Steroid 21-hydroxylase


Steroid 21-hydroxylase (or steroid 21-monooxygenase or 21α-Hydroxylase; ) is a cytochrome P450 enzyme which is involved with the biosynthesis of the steroid hormones aldosterone and cortisol.

The CAS number is 9029-68-9.

21-Hydroxylase is encoded by the gene CYP21B.


It catalyses the hydroxylation of the carbon atom 21 in steroids (adding an "–OH"), which is necessary with the formation of these hormones.


Clinical significance

A defect within the CYP21B gene causes a disturbance of the development of the enzyme, which leads to congenital adrenal hyperplasia due to 21-hydroxylase deficiency. A related pseudogene is located near this gene; gene conversion events involving the functional gene and the pseudogene are thought account for many cases of steroid 21-hydroxylase deficiency.


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