Dehydroepiandrosterone pathway

Figure 42-5. Pathways of testosterone biosynthesis. The pathway on the left side of the figure is called the or dehydroepiandrosterone pathway the pathway on the right side is called the A" or progesterone pathway. The asterisk indicates that the 17a-hydroxy-lase and 17,20-lyase activities reside in a single protein, P450cl7.

The major androgen or androgen precursor produced by the adrenal cortex is dehydroepiandrosterone (DHEA). Most 17-hydroxypregnenolone follows the glucocorticoid pathway, but a small fraction is subjected to oxidative fission and removal of the two-carbon side chain through the action of 17,20-lyase. The lyase activity is actually part of the same enzyme (P450cl7) that catalyzes 17tt-hydroxylation. This is therefore a dual function protein. The lyase activity is important in both the adrenals and... 

FIGURE 52-6 Biosynthesis/metabolism of steroids in the CNS. The conversion of delta5P to dehydroepiandrosterone (DHA) is postulated but not demonstrated. D5P and DHA inhibit and 3oc,5oc-THP potentiates GABAa receptor function, as summarized in Figure 52-7. Solid arrows indicate demonstrated pathways dotted arrows indicate possible pathways. Metabolic inhibitors of enzymes are indicated by . (Redrawn from [12], with permission.)... 

Brown RC, Cascio C, Papadopoulos V. 2000. Pathways of neurosteroid biosynthesis in cell lines from human brain regulation of dehydroepiandrosterone formation by oxidative stress and beta-amyloid peptide. J Neurochem 74 ... 

Estriol is also produced by another pathway in the syncytiotrophoblast cells of the placenta DHEAS from fetal adrenal is converted to 16a-hydroxydehydroepiandrosterone sulfate in the fetal liver, followed by removal of the sulfated chain to produce 16a-hydroxy-dehydroepiandrosterone, which is then aromatized to estriol. Estriol is the predominant estrogen produced during pregnancy, and almost all of tiie estriol and estradiol produced by the placental syncytiotrophoblast enters the maternal circulation. By the 7th week of gestation, the placenta produces the majority of the estrogen in the maternal circulation. 

The intermediates employed in the biosynthesis of androgens belong to two structurally different chemical classes, a,)3-unsaturated ketones (progesterone) and /3,y-unsatui ated alcohols (pregnenolone and dehydroepiandrosterone). The biosynthetic pathways in the production of androgens are also different [390]. 

Metabolic studies carried out with isotopically labeled dehydroepi-androsterone sulfate [312] showed that this conjugated steroid may follow an indirect metabolic pathway initiated by the hydrolysis of the sulfate group. In the course of the metabolism the conjugated steroid thus becomes a free steroid first and the free steroid may undergo further metabolism. On the other hand, dehydroepiandrosterone sulfate may follow a direct metabolic pathway without a break of the ester group. 

In most mammals, estrogens (female sex steroid hormones) are synthesized from cholesterol using the parent ring structure, cyclopentanoperhydrophenan-threne of the estrane series. The steroidogenic pathway includes the production of the androgenic precursors dehydroepiandrosterone and androstenedione, the latter of which is converted to testosterone, then to estradiol-17/i. This requires aromatization of these andogenic precursors by an aromatase enzyme complex. The major source of estrogen in postmenopausal women is the conversion of androstenedione to estrone by aromatase activity... 

Fig. 1 Human steroidogenic pathway from cholesterol to adrenal or gonadal steroid products. CYP cytochrome P450 enzyme, ftSfthydroxysteroid dehydrogenase, DHEA dehydroepiandrosterone, Oft hydroxy, arom aromatase...

DHEA (dehydroepiandrosterone) is another nonprescription antidepressant that has gone through cycles of popularity (Figure 6.2). DHEA, a steroid, is made from cholesterol in the adrenal gland and is part of the biochemical pathway in which hormones such as estrogen and testosterone are end-products. While its properties as a peripheral hormone are well characterized, less is known about DHEA s function in the brain. Media interest in DHEA arose when it was shown to prevent or slow memory loss in older populations. Further research showed that DHEA could influence neuronal function in the hippocampus, a region important for memory formation. However, several clinical studies showed mixed results when DHEA was used in... 

F. 34.23. Synthesis of the steroid hormones. The rings of the precursor, cholesterol, are lettered. Dihydrotestosterone is produced from testosterone by reduction of the carbon-carbon double bond in ring A. Structural changes between the precursor and final hormone are noted in blue. DHEA = dehydroepiandrosterone. The dashed lines indicate alternative pathways to the major pathways indicated. The starred enzymes are those that may be defective in the condition congenital adrenal hyperplasia. 

In another pathway, 17a,20a-dihydroxycholesterol. or 17a,20a-trihydroxycholesterol is an intermediate between cholesterol and dehydroepiandrosterone. Androgens are also assumed to be synthesized in liver and nongonadal tissue from 21-hydroxylated derivatives such as 3j8,17a,21 -trihydroxy-5a-pregnane-11,20-dione and 11-deoxycorticosol, which by splitting of the side chain yield dehydroepiandrosterone or androst-4-ene- 3,17-dione. 

The development of the various adrenal enzymes in the fetal adrenal may also be of considerable significance in interpreting the lack of activity observed after birth. jS-Hydroxysteroid dehydrogenase is low in fetal liver thus, the pathway prevails (pregnenolone— dehydroepiandrosterone), and cortisol synthesis is low. The relatively large adrenals found in utero may result from compensatory hyperplasia in response to low cortisol secretion. In early embryonic life, 21- and 11-hydroxylase activities are low, but they increase with the age of the fetus. 

The discovery of dehydroepiandrosterone sulfate secretion gave a new incentive to the whole study of conjugation, and it soon became evident that dehydroepiandrosterone sulfate could not only be biosynthesized from sulfo conjugated percursors along a direct biosynthetic pathway, but could also undergo further metabolism, with or without hydrolysis of the sulfate moiety (i.e., indirect or direct metabolism) and act as a privileged precursor of active steroids. 

Roberts et al. (1961) first described the conversion of dehydroepian-drosterone sulfate into androsterone and 5/3-androsterone glucuronides in vivo, thereby establishing that dehydroepiandrosterone sulfate can be split in vivo and that there is a dynamic equilibrium dehydroepiandrosterone sulfate dehydroepiandrosterone (Fig. 4). The conversion of dehydroepiandrosterone sulfate into androsterone and 5 -androsterone sulfates can only exist via an indirect pathway (Baulieu et al., 1965) (Fig. 5), but dehydroepiandrosterone sulfate can be transformed both directly and indirectly into epiandrosterone sulfate (Fig. 6). 

Fig. 6. 3a- H- C-dehydroepiandrosterone sulfate. Direct pathway to H- C-epiandrosterone sulfate, and indirect major pathway to C-epiandrosterone sulfate.

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