Sterol 27-hydroxylase (CYP27A1) (CYP27A1)

Sterol 27-hydroxylase (CYP27A1) is a mitochondrial cytochrome P450 enzyme that initiates the alternative, or acidic, pathway of bile acid synthesis by converting cholesterol into 27-hydroxycholesterol (UniProt P12659). This pathway is a vital component of cholesterol homeostasis, particularly in the brain and vascular endothelium, where it facilitates the removal of excess cholesterol (Griffiths & Wang, 2020). CYP27A1 also plays a significant role in Vitamin D metabolism, acting as a 25-hydroxylase to produce the prohormone 25-hydroxyvitamin D3 (PubChem CID 1593). Mutations in the CYP27A1 gene cause cerebrotendinous xanthomatosis (CTX), a rare autosomal recessive disorder characterized by the accumulation of cholestanol and cholesterol in the central nervous system and tendons, leading to progressive neurological symptoms and cataracts (NIH GARD). Therapeutic intervention typically involves bile acid replacement therapy, which restores feedback inhibition on the classical bile acid synthesis pathway to prevent the buildup of neurotoxic metabolites. Beyond CTX, the pathway is studied for its involvement in atherosclerosis and hormone-dependent cancers, as its oxysterol products can modulate nuclear receptor activity and cell signaling.

Other names

Sterol 26-hydroxylaseVitamin D3 25-hydroxylaseCYP27CP27Alternative bile acid synthesis pathwayAcidic bile acid synthesis pathwayCholesterol 27-hydroxylase5-beta-cholestane-3-alpha,7-alpha,12-alpha-triol 27-hydroxylase

The CYP27A1-mediated alternative bile acid synthesis pathway is targeted primarily through substrate replacement or feedback inhibition. In patients with CYP27A1 deficiency, exogenous bile acids like chenodeoxycholic acid (CDCA) act as ligands for the farnesoid X receptor (FXR), which suppresses the expression of CYP7A1, the rate-limiting enzyme of the classical pathway (Björkhem, 2013). This suppression prevents the accumulation of toxic cholestanol and bile alcohols that otherwise result from the shunting of cholesterol through the classical pathway in the absence of functional CYP27A1 (NIH GARD). Additionally, the pathway's product, 27-hydroxycholesterol, serves as an endogenous selective estrogen receptor modulator (SERM) and LXR ligand, making it a target for modulating cardiovascular and oncological processes (Griffiths & Wang, 2020).