Human Physiology, 14th edition (2016)

(Tina Sui) #1
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
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