In chemistry and biology, Steroids are a type of lipid, characterized by a carbon skeleton with four fused rings. Hundreds of distinct steroids have been identified in plants and animals. Their most important role in most living systems is as hormones.

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

In conversation, 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.

Different steroids vary in the functional groups attached to these rings. Cholesterol is an important steroid, being a common component of animal cell membranes. However, a high level of it can cause various conditions and diseases, including 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, manufactured by bodybuilding supplement companies

Steroid hormones have 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

Steroid hormones are synthesized from cholesterol. Cholesterol itself is absorbed from food or synthesized (mainly in the liver) from the intermediates of the citric acid cycle. There are 21 (later 22) carbon atoms in the hormone during its synthesis, and modifications are numbered after the C atom at which the change happens. The synthetic function of tissues depends on the hormones expressed therein. For example: the adrenal cortex expresses high levels of 21-hydroxylase. Most enzymes that catalyse steroid synthesis (anabolic enzymes) are located in the mitochondrion, as are many of the catabolic enzymes that chemically degrade steroids.

External links

• Michael W. King's Medical Biochemistry (http://www.indstate.edu/thcme/mwking/steroid-hormones.html). 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 receptors. However, retinoic acid does not have the structure of a steroid. See: Steroids and Related Hydrophobic Molecules (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=retinoids+AND+stryer%5Bbook%5D+AND+217301%5Buid%5D&rid=stryer.section.4466#4467).