505
CHAPTER THIRTEEN
Endocrine System
(tyrosine hydroxylase) in the secretory cells catalyzes a reac-
tion that converts tyrosine into a substance called
dopa.
A
second enzyme (dopa decarboxylase) catalyzes a reaction that
modif es dopa into dopamine, and a third enzyme (dopamine
betahydroxylase) catalyzes a reaction that alters dopamine
to Form norepinephrine. Still another enzyme (phenyletha-
nolamine N-methyltransFerase) then catalyzes conversion oF
norepinephrine to epinephrine. About 15% oF the norepineph-
rine is stored unchanged. The hormones occupy tiny vesicles
(chromaFf n granules), much like neurotransmitters are stored
in vesicles in neurons.
(ep
ı˘-neF
rin), also called adrenalin and
norepinephrine
(nor
ep-ı˘-neF
rin), also called noradrenalin. Both oF these
substances are a type oF amine called a
catecholamine,
and
they have similar molecular structures and physiological
Functions
(fig. 13.30)
. Epinephrine is synthesized From
norepinephrine.
The synthesis oF catecholamines begins with the amino
acid tyrosine. In the f rst step oF the pathway, an enzyme
Adrenal gland
Kidney
Adrenal cortex
Adrenal
medulla
(a)
Zona glomerulosa
Connective tissue capsule
(b)
Surface of
adrenal gland
Zona fasciculata
Zona reticularis
Adrenal medulla
Adrenal cortex
Cortex
Medulla
Capsule
Zona
glomerulosa
Zona
fasciculata
Zona
reticularis
Chromaffin
cells
Epinephrine
CCNC
H
3
OH H
HH
H
HO
HO
Norepinephrine
CCN
H
2
OH H
HH
HO
HO
FIGURE 13.28
Adrenal glands. (
a
) An adrenal gland consists of an outer cortex and an inner medulla. (
b
) The cortex consists of three layers, or
zones, of cells.
FIGURE 13.29
Light micrograph of the adrenal medulla and the
adrenal cortex (75×).
FIGURE 13.30
Epinephrine and norepinephrine have similar
molecular structures and similar functions.
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