Transcortin

Transcortin, also known as corticosteroid-binding globulin (CBG) or serpin A6 is a is_associated_with::protein that in humans is encoded by the SERPINA6 is_associated_with::gene. It is an is_associated_with::alpha-globulin.

Function
This gene encodes an is_associated_with::alpha-globulin protein with is_associated_with::corticosteroid-binding properties. This is the major transport protein for is_associated_with::glucocorticoids and is_associated_with::progestins in the blood of most vertebrates. The gene localizes to a chromosomal region containing several closely related serine protease inhibitors (is_associated_with::serpins) which may have evolved by duplication events.

Binding
Transcortin binds several steroid hormones at high rates:


 * is_associated_with::Cortisol - Approximately 75% of the cortisol in circulation is bound to transcortin. (The rest is bound to is_associated_with::serum albumin.) Cortisol is thought to be biologically active only when it is not bound to transcortin.
 * is_associated_with::Cortisone
 * Deoxycorticosterone (DOC)
 * is_associated_with::Corticosterone - About 78% of serum corticosterone is bound to transcortin.
 * is_associated_with::Aldosterone - Approximately 17% of serum aldosterone is bound to transcortin, while another 47% is bound to serum albumin. The remaining 36% is free.
 * is_associated_with::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.
 * is_associated_with::17-Hydroxyprogesterone

In addition, approximately 4% of serum is_associated_with::testosterone is bound to transcortin. A similarly small fraction of serum is_associated_with::estradiol is bound to transcortin as well.

Synthesis
Transcortin is produced by the is_associated_with::liver and is increased by is_associated_with::estrogens.

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, unbound plasma cortisol in term pregnancy is approximately 2.5 times that of nonpregnant women.