Endocrine Glands 319
1. Amines. These are hormones derived from the amino
acids tyrosine and tryptophan. They include the hormones
secreted by the adrenal medulla, thyroid, and pineal glands.
2. Polypeptides and proteins. Proteins are large polypeptides,
so the distinction between the two categories is somewhat
arbitrary. Antidiuretic hormone is a polypeptide with nine
amino acids ( table 11.2 ), too small to accurately be called a
protein. If a polypeptide chain is larger than about 100 amino
acids, such as growth hormone with 191 amino acids, it can
be called a protein. Insulin blurs the two categories, because
it is composed of two polypeptide chains derived from a sin-
gle, larger molecule (see chapter 3, fig. 3.23).
3. Glycoproteins. These molecules consist of a protein bound
to one or more carbohydrate groups. Examples are follicle-
stimulating hormone (FSH) and luteinizing hormone (LH).
4. Steroids. Steroid hormones are derived from cholesterol
after an enzyme cleaves off the side chain attached to the
five-carbon “D” ring ( fig. 11.2 ). Steroid hormones include
testosterone, estradiol, progesterone, and cortisol.
In terms of their actions in target cells, hormone mol-
ecules can be divided into those that are polar, and therefore
water-soluble, and those that are nonpolar, and thus insoluble
in water. (For a discussion of water solubility, see chapter 2
In ovaries
In adren
als
In
testes
Cholesterol
Pregnenolone Progesterone
Secreted by
corpus luteum
of ovaries
Cortisol (hydrocortisone)
Adrenal
cortex
Corpus
luteum
Ovary
Follicles
Ovary in ovary
Secreted by adrenal cortex
Secreted by
follicles of ovaries
Estradiol-17β
Androstenedione Testosterone
Testis
Seminiferous
tubules
Interstitial
Spermatic (Leydig) cells
cord
Secreted by Leydig
cells of testes
HO
HO
HO
HO
OH
OH
OH
O
CH 2 OH
CO
CH 3
C
CH 3
CO O
O
O O
O
BA
CD
Figure 11.2
Simplified biosynthetic
pathways for steroid
hormones. Notice that progesterone (a hormone
secreted by the corpus luteum of the ovaries) is a
common precursor of all other steroid hormones and that
testosterone (the major androgen secreted by the Leydig
cells of the testes) is a precursor of estradiol-17 b , the
major estrogen secreted by the follicles of the ovaries.
Hormone Structure Gland Primary Effects
Antidiuretic
hormone
9 amino
acids
Posterior
pituitary
Water retention and
vasoconstriction
Oxytocin 9 amino
acids
Posterior
pituitary
Uterine and mam-
mary contraction
Insulin 21 and 30
amino acids
(double
chain)
Beta cells
in islets of
Langerhans
Cellular glucose up-
take, lipogenesis,
and glycogenesis
Glucagon 29 amino
acids
Alpha cells
in islets of
Langerhans
Hydrolysis of stored
glycogen and fat
ACTH 39 amino
acids
Anterior
pituitary
Stimulation of
adrenal cortex
Parathyroid
hormone
84 amino
acids
Parathyroid Increase in blood
Ca^21 concentration
FSH, LH,
TSH
Glyco-
proteins
Anterior
pituitary
Stimulation of
growth, develop-
ment, and secre-
tory activity of
target glands
Table 11.2 | Examples of Polypeptide and
Glycoprotein Hormones