486
UNIT THREE
In some cases, steroid hormones may repress a particu-
lar gene, so transcription of it does not occur. The cellular
response results from decreased levels of a particular protein.
Nonsteroid Hormones
A nonsteroid hormone, such as an amine, peptide, or protein,
usually combines with speciF c receptor molecules on the tar-
get cell membrane. Each receptor molecule is a protein that
has a
binding site
and an
activity site.
The hormone combines
with the binding site, which causes the receptor’s activity site
to interact with other membrane proteins. Receptor binding
where it directs synthesis of speciF
c proteins, which may
be enzymes, transport proteins, or even hormone recep-
tors. The activities of these hormones produce the cellular
changes associated with the particular hormone (
f
g. 13.5
,
table 13.4
, and Clinical Application 13.1). An example is the
steroid hormone
aldosterone
(al
do-ster-o
¯n
, al-dos
ter-o
¯n),
from the adrenal gland, whose action is to stimulate the kid-
neys to retain sodium. In response to aldosterone, cells that
form tubules in the kidney begin to synthesize more Na
+
/K
+
pumps. These are the proteins that actively transport these
ions across the cell membrane, retaining sodium.
O
H
3
C
(a)
Cortisol
(c)
Parathyroid hormone (PTH)
(d)
Oxytocin
OH
HO
C
HO
(b)
Norepinephrine
HH
OH H
CN
H
2
C
H
H
CC
H
H
C
H
H
C
H
H
H
H
H
H
C
C
C
C
C
(e)
Prostaglandin PGE
2
H
OH
H
HHO
H
C
H
H
H
H
C
H
C
C
H
H
CC
H
H
C
O
OH
H
H
H
H
C
HO
H
3
C
CO
CH
2
OH
C
O
C
Ala
Val
Val
Val
Asp
His
Lys
Lys
Ser
Arg
Gly
Arg
Arg
Asp
Ser
Glu
Pro
Arg
Asp
Ala
Ser
Glu
Glu
Phe
Ile
Gly
Asp
Lys
His
His
Ser
Leu
Leu
Met
Ala
Pro
Pro
Ala
Ala
Lys
Lys
Lys
Glu
Glu
Leu
Asp
Ser
Gly
Glu
Glu
Glu
Lys
Arg
Lys
Lys
Gly
Try
Cys
Cys
Asp
Glu
Ile
Pro
Leu
Gly
His
Ileu
Met
Glu
Ser
Phe
Ala
Val
Leu
Glu
Ser
Asp
Val
Val
Ile
Leu
Leu
Leu
Asp
Tyr
Tyr
Lys
Glu
Leu
Val
Lys
Gly
Glu
S
S
FIGURE 13.4
Structural formulas of (
a
) a steroid hormone (cortisol) and (
b
) an amine hormone (norepinephrine). Amino acid sequences of
(
c
) a protein hormone (PTH) and (
d
) a peptide hormone (oxytocin). Structural formula of (
e
) a prostaglandin (PGE
2
).
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