General features of hormones for medical and biology undergraduates

1. Hormones
General Features
R.C. Gupta
Professor and Head
Department of Biochemistry
National Institute of Medical Sciences
Jaipur, India

2. Higher animals and human beings possess
a diversity of cells, tissues, organs and
systems for specialized functions
They require mechanisms by which
different cells can communicate with
each other
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3. EMB-RCG
Intercellular communication occurs
through:
Nervous system and endocrine system
also communicate with each other
Nervous system Endocrine system

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Electrical-chemical signals
transmitted through fixed
structural routes
Chemical signals transmitted
in the form of mobile
hormone molecules
Nervous
system
Endocrine
system

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Systemic effects
Ultimate hormone
↓
Target endocrine gland
↓
↓ Tropic hormone
Anterior pituitary
↓ Releasing hormone
Hypothalamus
↓
External or internal signal

6. Hormones are present in blood in very
small concentrations yet they produce
profound biological effects
The action of a hormone occurs
through a cascade of events in which the
signal is amplified at a number of stages
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7. Amplification
Glucagon-receptor complex
Inactive
adenylate
cyclase
Active
adenylate
cyclase
ATP cAMP
Glycogen Glucose–1–P Glucose
Amplification
Inactive protein
kinase A
Amplification
Amplification
Active phosphorylase
Active protein
kinase A
Inactive phosphorylase
kinase b
Active phosphorylase
kinase b
Inactive phosphorylase

8. Definition
The target cells may be distant or nearby
The hormone acts on its target cells which
possess specific receptors for the hormone
A molecule secreted by some cells that
controls and regulates the activity of certain
cells or organs

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Depending on the distance between
the target cell and the hormone-
secreting cell, the hormone may be:
Endocrine hormone
Paracrine hormone
Autocrine hormone

10. The endocrine hormones are
secreted by endocrine (ductless)
glands, and act on distant cells
The paracrine hormones act on
cells in near vicinity of the
hormone-secreting cells
The autocrine hormones act on the
cells secreting the hormone
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11. Hormone receptors
Target cells possess receptors for hormones
Chemically, the receptors are proteins
A receptor is specific for a particular hormone
It has at least two domains, a recognition
domain and a signal domain

12. The receptor may be located in the cell
membrane or inside the cell
The intracellular receptors may be located
in the cytosol or in the nucleus
A given cell may possess receptors for
several hormones

13.  Nucleic acids include deoxyribonucleic acid
(DNA) and ribonucleic acid (RNA)
 Both are information molecules
 DNA stores genetic information
 The entire genetic material present in the DNA of
an organism is known as its genome
s
1, 2, 3, 4,
5 and 6
are hormone
receptors
Nucleus Cell membrane

14. The number of receptors is not fixed
The receptors can undergo:
Up-regulation (increase in
the number of receptors)
Down-regulation (decrease
in the number of receptors)

15. Classification of hormones
Hormones may be classified
on the basis of:
Chemical nature
Solubility
Location of receptors and
nature of second messengers
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On chemical basis, hormones can
be divided into:
Proteins, peptides
and amino acid
derivatives
Steroids

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On the basis of solubility, hormones
can be divided into:
Hydrophilic
hormones e.g.
proteins, peptides
and
catecholamines
Hydrophobic
hormones e.g.
steroid hormones
and thyroid
hormones

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The classification based on the location
of receptors and nature of second
messengers is more comprehensive
On this basis, the hormones may be
divided into two groups:
Group I hormones Group II hormones

19. Group I hormones
These hormones are lipophilic
Examples are steroid hormones,
thyroid hormones, calcitriol etc
Being lipophilic, they require carrier
proteins to transport them in blood
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20. The receptors for glucocorticoids and
mineralocorticoids are present in the
cytosol
The receptors for oestrogen,
progesterone, thyroid hormones and
calcitriol are present in the nucleus
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21. Receptors of group I hormones are
intracellular
Being lipid-soluble, the hormones can
easily traverse the cell membrane
Mechanism of action of Group I
hormones

22. The hormones bind to their intracellular
receptors
The hormone-receptor complex is mobile
It does not require any second
messenger to carry the signal

23. The receptor possesses:
One domain to recognize and bind
the hormone
Another domain to recognize a
specific sequence in the DNA
The DNA sequence is called
hormone response element (HRE)

24. There are different response
elements for different hormones like:
Glucocorticoid response
element (GRE)
Mineralocorticoid response
element (MRE)
Thyroid hormone response
element (TRE)

25. The HREs are located just upstream of
some structural genes and their promoters
The hormone-receptor complex goes to
the nucleus and binds to the HRE
This increases the transcription and
translation of specific genes

26. The proteins synthesized produce the
biological effects attributed to the hormone
For example, glucocorticoids increase the
expression of genes encoding gluconeogenic
enzymes
Increased synthesis of these enzymes
increases gluconeogenesis

27.  Nucleic acids include deoxyribonucleic acid
(DNA) and ribonucleic acid (RNA)
 Both are information molecules
 DNA stores genetic information
 The entire genetic material present in the DNA of
an organism is known as its genome
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28. Group II hormones
Group II hormones include proteins,
peptides and catecholamines
Being hydrophilic, they do not require
carriers to transport them in blood
Their receptors are trans-membrane
proteins

29. The second messenger is produced by an
effector
The hormone binds to its receptor on the
cell surface, and does not enter the cell
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A second messenger is required to carry
the message inside the cell

30. On hormone-receptor binding, a signal
transducer is activated
The signal transducer goes to, and
activates, an effector
The effector produces a second
messenger
Mechanism of action of Group II
hormones

31. The second messengers include:
Cyclic AMP (cAMP)
Cyclic GMP (cGMP)
Ca++ and/or phosphoinositides
Tyrosine kinase
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32. cAMP
The hormones using cAMP as a second
messenger include:
• TSH
• ACTH
• FSH
• Glucagon
• Epinephrine acting through a2-, b1-
and b2-adrenergic receptors

33. cAMP is produced in the cell from ATP by
adenylate cyclase, a membrane-bound
enzyme that is normally inactive
Binding of the hormone to its receptor
activates adenylate cyclase
Adenylate cyclase
ATP cAMP + PPi

34. Concentration of cAMP in the cell
increases
This produces the biological effects
Most of these effects occur due to cAMP-
induced activation of protein kinase A

35. For example, activation of protein
kinase A in liver by glucagon:
Increases glycogenolysis
Decreases glycogenesis

36. Adenylate cyclase (effector) is not in
contact with the hormone receptor
The signal is carried from the receptor to
the effector by the signal transducer
(receptor-effector coupling)
The signal transducer is a G-protein

37. Receptor
G-Protein

38. G-protein is a trimer made up of an a-
subunit, a b-subunit and a g-subunit
The a-subunit has a site which can be
occupied by GDP or GTP
Normally, it is occupied by GDP
When the hormone binds to its receptor,
GDP is displaced by GTP

39. Hormone

40. The GTP-bearing a-subunit dissociates
from the b- and g-subunits
The a-subunit goes and binds to
adenylate cyclase
Adenylate cyclase is activated
Active adenylate cyclase produces cAMP

41. A.
B.
C.
Hormone
Receptor
GDP
GDP
GTP
G-Protein
Cell membrane
Adenylate cyclase
GTP
ATP cAMP + PPi
a

42. a-Subunit has intrinsic GTPase activity
which slowly hydrolyses GTP into GDP
When GTP is converted into GDP, the a-
subunit dissociates from the effector
It goes back to and re-associates with the
b- and g-subunits

43. Pi
GDP
GDP
D.
E.

44. The G-proteins which stimulate the
effector are known as stimulatory
G-proteins or Gs-proteins
There are inhibitory G-proteins or Gi-
proteins also which inhibit the effector

45. cGMP
The hormones using cGMP as a second
messenger include:
Atrial natriuretic peptide (ANP)
Nitric oxide (NO) etc
cGMP is formed from GTP by guanylate
cyclase

46. EMB-RCG
GTP
cGMP
GMP
Guanylate
cyclase
cGMP phospho-
diesterase
PPi

47. ANP activates the membrane-bound form
NO activates the cytosolic form
Membrane-bound form
Cytosolic (soluble) form
Two forms of guanylate cyclase are
known:

48. Guanylate cyclase activity is present in the
cytoplasmic portion of the ANP receptor
This is switched on when ANP binds to its
receptor on the cell surface

49. cGMP is second messenger for NO also
NO is the only hormone known so far
which is a gas
NO is synthesized from arginine by nitric
oxide synthase (NOS)

50. 2 H2N—C—N—CH2—CH2—CH2—CH—COOH
HN H NH2
Arginine
Citrulline
4 O2 + 3 NADPH + 3 H+
2 NO + 3 NADP+ + 4 H2O
Nitric oxide
synthase
II I I
2 H2N—C—N—CH2—CH2—CH2—CH—COOH
II
O H NH2
I I

51. Guanylate cyclase, activated by ANP and
NO, converts GTP into cGMP
cGMP activates protein kinase G
Protein kinase G phosphorylates some
target proteins
This produces the biological effects

52.  Nucleic acids include deoxyribonucleic acid
(DNA) and ribonucleic acid (RNA)
 Both are information molecules
 DNA stores genetic information
 The entire genetic material present in the DNA of
an organism is known as its genome

53. Nitric oxide synthase (NOS) has three
isoforms – iNOS, nNOS and eNOS
iNOS is inducible, and is present in
macrophages and neutrophils
nNOS is present in neurons
eNOS is present in endothelial cells

54. NO synthesized in phagocytes aids in the
destruction of phagocytosed cells
NO synthesized in neurons acts as a
neurotransmitter
NO formed in endothelial cells diffuses into
adjacent smooth muscles
It causes vasodilatation by relaxing the
smooth muscles

55. Ca++ and/or phosphoinositides
Hormones using Ca++ and/or phospho-
inositides as second messenger include:
• Oxytocin
• Gastrin
• Cholecystokinin
• Epinephrine acting through a1-adrenergic
receptors
Binding of these hormones to their
receptors activates phosphoinositidase

56. Phosphoinositidase is a membrane-bound
enzyme
It is also known as phospholipase C
Signal is transmitted from the receptor to
phospholipase C by a G-protein

57. Activated phospholipase C acts on phos-
phatidyl inositol-4,5-biphosphate (PIP2)
PIP2 is present in the cell membrane
It is formed by phosphorylation of phos-
phatidyl inositol

58. CH2 — O —C — R1
CH — O — C — R2
CH2 — O —P — O
|
OH
O
||
O
||
O
||
H
H
H
OH
OH OH
H H
OH
H
Phosphatidyl inositol (PI)
OH
CH2— O — C — R1
CH — O — C — R2
CH — O — P — O2
O
||
O
||
O
||
H
H
H
O
OH OH
H H
O
H
Phosphatidyl inositol-4,5-biphosphate (PIP2)
OH
P
|
PI 3-kinase
2 ATP 2 ADP
P
|
|
OH

59. Phospholipase C hydrolyses phosphatidyl
inositol-4,5-biphosphate (PIP2) into:
Inositol tri-
phosphate (IP3)
Diacylglycerol
(DAG)

60. CH2 — O —C — R1
CH — O — C — R2
|
CH2 — OH
O
||
O
||
Diacylglycerol
(DAG)
Inositol-1,4,5-triphosphate (IP3)
H
H
O
OH OH
H H
O
HOH
P
|
P
|
O
H
P
|
H2O
Phospholipase C
CH2— O — C — R1
CH — O — C — R2
CH2 — O —P — O
O
||
O
||
O
||
H
H
H
O
OH OH
H H
O
H
Phosphatidyl inositol-4,5-biphosphate (PIP2)
OH
P
|
P
|
|
OH

61. IP3 releases calcium ions from its intra-
cellular stores e.g. endoplasmic reticulum
Calcium ions combine with calmodulin,
and activate calmodulin kinase
DAG activates protein kinase C

62. Activated calmodulin kinase and protein
kinase C cause phosphorylation of some
target proteins
The phosphorylated proteins produce
the biological effects

63. Hormone
Receptor
Phosphatidyl inositol-4, 5-biphosphate
Endoplasmic
reticulum
Inositol
triphosphate
Diacylglycerol
Calmodulin
Ca++–Calmodulin
Calmodulin kinase Protein kinase C
Phosphorylation of target proteins
Biological effects
+
Ca++
+
+
Phospholipase C
G-
Protein

64. The hormones acting via tyrosine kinase
include insulin, growth hormone, prolactin
etc
These hormones act by phosphorylating
tyrosine residues of some target proteins
in the cell
Tyrosine kinase

65. EMB-RCG
Most of the growth factors also act by
tyrosine phosphorylation, for example:
Insulin-like growth factor-1 (IGF-1)
Insulin-like growth factor-2 (IGF-2)
Nerve growth factor (NGF)
Epidermal growth factor (EGF)
Platelet-derived growth factor (PDGF)

66. Receptors for hormones acting via tyrosine
kinase are trans-membrane
The extracellular portion of the receptor has
got the hormone-binding domain
The cytoplasmic portion has got tyrosine
kinase domain

67. When the hormone binds to the receptor,
tyrosine kinase domain becomes active
Tyrosine kinase phosphorylates some
tyrosine residues in the receptor itself
The receptor becomes auto-phosphorylated

68. The auto-phosphorylated receptor
phosphorylates the tyrosine residues of
some target proteins
The target proteins become active and
produce the biological effects

69. An example of this subgroup of hormones
is insulin
Insulin receptor is made of two a-chains
and two b-chains
The a-chains are linked to each other by
disulphide bonds

70. The a-chains are linked to b-chains by
disulphide bonds
The a-chains are entirely extracellular
The b-chains are partly extracellular, partly
membrane-embedded and partly cytosolic

71. The a-chains possess the insulin-binding
site
The cytoplasmic portion of b-chains
possesses tyrosine kinase domain
The tyrosine kinase domain is normally
inactive

72. HOOC‒
H2N
I
NH2
I
‒COOH
H2N‒ ‒NH2
I
HOOC
I
COOH
I
S
I
S
I
I
S
I
S
I
‒S‒S‒
‒‒ Tyrosine kinase domain
a-Chain ‒‒
b-Chain ‒‒
Cell
membrane ‒
Insulin receptor

73. Binding of insulin to the a-chains switches
on the tyrosine kinase activity of b-chains
Active tyrosine kinase phosphorylates
some tyrosine residues in the b-chains
The receptor becomes auto-phosphorylated

74. Autophosphorylated insulin receptor
I
S
I
S
I
I
S
I
S
I
‒S‒S‒
Insulin
P P
P P

75. The auto-phosphorylated receptor phospho-
rylates tyrosine residues of some target
proteins
These include insulin receptor substrate-1
(IRS-1), insulin receptor substrate-2 (IRS-2)
etc
This initiates a cascade of reactions
culminating in the varied biological effects of
insulin

76. Minute amounts of hormones produce
profound biological effects
Therefore, regulation of hormone secretion
should be very precise
Hormone secretion should increase when
its biological effect is required
It should decrease when the desired
biological effect has been achieved
Regulation of hormone secretion

77. Hypothalamo-hypophyseal-endocrine axis
is one regulatory cascade
It is involved in the regulation of several
hormones
Feedback inhibition is another important
mechanism
A rise in hormone concentration causes
feedback inhibition of hormone secretion

78. As an example, secretion of thyroid hormones
is regulated by both the mechanisms
When there is need for thyroid hormones,
hypothalamus secretes thyrotropin-releasing
hormone (TRH)
TRH stimulates anterior pituitary to secrete
thyroid stimulating hormone (TSH)

79. TSH acts on thyroid gland and stimulates
the secretion of thyroid hormones
Raised blood level of thyroid hormones
causes feedback inhibition of:
TRH secretion
TSH secretion
Secretion of thyroid hormones

80. Hypothalamus
Anterior pituitary
Thyroid gland
Thyrotropin-releasing hormone
Thyroid-stimulating hormone
Thyroxine and
tri-iodothyronine
+
+
–
–
–

81. In some cases, feedback regulation is
exercised by some metabolite affected by
the hormone
An example is regulation of
parathormone (PTH) secretion
A decrease in plasma calcium
concentration evokes PTH secretion

82. PTH initiates a series of reactions leading
to a rise in plasma calcium concentration
Rise in plasma calcium level causes
feedback inhibition of PTH secretion

83. Plasma calcium
Parathyroid glands
Parathrormone Θ

84. Hormones are present in blood in very
minute concentrations
Usual techniques are not suitable for
measuring hormone concentrations
Yet measurements are often required for
diagnosis of endocrine disorders
Assay of hormone concentrations

85. In the past, hormone concentrations were
measured by bio-assay
Biological effect of the hormone was
measured in live animals or in tissues
This lacked sensitivity

86. Modern techniques are generally based
on antigen-antibody reaction
These immunochemical techniques are
quick, accurate and sensitive

87. The common immuno-
chemical techniques are:
Radio-immuno-assay
Enzyme-immuno-assay
Chemiluminescence-immuno-assay

88. Antibodies are prepared against the
hormone
The hormone is labeled with a radio-
active isotope e.g. 125I
Labeled hormone, unlabeled hormone and
the antibody are added to a test tube
Radio-immuno-assay (RIA)

89. The amount of antibody is kept very low
Labeled and unlabeled hormone compete
with each other to bind to the antibody
The degree of binding depends upon their
relative concentrations

90. The amount of antibody bound to labeled
hormone is quantitated by measuring
radio-activity
Radio-activity emitted by 125I can be
measured by a gamma counter

91. Briefly, the steps are:
A series of tubes, marked S1, S2, S3, S4 etc
(standards) and U (unknown), is set up
A fixed, and relatively small, amount of
antibody (Ab) is added to each tube
A fixed amount of labeled hormone (Ag*)
is added to each tube
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92. Unlabeled hormone (Ag) is added to
tubes marked S1, S2, S3 , S4 etc in
increasing amounts (C1, C2, C3 , C4 etc)
Patient’s serum having an unknown
amount of unlabeled hormone (CU) is
added to the tube marked U
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93. Amount of antibody
(Ab) x x x x x
Amount of labeled
hormone (Ag*) y y y y y
Amount of unlabeled
hormone (Ag) C1 C2 C3 C4 ‒
Amount of hormone
in patient’s serum – – – – CU
S1 S2 S3 S4 U

94. The tubes are incubated for a fixed period
Some Ab combines with Ag and some
with Ag*
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Ag and Ag* compete with each other
to bind to the limited amount of Ab

95. The tubes are centrifuged
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Ag-Ab and Ag*-Ab complexes settle at
the bottom

96. Amounts of Ag-Ab and Ag*-Ab complex in a
tube depend upon amounts of Ag* and Ag
If the amount of Ag is less than that of Ag*,
less Ag-Ab complex will be formed than
Ag*-Ab complex
As the amount of Ag increases, more Ag-Ab
and less Ag*-Ab complex will be formed

97. Ag*-Ab complex is radio-active while
Ag-Ab complex is not
The concentration of Ag*-Ab complex in
different tubes will be different
Its concentration will be inversely
proportional to the concentration of Ag

98. Ag + Ag* + Ab → Ag-Ab + Ag*-Ab + Ag* + Ag
The supernatant containing unbound Ag
and Ag* is removed
Since antibody concentration is low,
some unbound Ag and Ag* will also
remain in each tube
Radioactivity is measured in each tube

99. A calibration curve is prepared by plotting
radioactivity in S1, S2, S3 etc against the
concentrations of Ag (C1, C2, C3 etc)
The concentration of hormone in the
patient’s serum (CU) can be read off from
the calibration curve

100. Hormone concentration →
C1 C2
Cu
C3 C4
↑
Radio-
activity
Calibration curve

101. The principle of EIA is similar to that of RIA
Instead of an isotope, an enzyme is used
as a label
The common enzymes used as label are
peroxidase and alkaline phosphatase
Enzyme-immuno-assay (EIA)

102. Instead of measuring radioactivity, the
enzyme concentration is measured
For measuring enzyme concentration,
substrate of the enzyme is added
The amount of product formed is
determined after a fixed interval

103. A common form of EIA is enzyme-linked
immuno-sorbent assay (ELISA)
In ELISA, the antibody is fixed on a solid
support
The solid support may be wells molded in
a plastic plate

104. An ELISA plate with wells molded in it

105. Antibody concentration in the wells is
much higher than antigen (hormone)
concentration
The wells are labeled S1, S2, S3, S4 etc for
standard solutions
The well labeled U is for unknown i.e.
patient’s serum

106. The steps are:
Standard hormone solutions in increasing
concentrations (C1, C2, C3 etc) are added
to the wells marked S1, S2, S3 etc
Patient’s serum having an unknown
hormone concentration (CU) is added
to the well U
Hormone molecules bind to the fixed
antibody molecules
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107. A second antibody recognizing a different
epitope of the antigen (hormone) is tagged
with an enzyme
It is added to each well in a relatively large
amount
The enzyme-linked antibody also binds to
the hormone
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108. One
epitope
│
│
One
antibody
│
Second
antibody
Enzyme
│
ANTIGEN
Second
epitope
│

109. Complex

110. The unbound enzyme-linked antibody
molecules are washed off
Each well now contains complexes of the
fixed antibody, the antigen and the
enzyme-linked antibody
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111. A fixed amount of substrate of the
enzyme is added to each well
The enzyme converts the substrate
into a coloured product
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112. Substrate → Product
Antibody fixed in
well
Antigen added;
binds to antibody
Enzyme-linked
antibody added;
binds to antigen
Substrate added;
converted into
product




113. The absorbance is proportional to the
enzyme concentration
The enzyme concentration is proportional to
the hormone concentration
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After a fixed incubation period, intensity of
colour (absorbance) is measured in each
well

114. A calibration curve is plotted between
the known concentrations of hormone
(C1, C2, C3 etc) and the absorbance
The hormone concentration in the
patient’s serum (CU) can be read off
from the calibration curve
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115. Hormone concentration →
C1 C2 C3 C4
CU
↑
Absorbance
Calibration curve

116. CLIA is based on chemiluminescence i.e.
emission of light driven by a chemical
reaction
Principle of CLIA is similar to that of EIA
Instead of an enzyme, a chemiluminescent
substance is used as a label
Chemiluminescence-immuno-assay (CLIA)

117. Chemiluminescent
substances include:
Luminol
Isoluminol
Acridium esters

118. The resulting emission of light is measured
The chemiluminescent substance is
usually oxidised to form an excited
intermediate
When this intermediate returns to the
ground state, it releases photons

119. Luminol + H2O2
↓
3-Aminophthalate + H2O + Light
Peroxidase
The reaction will occur on addition of H2O2
Luminol can be used as a label

120. Luminol and peroxidase
before adding H2O2
Chemiluminescence
after addition of H2O2
Chemiluminescence is measured by a luminometer

121. CLIA is extremely sensitive, quick and
relatively economical
It is linear over a wide range of
concentrations of the analyte
Shelf-life of CLIA reagents is fairly long