3 Биологические мембраны. Обмен веществом (1160072), страница 20

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Two natural productsderived from fungi, lovastatin and compactin, have shown promisein treating patients with familial hypercholesterolemia. Both are competitive inhibitors of HMG-CoA reductase and thus inhibit cholesterolsynthesis. Lovastatin treatment lowers serum cholesterol by as muchas 30% in individuals who carry one copy of the gene for familial hypercholesterolemia. When combined with an edible resin that binds bileacids and prevents their reabsorption from the intestine, the drug iseven more effective.Steroid Hormones Are Formed by Side ChainCleavage and OxidationAll steroid hormones in humans are derived from cholesterol (Fig.

2041). Two classes of steroid hormones are synthesized in the cortex ofthe adrenal gland: mineralocorticoids, which control the reabsorption of inorganic ions (Na+, Cl~, and HCO3 ) by the kidney, and glucocorticoids, which help regulate gluconeogenesis and also reduce theinflammatory response. The sex hormones are produced in male andfemale gonads and the placenta. They include androgens (e.g., testosterone) and estrogens (e.g., estradiol), which influence the development of secondary sexual characteristics in males and females, respectively, and progesterone, which regulates the reproductive cycle infemales.

The steroid hormones are effective at very low concentrations,and they are therefore synthesized in relatively small quantities. Incomparison with the bile salts, their production consumes relativelylittle cholesterol.The synthesis of these hormones requires removal of some or all ofthe carbons in the "side chain" that projects from C-17 of the D ring ofcholesterol. Side chain removal takes place in the mitochondria of tissues that make steroid hormones.

It involves first the hydroxylation oftwo adjacent carbons in the side chain (C-20 and C-22) then cleavage ofthe bond between them (Fig. 20-42). Formation of the individual hormones also involves the introduction of oxygen atoms. All of the hydroxylation and oxygenation reactions in steroid biosynthesis are catalyzed by mixed-function oxidases (Box 20-1) that use NADPH, O2, andmitochondrial cytochrome P-450.ProgesteroneCholesterol*PregnenoloneProgesteroneCortisol ^(glucocorticoid)Affects proteinand carbohydratemetabolism;suppresses immuneresponse, inflammation,and allergic responses.•TestosteroneCorticosterone(mineralocorticoid)^EstradiolAldosterone(mineralocorticoid)Regulate reabsorptionofNa\er,HCO3-inthe kidney.Male and female sexhormones.

Influencesecondary sexual characteristics; regulatefemale reproductivecycle.Figure 20-41 Some steroid hormones derived fromcholesterol. The structures of some of these compounds are shown in Fig. 9-15..

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