The Endocrine System
intermediates form, which must be further modiﬁ ed before
the ﬁ nal steroid hormone is produced. These modiﬁ
which also include a dehydrogenation reaction mediated by
an enzyme different from the P450s, occur in both the mito-
chondria and in the smooth endoplasmic reticulum. Thus, the
intermediates are shuttled back and forth between the two
organelles (step 6). The ﬁ nal product depends upon the cell
type and the types and amounts of the enzymes it expresses.
Cells in the ovary, for example, express large amounts of the
enzyme needed to convert testosterone to estradiol, whereas
cells in the testes do not express signiﬁ cant amounts of this
enzyme and thus make primarily testosterone.
Once formed, the lipophilic steroid hormones are not
stored in the cytosol, but instead diffuse out through the lipid
bilayer of the plasma membrane into the interstitial ﬂ
from there into the circulation (step 7). Because of their lipid
nature, steroid hormones are not highly soluble in blood, and
thus they are largely transported in plasma bound to carrier
proteins such as albumin.
The next sections describe the pathways the adrenal cortex
and gonads follow for steroid synthesis. Those for the placenta
are somewhat unusual and are brieﬂ y discussed in Chapter 17.
Hormones of the Adrenal Cortex
The ﬁ ve major hormones secreted by the adrenal cortex are
aldosterone, cortisol, corticosterone, dehydroepiandros-
terone (DHEA), and androstenedione (
is known as a
effects are on salt (mineral) balance, mainly on the kidneys’
handling of sodium, potassium, and hydrogen ions. Its
actions are described in detail in Chapter 14. Brieﬂ y, pro-
duction of aldosterone is under the control of a circulating
located on inner
(a) Schematic overview of steps involved in steroid
synthesis. See text for key to numbered steps. PKA:
protein kinase A; LDL: low-density lipoprotein.
(b) The ﬁ ve hormones shown in boxes are the major
hormones secreted from the adrenal cortex.
Dehydroepiandrosterone (DHEA) and androstenedione are
androgens—that is, testosterone-like hormones. Cortisol
and corticosterone are glucocorticoids, and aldosterone is a
mineralocorticoid that is only produced by one part of the