8. Structure and biological activities of steroids
Learning objectives
- What are steroids?
- What is the structure of steroids?
- What are bile acids?
- What are the functions of bile acids?
- Which organs are involved in vitamin D synthesis?
General
Steroids are a type of organic molecule with a specific steroid structure. The steroid core is composed of seventeen carbon atoms bonded into four rings. The rings are denoted A, B, C and D. The carbon atoms are counted according to a standardized IUPAC system.
Many important biological molecules are steroids, like cholesterol, vitamin D, and bile acids. A group of hormones called steroid hormones have steroid structure and are very important. Steroid hormones are further described in topic 31. Cholesterol is further described in topic 9.
Note that the word “steroids” in the medical practice usually refers to drugs which have corticosteroid-like effects.
Bile acids
Bile acids are steroid acids produced by the liver and excreted into the bile. Cholesterol is converted into primary bile acids in the liver. Primary bile acids are released, as a part of bile, into the duodenum after eating. Bacteria in the intestinal lumen converts primary bile acids into secondary bile acids.
Bile acids are detergents. They allow for the emulsion and absorption of fat from the diet. 95% of secondary bile acids are reabsorbed together with the food. This is called the enterohepatic circulation.
Bile acids also allow for the excretion of cholesterol and bilirubin into the bile.
A deficiency of bile acids can cause fat malabsorption, and the formation of gall stones in the gallbladder or biliary tract.
Vitamin D
Vitamin D is a hormone which is important in regulating calcium homeostasis. The body can synthesize vitamin D from cholesterol, but many organs are involved.
In the liver cholesterol is converted into 7-dehydrocholesterol. This compound is transported to the skin. UV radiation hitting the 7-dehydrocholesterol in the skin cleaves it to cholecalciferol.
Cholecalciferol is transported to the liver, where it is hydroxylated to calcidiol. Calcidiol is transported to the kidneys, which catalyse the final step of vitamin D synthesis; hydroxylation of calcidiol to calcitriol (1,25-dihydroxycholecalciferol).
Dolichol
Dolichol is a molecule which is involved in co-translational modification of proteins. It is synthesised not from a steroid, but from a steroid precursor called farnesyl-PP.
Summary
- What are steroids?
- Steroids are biological molecules with a steroid structure
- What is the structure of steroids?
- The steroid core is composed of seventeen carbon atoms bonded into four rings, A, B, C, and D
- What are bile acids?
- Bile acids are steroid acids produced by the liver and released into the bile
- What are the functions of bile acids?
- Bile acids are detergents, increasing the digestion and absorption of dietary fat
- Which organs are involved in vitamin D synthesis?
- The liver, skin, and kidney