Conversion of Cholesterol into Bile Acids
Primary Location
Liver
Steps Involved
Initial Step
Cholesterol is converted to 7α-hydroxycholesterol by the enzyme cholesterol 7α-hydroxylase (CYP7A1), which is the rate-limiting step in bile acid synthesis.
Subsequent Reactions
Further hydroxylation, oxidation, and conjugation reactions lead to the production of cholic acid and chenodeoxycholic acid, the two primary bile acids.
Formation of Bile Salts
These bile acids are conjugated with either glycine or taurine, forming bile salts, which enhances their solubility and effectiveness in emulsifying dietary fats.
Storage and Release
Bile salts are stored in the gallbladder and released into the small intestine during digestion to aid in the digestion and absorption of fats.
Conversion of Cholesterol into Steroid Hormones
Primary Location: Steroidogenic cells in the adrenal cortex, gonads, and placenta.
Steps Involved
Cholesterol Transport
Cholesterol is transported from the outer to the inner mitochondrial membrane by steroidogenic acute regulatory protein (StAR).
Initial Conversion
Cholesterol is converted into pregnenolone by the enzyme cytochrome P450 side-chain cleavage enzyme (CYP11A1 or P450scc). This is the rate-limiting step in steroid hormone synthesis.
Steroid Hormone Synthesis
Pregnenolone is then metabolized into various steroid hormones, including glucocorticoids, mineralocorticoids, and sex hormones (androgens, estrogens, progestogens) through a series of hydroxylation, oxidation, and isomerization reactions.
Conversion of Cholesterol into Vitamin D
Process Overview
Involves processes in the skin, liver, and kidneys to produce active vitamin D.
Steps Involved
Skin Reaction
In the skin, 7-dehydrocholesterol is converted to previtamin D3 upon exposure to UVB radiation. Previtamin D3 then spontaneously isomerizes to vitamin D3 (cholecalciferol).
Liver Conversion
In the liver, cholecalciferol is hydroxylated by the enzyme 25-hydroxylase (CYP2R1) to form 25-hydroxycholecalciferol (calcifediol).
Kidney Conversion
In the kidneys, calcifediol is further hydroxylated by 1α-hydroxylase (CYP27B1) to produce the biologically active form, 1,25-dihydroxycholecalciferol (calcitriol).
Role of Calcitriol
Calcitriol plays a crucial role in regulating calcium and phosphate homeostasis. It enhances the absorption of calcium and phosphate from the intestines and promotes bone health by regulating their levels in the blood.