(tyrosine hydroxylase) in the secretory cells catalyzes a reac-
tion that converts tyrosine into a substance called
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.
rin), also called adrenalin and
rin), also called noradrenalin. Both oF these
substances are a type oF amine called a
they have similar molecular structures and physiological
. Epinephrine is synthesized From
The synthesis oF catecholamines begins with the amino
acid tyrosine. In the f rst step oF the pathway, an enzyme
Connective tissue capsule
Adrenal glands. (
) An adrenal gland consists of an outer cortex and an inner medulla. (
) The cortex consists of three layers, or
zones, of cells.
Light micrograph of the adrenal medulla and the
adrenal cortex (75×).
Epinephrine and norepinephrine have similar
molecular structures and similar functions.