O

carbon 18 increases the mineralocorticoid potency of corticosterone by a factor of 200 and only slightly decreases glucocorticoid activity.

5. Cortisol differs from corticosterone only by the presence of a hydroxyl group at carbon 17. Cortisol is the most potent of the naturally occurring glucocorticoids. It has 10 times as much glucocorticoid activity as aldosterone, but less than 0.25% of aldosterone's mineralocorticoid activity in normal human subjects. Synthetic glucocorticoids with even greater potency than cortisol are available for therapeutic use.

6. Cortisone differs from cortisol only in that the substituent on carbon 11 is a keto group rather than a hydroxyl group. Cortisone is produced from cortisol at extraadrenal sites by oxidation of the hydroxyl group on carbon 11 and circulates in blood at about one-fifth the concentration of cortisol. Oxidation of cortisol to cortisone profoundly lowers its affinity for adrenal steroid hormone receptors and hence inactivates it. Cortisol is oxidized to cortisone in mineralocorticoid target cells, and cortisone can be reduced to cortisol in the liver and other glucocorticoid target tissues. This so-called cortisol-cortisone shuttle is catalyzed by two enzymes, 11-hydroxysteroid dehydrogenase (HSD)-I and HSD-II, which are products of different genes and are expressed in different tissues. HSD-I can catalyze the reaction in either direction, and hence may activate or inactivate the hormone. HSD-II, which is expressed in all mineralocorticoid target tissues, catalyzes only the oxidation of cortisol to cortisone (Fig. 5).

Steroids in the 19-carbon series usually have androgenic activity and are precursors of the estrogens (female hormones). Hydroxylation of either pregnenolone or progesterone at carbon 17 is the critical prerequisite for cleavage of the C20,21 side chain to yield the adrenal androgens dehydroepiandrosterone or androstenedione (see Fig. 4). The principal testicular androgen is testosterone, which has a hydroxyl group rather than a keto group at carbon 17. Although 19 carbon androgens are

17a-hydroxypregnenolone 17a-hydroxyprogesterone

Cortisol (active)

cortisone (inactive)

FIGURE 5 The cortisol-cortisone shuttle. Two enzymes, 11-hyrdox-ysteroid dehydrogenase (HSD-I and HSD-II), catalyze the inactivating conversion of cortisol to cortisone. HSD-I can also catalyze conversion of the inactive cortisone to cortisol.

Cortisol (active)

cortisone (inactive)

FIGURE 5 The cortisol-cortisone shuttle. Two enzymes, 11-hyrdox-ysteroid dehydrogenase (HSD-I and HSD-II), catalyze the inactivating conversion of cortisol to cortisone. HSD-I can also catalyze conversion of the inactive cortisone to cortisol.

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