Lipids as Signals, Cofactors, and Pigments:- Steroid Hormones Carry Messages between Tissues
Steroids are oxidized derivatives of sterols; they have the sterol nucleus but lack the alkyl chain attached to ring D of cholesterol, and they are more po lar than cholesterol. Steroid hormones move through the bloodstream (on protein carriers) from their site of production to target tissues, where they enter cells, bind to highly specific receptor proteins in the nucleus, and trigger changes in gene expression and metabolism. Because hormones have very high affinity for their receptors, very low concentrations of hormones (nanomolar or less) are sufficient to produce responses in target tis sues. The major groups of steroid hormones are the male and female sex hormones and the hormones produced by the adrenal cortex, cortisol and aldosterone (Fig. 10–19). Prednisone and prednisolone are steroid drugs with potent anti-inflammatory activities, mediated in part by the inhibition of arachidonate release by phospholipase A2 and consequent inhibition of the synthesis of leukotrienes, prostaglandins, and throm boxanes. They have a variety of medical applications, including the treatment of asthma and rheumatoid arthritis.
FIGURE 10–19 Steroids derived from cholesterol. Testosterone, the male sex hormone, is produced in the testes. Estra diol, one of the female sex hormones, is produced in the ovaries and placenta. Cortisol and aldosterone are hormones synthesized in the cortex of the adrenal gland; they regulate glucose metabolism and salt excretion, respectively. Prednisolone and prednisone are synthetic steroids used as antiinflammatory agents.
