Conversion of a53pHydroxysteroids to a43Ketosteroids

After cholesterol side-chain cleavage, two different pathways (A5 and A4) have been identified, which are defined by the position of one of the double bonds in the steroid molecule. In the A4 pathway, pregnenolone, which has a double bond between carbons 5 and 6 of the steroid backbone, is converted to progesterone, which has a double bond between carbons 4 and 5. An isomerization reaction shifts the position of the double bond from between carbons 5 and 6 to between carbons 4 and 5. In the A5 pathway, A5 to A4 isomerization does not occur until the last step, in which androstenediol is converted to testosterone. The intermediates of the two pathways are A5-3P-hydroxysteroids and A4-3-ketosteroids, respectively. The conversion of A5-3P-hydroxysteroids to A4-3-ketosteroids is accomplished by 3P-HSD enzyme, which uses NAD+ as a cofactor, sequentially catalyzing a 3P dehydrogena-tion and isomerization (48). The 3P-HSD enzymes can act on three A5 intermediates, pregnenolone, 17a-hydroxypregnenolone, and dehydroepiandrosterone, converting them, respectively, to the A4 steroids progesterone, 17a-hydroxyprogesterone, and androstenedione. The predominant A5 to A4 conversion occurs most commonly with either pregnenolone or dehydroepiandrosterone as substrates rather than 17a-hydrox-ypregnenolone. Whether the A5- or A4-pathway is followed from pregnenolone to

Cholesterol Testosterone Cofactors

Fig. 4. Pathway of testosterone biosynthesis. Of the four possible biosynthetic pathways, the first or A5 predominates in the rabbit and human, whereas the second or A4 predominates in rodents. CHOL, cholesterol; PREG, pregnenolone; 17-PREG, 17-hydroxypregnenolone; DHEA, dehydroepiandros-terone; ADIOL, androstenediol; PROG, progesterone; 17-PROG, 17-hydroxyprogesterone; A4, androstenedione;T, testosterone; DHT, dehydrotestosterone; E2 17P-estradiol.

Fig. 4. Pathway of testosterone biosynthesis. Of the four possible biosynthetic pathways, the first or A5 predominates in the rabbit and human, whereas the second or A4 predominates in rodents. CHOL, cholesterol; PREG, pregnenolone; 17-PREG, 17-hydroxypregnenolone; DHEA, dehydroepiandros-terone; ADIOL, androstenediol; PROG, progesterone; 17-PROG, 17-hydroxyprogesterone; A4, androstenedione;T, testosterone; DHT, dehydrotestosterone; E2 17P-estradiol.

testosterone depends on the species and developmental status of the Leydig cell. In humans, the A5 pathway is predominant (49-52). Mammalian 3P-HSDs comprise a large family of enzymes (53,54). In the human branch of this family, two genes encode two corresponding, type I and II, 3P-HSD enzymes. Both types dehydro-genate 3P-hydroxysteroids and isomerize C21 and C19 steroids. Type I 3P-HSD is exclusively present in placenta and skin, whereas type II is the predominant form expressed in the adrenal, ovary, and testis. In the testis, type II 3P-HSD activity is localized exclusively in Leydig cells (37,55).

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