathway (Fig. 16.10). This begins with
the oxidative cleavage of the side-chain on C-20
of cholesterol by an NADPH-dependent mitochondrial hydroxylase with cytochrome P-450.
This yields isohexanal and pregnenolone. The
ensuing metabolism takes place in the endoplasmic reticulum. Progesterone arises by conversion of the 5-en-3~-hydroxy structure of pregnenolone to the typical 4-en-3-oxo structure of the
biologically active steroids by 3~-hydroxy steroid
dehydrogenase and ~5/~4-isomerase. Metabolic
pathways lead from progesterone to corticosterone and aldosterone, and from 17a-hydroxy progesterone to cortisol and cortisone. The biosynthesis of testosterone from pregnenolone requires, on the one hand, the sequence of 17ahydroxylation, oxidative cleavage of the sidechain on C-17 and reduction of the 17-oxo group
to 17a-hydroxyl, and, on the other hand, the conversion of the 5-en-4~-hydroxy to the 4-en-3-oxo
structure. Depending on the order of these
reactions, one can distinguish the ~5 pathway,
from pregnenolone to testosterone via 5androstendiol, and the ~ 4 pathway, via 4androstendione (Fig. 16.10). The rat and other
rodents mainly use the ~ 4 pathway, humans and
16.3.2 Steroid-Binding Proteins
639
rabbits use the ~5 route, and goats and many
other mammals have both pathways. Aromatization of the A ring and the consequent cleavage of
the C-19 methyl group leads from testosterone to
oestradiol-17~, and from androstendione to oestrone [39, 74].
All steroid-forming tissues have the same
enzyme spectrum of dehydrogenases, reductases,
hydroxylases and isomerases, although perhaps
with different relative activities. The species- and
organ-specific spectrum of products arises from
the preference for certain synthetic pathways and
from cessation of synthesis at different steps. In
addition to the steroids that are typical of the
organ, lower amounts of other steroids may also
be produced, e.g. sex hormones in the adrenal
cortex, oestrogen in the testis, and androgen in
the ovary. The synthesis of atypical steroids may
increase markedly in tumours. Certain steroidmetabolizing enzymes are also found in other tissues; noteworthy in this context is the reduction
of testosterone to dihydrotestosterone in associated glands of the male genital tract, and the
formation of oestrogens from testosterone or
androsterone in the liver, brain and blood of
mammals. The inactivation of steroid hormones
occurs mainly in the liver, but in mammals and
reptiles it also occurs in the kidney. The degradation p'athways are not yet known in detail, even
for the mammals. Important steps in inactiviation, however, include the reduction of the 4en-3-oxo configuration and the 20-oxo group. The
newly formed 3-hydroxy group is esterified with
glucuronate or sulphate and the conjugates are
excreted in the urine [9, 39, 74].
16.3.2 Steroid-Binding Proteins
Because of their relatively low solubility in water,
the steroid hormones can only be transported in
the blood plasma when they are bound to proteins. Serum albumin binds steroids unspecifically
with relatively low affinity, and most vertebrates
have specific steroid-binding plasma proteins
[34]. The difference between the affinity constants of the binding proteins (10- 8 moUl) and the
intracellular receptors (10- 9 moUI) thermodynamically favours the uptake of steroid molecules into
the cell. Two types of steroid-binding globulins
are usually found in the vertebrates from the teleosts to the mammals; of these, the corticosteroidbinding globulin (CBG) or trans cortin binds all
C21 steroids, including progesterone, and the
hormone-binding globulin (SHBG) binds Cl8 and
bind more firmly to chromatin than do those of
the mammals, with a tissue osmolality of about
300 mOsmoUkg water. The oestrogen receptor of
the dogfish is eluted from DNA-cellulose with
0.55 moUl NaCl, compared with 0.22 moUI
required for the freshwater ray Potamotrygon sp.
and the mammals [16]. The sequence similarity
between members of the family is 2::42 % in the
DNA-binding domain C, usually 2::15 % in the
hormone-binding domain E, and negligible in the
other domains [28, 127]. Sequence comparisons
between receptors of the same type show, for
example, 81-82 % agreement between, the
mouse progesterone receptor and those of
humans and rabbits, and 50 % agreement with
that of the chicken [104]; the thyroid receptor of
the frog Xenopus laevis agrees 92 % with that of
the chicken [11]. The human and chicken oestrogen receptors have 80 % of their 589 amino acids
in common, and the corresponding receptor from
the rainbow trout shows 92-95 % similarity to
other species in domain E, but lacks domain A
[66,88].
16.3.1 Biosynthesis of Steroid Hormones
All vertebrate steroid hormones share a common
synthesis pathway (Fig. 16.10). This begins with
the oxidative cleavage of the side-chain on C-20
of cholesterol by an NADPH-dependent mitochondrial hydroxylase with cytochrome P-450.
This yields isohexanal and pregnenolone. The
ensuing metabolism takes place in the endoplasmic reticulum. Progesterone arises by conversion of the 5-en-3~-hydroxy structure of pregnenolone to the typical 4-en-3-oxo structure of the
biologically active steroids by 3~-hydroxy steroid
dehydrogenase and ~5/~4-isomerase. Metabolic
pathways lead from progesterone to corticosterone and aldosterone, and from 17a-hydroxy progesterone to cortisol and cortisone. The biosynthesis of testosterone from pregnenolone requires, on the one hand, the sequence of 17ahydroxylation, oxidative cleavage of the sidechain on C-17 and reduction of the 17-oxo group
to 17a-hydroxyl, and, on the other hand, the conversion of the 5-en-4~-hydroxy to the 4-en-3-oxo
structure. Depending on the order of these
reactions, one can distinguish the ~5 pathway,
from pregnenolone to testosterone via 5androstendiol, and the ~ 4 pathway, via 4androstendione (Fig. 16.10). The rat and other
rodents mainly use the ~ 4 pathway, humans and
16.3.2 Steroid-Binding Proteins
639
rabbits use the ~5 route, and goats and many
other mammals have both pathways. Aromatization of the A ring and the consequent cleavage of
the C-19 methyl group leads from testosterone to
oestradiol-17~, and from androstendione to oestrone [39, 74].
All steroid-forming tissues have the same
enzyme spectrum of dehydrogenases, reductases,
hydroxylases and isomerases, although perhaps
with different relative activities. The species- and
organ-specific spectrum of products arises from
the preference for certain synthetic pathways and
from cessation of synthesis at different steps. In
addition to the steroids that are typical of the
organ, lower amounts of other steroids may also
be produced, e.g. sex hormones in the adrenal
cortex, oestrogen in the testis, and androgen in
the ovary. The synthesis of atypical steroids may
increase markedly in tumours. Certain steroidmetabolizing enzymes are also found in other tissues; noteworthy in this context is the reduction
of testosterone to dihydrotestosterone in associated glands of the male genital tract, and the
formation of oestrogens from testosterone or
androsterone in the liver, brain and blood of
mammals. The inactivation of steroid hormones
occurs mainly in the liver, but in mammals and
reptiles it also occurs in the kidney. The degradation p'athways are not yet known in detail, even
for the mammals. Important steps in inactiviation, however, include the reduction of the 4en-3-oxo configuration and the 20-oxo group. The
newly formed 3-hydroxy group is esterified with
glucuronate or sulphate and the conjugates are
excreted in the urine [9, 39, 74].
16.3.2 Steroid-Binding Proteins
Because of their relatively low solubility in water,
the steroid hormones can only be transported in
the blood plasma when they are bound to proteins. Serum albumin binds steroids unspecifically
with relatively low affinity, and most vertebrates
have specific steroid-binding plasma proteins
[34]. The difference between the affinity constants of the binding proteins (10- 8 moUl) and the
intracellular receptors (10- 9 moUI) thermodynamically favours the uptake of steroid molecules into
the cell. Two types of steroid-binding globulins
are usually found in the vertebrates from the teleosts to the mammals; of these, the corticosteroidbinding globulin (CBG) or trans cortin binds all
C21 steroids, including progesterone, and the
hormone-binding globulin (SHBG) binds Cl8 and