
iron protein adrenodoxin, which accepts electrons
from flavoprotein and transfers them to the cyto-
chrome P450 protein. The microsomal hydroxylase
system lacks the adrenodoxin component. Cyto-
chrome P450 hydroxylases (also known as mixed-
function oxidases) catalyze oxidation-reduction
reactions in which one atom of the substrate (O
2
)
ends up on the steroid as a part of the hydroxyl and
the other oxygen atom as part of water (H
2
O); the
necessary reducing equivalents are derived from
NADPH.
Transport
0015 All steroid hormones, which are quite water-
insoluble, circulate in blood, bound to plasma pro-
teins called steroid hormone-binding globulins. The
androgens and estrogens are both transported by a
plasma b-globulin, which is a hepatic glycoprotein
with a molecular weight of 94 000. This carrier pro-
tein has a high affinity (K
d
1.5 10
10
mol l
1
) for
testosterone, 5a-DHT, and estradiol but not for
progesterone or cortisol. The glucocorticoids are
systemically transported via the corticosteroid-bind-
ing protein (CSBP) or transcortin, which is also a
hepatic protein. Similarly, the vitamin D sterols are
bound to the vitamin D-binding protein (DBP), and a-
globulin. The primary role of all these different sterol-
binding proteins is to provide a reservoir of bound
hormone which can effectively dampen oscillations in
the ‘free’ concentrations of hormones.
Catabolism and Excretion of Steroid Hormones
0016 The catabolic pathways that result in the inactivation
of the steroid hormones occur mainly in the liver.
Conjugation of the hydrophobic steroids either with
sulfate or glucuronides renders the hormone more
hydrophilic for easy excretion in the urine. The
excreted species are usually polyhydroxyl forms
with the particular glucuronide or sulfate: these con-
jugations are mediated by sulfokinase and glucuronyl
transferase enzymes.
Functions of Steroid Hormones
0017 Steroid hormones secreted by the endocrine glands
exert profound influences on cell growth, develop-
ment, and physiology, chiefly by controlling the
synthesis of particular proteins. All steroid hormones
modulate cellular responses by either inducing or
repressing the transcription of specific proteins at
the genetic level.
0018 A general result of glucocorticoid action is a
direct effect of increased glucogen, especially in the
liver, and increases in the levels of circulating glucose.
Prolonged high levels of glucocorticoids can lead to
the death of susceptible cells which accounts for
muscle wasting and immunodeficiency. As a powerful
antiinflammatory agent, glucocorticoids may act
on many cells to induce the synthesis of the protein(s)
lipomodulin. Lipomodulin acts as an inhibitor of
phospholipase A
2
and thereby prevents the release
of prostaglandins, prostacyclins, and leukotrienes,
some of which mediate processes of inflammation
and pain.
0019Mineralocorticoids such as aldosterone promote
sodium reabsorption by the kidney tubules, usually
followed by changes in water balance; in terms of
function they mesh with the vasopressin system and
atrial natriuretic factor which also control water
absorption and excretion by the kidney and also
participate in the regulation of blood pressure.
0020The biological responses of androgens are divisible
into four categories: (1) growth-promoting or andro-
genic effect on the male reproductive tract; (2) stimu-
lating or anabolic effect on body weight (skeletal
growth, skeletal muscle growth, subcutaneous fat
distribution, growth of prostate and seminal vesicle);
(3) development of male secondary sexual character-
istics (growth of external genitalia, enlargement of
larynx and thickening of vocal cords, hair growth
distribution); and (4) actions in the central nervous
system and brain (differentiation of brain cortex,
hypothalamus, initiation of puberty).
0021The dominant effect of estrogens occurs in the
female reproductive tract. Significant biological activ-
ity of estrogens also occurs in the hypothalamus,
brain, and mammary tissue. Estrogens primarily
initiate the transcription of specific proteins in the
uterus, and affect uterine cell proliferation and differ-
entiation. A classical developmental role of estrogen
in the liver of the adult hen is in the induction
of vitellogenin, which is transported to the ovary
and cleaved to yield egg proteins, phosvitin, and
lipovitellin.
0022The biological activity of progesterone is restricted
to the female reproductive tract and mammary tissue.
Some of the effects mediated by progesterone include
thermogenesis in women, regulation of egg move-
ment through the fallopian tubes, preparation of
uterus to receive the blastocyst, alteration of electrical
activity in the brain, control of uterine contraction
(at parturition), and generation of the secretory
system of breast during pregnancy.
0023The steroid hormone 1,25(OH)
2
D
3
is principally
involved in the regulation of mineral (calcium
and phosphorus) metabolism, bone growth, and
differentiation. A detailed account of the manifold
effects of vitamin D is given in a subsequent article.
(See Cholecalciferol: Properties and Determination;
Physiology.)
HORMONES/Steroid Hormones 3171