320
Chapter 11
Roug
ough
ugh
gh
endoplasmic
plasmic
asmi
reticulum
Secretory
S
vesicles
Nucleus
Plasma membrane
Synthesis
Packaging
Storage
Secretion
Prohormone
Hormone
Hormone
Hormone
(and any "pro"
fragments)
Golg
Golgi
apparatus
apparat
ar fluid
lular
acellul
Intracel
Intra
In
O
OH
O
OH
C
HO
CH
2
OH
CH
2
OH
HO
OH
O
O
O
C
HO
C
H
Cortisol
Aldosterone
Testosterone
HO
CH
3
CH
3
Cholesterol
Estradiol
O
Figure 11–3
Typical synthesis and secretion of peptide hormones. Secretion of
stored secretory vesicles occurs by the process of exocytosis.
Figure 11–3
physiological
inquiry
What is the advantage of packaging peptide hormones in
secretory vesicles?
Answer can be found at end of chapter.
Figure 11–4
Structures of representative steroid hormones and
their structural relationship to cholesterol.
gonads
(testes and ovaries) as well as by the placenta during
pregnancy.
Figure 11–4
shows some examples of steroid hor-
mones. In addition, the hormone 1,25-dihydroxyvitamin D,
the active form of vitamin D, is a steroid.
The general process of steroid hormone synthesis is illus-
trated in
Figure 11–5
. In both the gonads and the adrenal
cortex, the cells are stimulated by the binding of a pituitary
gland hormone to its receptor (step 1 in Figure 11–5). These
receptors are linked to G-proteins, which activate adenylyl
cyclase and thus cAMP production (step 2). The subsequent
activation of protein kinase A results in phosphorylation of
numerous cytosolic and membrane proteins (step 3), which
facilitate the subsequent steps in the process.
All of the steroid hormones are derived from cholesterol.
Cells producing steroid hormones synthesize some of their
own cholesterol for this purpose from molecules of acetate.
Typically, however, most of the cholesterol enters the cells in
the form of low-density lipoproteins (LDLs). Receptors on the
cell surface bind to circulating LDLs; when this occurs, the
LDL/receptor complex is internalized into the cell cytosol by
endocytosis. Once inside the cell, the cholesterol is stored in
an esterifi
ed form in large, non-membrane-bound lipid drop-
lets. When the cell is stimulated, free cholesterol is released
from the lipid droplet by the action of the enzyme
choles-
terol esterase,
which is activated by protein kinase A (step
4). Carrier proteins then transport the free cholesterol to the
mitochondria (step 5). Once at the mitochondria, the choles-
terol must be transported across the outer membrane to the
inner mitochondrial membrane, which contains the enzymes
required to process cholesterol into steroid hormones. These
enzymes, called
cytochrome P450s,
are a large family of
related proteins that can attach hydroxyl groups to carbon
atoms and that participate in cleaving carbon-carbon bonds.
In the case of cholesterol and steroid hormones, the carbon
atoms may be within the ring structure or in branches aris-
ing from the rings. As the P450 enzymes modify cholesterol,
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