Peptide hormones and catecholamines allow for rapid responses to environmental changes. They are stored in secretory vesicles and released via exocytosis within seconds or minutes in response to stimulation. This causes short-term effects that are terminated once the hormones are degraded. In contrast, steroid hormones and thyroid hormones are synthesized from cholesterol or thyroglobulin precursors within cells. They diffuse out of cells and circulate in the blood bound to carrier proteins. This allows their effects to last longer, from hours to days, but production and release takes longer than for peptide hormones and catecholamines. The different hormone types thus allow for both rapid short-term responses and longer-term regulatory effects.

1.  Peptide Hormones and Catecholamines:
 As we know organisms must be able to respond
instantly to many changes in their internal / external
environment. Such rapid responses – peptide
hormones and catecholamines
 Signaling cells that produce these hormones /
neurotransmitters store them in secretory vesicles just
under plasma membrane
 Supply of stored preformed signaling molecules – 1
day in case of peptide hormones and for several days
– in case of catecholamines
 All peptide hormones including insulin and ACTH
are synthesized as a part of longer propeptide –
cleaved by specific protease to generate active
molecules. Just after this, it is transported to a
secretory vesicles

2. Fig. 17.24 Lodish 5th
Fig. 5.51 Lodish 5th
Fig. 17.25 Lodish 5th
 Stimulation of signaling cell by a peptide hormones/
catecholamines causes immediate exocytosis of the
peptide hormones in surrounding medium /blood
 Signaling cells are also stimulated to synthesize the
hormone and replenish the cell’s supply
 Release of peptide hormones persist in the blood for
only seconds / minutes before being degraded by blood
and tissue proteases
 Released catecholamines are inactivated by different
enzymes/taken up by specific cells
 Initial actions of these signaling substances on target
cells also last only seconds/minutes

4.  Thus the catecholamines and some peptide hormones
mediate short responses that are terminate by their own
degradation
 Steroid Hormones and Thyroxin:
 Steroid hormone are synthesized from cholesterol
Fig. 2.37, Alberts 3rd Ed
 Steroid producing cells like those in adrenal cortex
store a small supply of hormone precursor but none of
the mature, active hormone
 On stimulation, the cells convert the precursor to
active hormone
Fig 42.4 Harper 26th Ed
Fig 42.3 Harper 26th Ed
Fig 48.4 Harper 24th Ed
Fig 42.5 Harper 26th Ed
Fig 42.7 Harper 26th Ed

10.  They diffuse across the plasma membrane into blood
 Similarly, thyroglobin – the iodinated precursor of
thyroxin is stored in thyroid follicles
 When cells lining these follicles are exposed to TSH,
they take up thyroglobin
 Controlled proteolysis by lysozymal enzymes yields –
T3, T4, and they are released into blood
Fig. 42.11 Harper 26th Ed
Fig. 46.1 Harper 24th Ed
Fig 46.3 Harper 24th Ed
 Release of these hormones by signaling cells takes
from hours  days
 They are poorly soluble in aqueous solution
therefore:

11. -Tgb is large iodinated, glycosylated
protein with molecular mass 660kDa
- Carbohydrates account for 8-10% of the
weight of thyroglobulin and iodide for
about 0.2 1% depending upon iodine
content in the diet
-Tgb composed of two large subunits
- It contains 115 tyrosine residues, each of
which is potential site of iodination
-Thyroid is able to concentrate I- against strong
electrochemical gradient
- This is an energy dependent process and is
linked to the Na+-K+ ATPase-dependent
thyroidal I- transporter

12.  They are transported in blood by carrier proteins
 They are not rapidly degraded as are tightly bound
 Thus cellular responses of thyroxine and steroid
hormones take a while to occur but last much longer
hours to days then do the effect mediated by
peptides hormones and catecholamine
Table 20.4 Lodish 3rd / Table 19.2 Lodish 2nd
 Hormones can be Classified based on their
Solubility and Receptor Location:
 Hormones fall into three broad categories:

14.  Small lipophilic molecules that defuse across
the plasma membrane and interact with intra-
cellular receptors
 Hydrophilic OR
 Lipophilic molecules that bind to cell surface
receptor: Prostaglandins
Fig 20.2 Lodish 4th Ed
 Effect on Many Hormones are Mediated
by Second Messengers:
Fig 19.4 Lodish 2nd Ed
 Feed-Back Control of Hormonal Levels:
 Synthesis/release of many hormones are subject to feed
back regulation

17.  This type of regulation is particularly important in co-
coordinating the action of multiple hormones on various
cell-types during growth and differentiation
 Often, levels of several hormones are interconnected by
feed back circuits, in which changes in the level of one
hormone affect the levels of other hormones
 One example involves the regulation of estrogen and
progesterone, steroid hormones that stimulate the growth
and differentiation of cells in the endometrium, the tissue
lining the interior of the uterus
 Changes in the endometrium prepares the organ to
receive and nourish the embryo
Fig 20.5 Lodish 3rd Ed
 Steroid Hormones, Thyroid Hormones,
Retinoid and Vitamin D bind to Intra-
cellular Receptor that are Ligand-Activated
Gene Regulatory Proteins:

18. (Nerve cells )
-The levels of both hormones are regulated by
complex feed back circuit involving several
hormones
-A key role is played by the anterior pituitary
gland
- An organ separated from but controlled by the
brain
in to the oviduct

19.  Intracellular receptor super family/steroid hormone
receptor super family
Page 38, Scientific American-Nov, 2007
Fig 24.13, Zubay
Fig 15.12, Alberts 3rd Ed
Page 38, Scientific American-Nov, 2007
Fig 15.13,Alberts 3rd Ed
 Cell-Surface Receptors can be Categorized
into Four Major Classes/Three Major
Classes:
Fig 20.4, Lodish 3rd Ed
Fig 15.14, Alberts 3rd Ed