Steroids

A steroid is a lipid characterized by a carbon skeleton with four fused rings. Different steroids vary in the functional groups attached to these rings. Hundreds of distinct steroids have been identified in plants and animals. Their most important role in most living systems is as hormones.

Steroid skeleton. Carbons 18 and above can be absent.

In human physiology and medicine, the most important steroids are cholesterol, the steroid hormones, and their precursors and metabolites.

In conversations, the term steroid is often used more restrictively in specific contexts. For instance, steroid in common medical usage by non-endocrinologists usually refers to corticosteroids, nearly always glucocorticoids. In an athletic or body-building context, steroid commonly refers to anabolic steroids.

Cholesterol is an important steroid alcohol, being a common component of animal cell membranes. However, a high level of it can cause various conditions and diseases, such as atherosclerosis. Most other steroids are synthesized from cholesterol. Also, various hormones, including vertebrate sex hormones, are steroids created from cholesterol.

There are also various specific categories of steroids, including:

• Anabolic steroids - used by athletes to increase performance
• Corticosteroids - affects metabolism and electrolyte excretion
• Sex hormones - androgens, estrogens, and progestagens
• Prohormones - precursors to actual steroid hormones. They were manufactured by bodybuilding supplement companies until they were banned from the supplement market.
• Phytosterols - steroids naturally occurring in plants.

Steroid hormones produce their physiological effects by binding to steroid hormone receptor proteins. The binding of steroid hormones to their receptors causes changes in gene transcription and cell function.

Synthesis

Steroids are synthesized biologically in the HMG-CoA reductase pathway, one of the two terpenoid metabolic pathways known to exist in most organisms. Their most direct hydrocarbon ancestor is the triterpene squalene, which is metabolised first into lanosterol, then into cholesterol.

Most steroid hormones are synthesized from pregnenolone, a derivative of cholesterol. Steroid hormones contain 21 or fewer carbon atoms. The first step in steroid hormone synthesis is the removal of the 6 carbon side-chain of cholesterol. The enzyme desmolase cleaves hydroxylated bonds at C-20 and C-22. Three molecules of O₂ and three molecules of NADPH are consumed in the conversion from cholesterol to pregnenolone. Adrenocorticotropic hormone (ACTH) stimulates this conversion.

Books on Steroids

• Dunks, Doubles, Doping: How Steroids are Killing American Athletics (ISBN 1592289029)

External links

• Michael W. King's Medical Biochemistry. Note: King's webage starts out by saying that retinoic acid is a steroid hormone. Steroids and retinoids are both hydrophobic, pass through cell membranes and bind to intracellular rwangaeceptors. However, retinoic acid does not have the structure of a steroid. See: Steroids and Related Hydrophobic Molecules.
• "Biochemistry" by Jeremy M. Berg, John L. Tymoczko and Lubert Stryer (2002) W. H. Freeman and Co. steroid topics in this text