Transcortin, also known as corticosteroid-binding globulin (CBG) or serpin A6, is a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...
produced in the liver in animals. In humans it is encoded by the SERPINA6
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
. It is an
alpha-globulin
Alpha globulins are a group of globular proteins in plasma that are highly mobile in alkaline or electrically charged solutions. They inhibit certain blood proteases and show significant inhibitor activity.
The alpha globulins typically have mo ...
.
Function
This gene encodes an
alpha-globulin
Alpha globulins are a group of globular proteins in plasma that are highly mobile in alkaline or electrically charged solutions. They inhibit certain blood proteases and show significant inhibitor activity.
The alpha globulins typically have mo ...
protein with
corticosteroid-binding properties. This is the major transport protein for
glucocorticoids and
progestins in the blood of most vertebrates. The gene localizes to a chromosomal region containing several closely related serine protease inhibitors (
serpins).
Binding
Transcortin binds several steroid hormones at high rates:
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Cortisol - Approximately 90% of the cortisol in circulation is bound to transcortin. (The rest is bound to
serum albumin.) Cortisol is thought to be biologically active only when it is not bound to transcortin.
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Cortisone
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Deoxycorticosterone (DOC)
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Corticosterone - About 78% of serum corticosterone is bound to transcortin.
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Aldosterone - Approximately 17% of serum aldosterone is bound to transcortin, while another 47% is bound to serum albumin. The remaining 36% is free.
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Progesterone - Approximately 18% of serum progesterone is bound to transcortin, while another 80% of it is bound to serum albumin. The remaining 2% is free.
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17α-Hydroxyprogesterone
In addition, approximately 4% of serum
testosterone is bound to transcortin.
A similarly small fraction of serum
estradiol is bound to transcortin as well.
Synthesis
Transcortin is produced by the
liver
The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...
and is increased by
estrogen
Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal a ...
s.
Clinical significance
Mutations in this gene are rare. Only four mutations have been described, often in association with fatigue and chronic pain.
This mechanism for these symptoms is not known. This condition must be distinguished from secondary hypocortisolism. Exogenous hydrocortisone does not appear to improve the fatigue.
Hepatic synthesis of corticosteroid-binding globulin more than doubles in pregnancy; that is, bound plasma cortisol in term pregnancy is approximately 2 to 3 times that of nonpregnant women.
See also
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Serpin
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Circaseptan, 7-day biological cycle
References
Further reading
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External links
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* The
MEROPS
MEROPS is an online database for peptidases (also known as proteases, proteinases and proteolytic enzymes) and their inhibitors. The classification scheme for peptidases was published by Rawlings & Barrett in 1993, and that for protein inhibi ...
online database for peptidases and their inhibitors
I04.954*
{{Use dmy dates, date=April 2017
Glycoproteins