Biology of Disease

7.2 Hormone Production

Amine hormones include adrenaline, noradrenaline (also called epinephrine
and norepinephrine respectively) and thyroid hormones. Their synthesis
involves a series of enzyme-catalyzed reactions in the cytoplasm of
endocrine cells. For example, thyroid hormones (Section 7.7) are synthesized
by iodination of tyrosine residues in the protein thyroglobulin found in
the thyroid. Most peptide hormones are synthesized as large inactive
prohormones, which are subsequently cleaved by enzymes to produce the
active hormone. Sometimes a number of hormones may be derived from
the same prohormone. Steroid hormones are synthesized by a sequence of
enzymatic reactions using cholesterol as a common precursor. The enzymes
responsible for conversion of cholesterol to hormone are located in the
smooth endoplasmic reticulum and mitochondria of cells. The presence
or absence of particular enzymes determines the type of steroid hormone
synthesized by that specific cell.

Amine hormones, such as adrenaline and noradrenaline, are stored in
secretory granules within the cytoplasm, but thyroid hormones are stored
within the thyroid follicles as components of thyroglobulin. Peptide hormones
are usually stored in membrane-bound vesicles within the cytoplasm of the
endocrine cell. Steroid hormones are not usually stored but are released upon
synthesis. However, lipid droplets may be found in the cytoplasm containing
precursor material for these hormones.

Hormones are released in response to nervous, hormonal or metabolic stimuli.
Hormones stored in granules are released when the granules move and fuse
with the plasma membrane. Some hormones, for example thyroxine, are
released continuously whereas others show diurnal variation and their release
varies during the day. For example, cortisol shows diurnal variation with levels
being high in the morning but low at night. The concentrations of hormones
in the plasma must be kept within narrow ranges for optimum function. A

HORMONE PRODUCTION

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HO OH

OH

O

CHOH

CH 2

CH 3

CH 3

NHCH 3

Gly- Ile-Val-Glu-Gln-Cys-Cys-Thr-Ser- Ile-Cys-Ser-Leu-Tyr-Gln-Leu-Glu-Asn-Tyr-Cys-Asn

Phe-Val-Asn-Gln-His-Leu-Cys-Gly-Ser-His-Leu-Val-Glu-Ala-Leu-Tyr-Leu-Val-Cys-Gly-Glu-Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Thr

SS

S

S

S

S

A chain

B chain

A)

B)

C)

Figure 7.3 Examples of hormones.

(A) adrenaline (epinephrine), (B) insulin and (C)

testosterone.