Q.39 Which one of the following enzymes converts testosterone to estradiol?
(A) Aromatase
(B) 3β-hydroxysteroid dehydrogenase
(C) 5α-reductase
(D) 17β-hydroxysteroid dehydrogenase
Aromatase is the enzyme that converts testosterone to estradiol through aromatization. This multiple-choice question from CSIR NET Life Sciences tests understanding of steroidogenesis enzymes.
Correct Answer
** (A) Aromatase **
Option Analysis
Aromatase (CYP19A1) catalyzes the final step in estrogen biosynthesis by cleaving the C10-C19 bond in androgens like testosterone, yielding estradiol and formic acid. This cytochrome P450 enzyme operates in three oxidative steps and predominates in granulosa cells, adipose tissue, and brain.
3β-Hydroxysteroid dehydrogenase (3β-HSD) converts Δ5-3β-hydroxysteroids (like pregnenolone or DHEA) to Δ4-3-keto steroids (like progesterone or androstenedione), essential for all steroid classes but not for aromatizing testosterone to estradiol.
5α-Reductase reduces testosterone’s C4-5 double bond to form dihydrotestosterone (DHT), a potent androgen, primarily in prostate, skin, and liver; it blocks aromatization by competing with aromatase.
17β-Hydroxysteroid dehydrogenase (17β-HSD) interconverts 17-ketosteroids and 17β-hydroxysteroids, such as androstenedione to testosterone or estrone to estradiol, but does not directly convert testosterone (already 17β-hydroxy) to estradiol.
Aromatase serves as the key enzyme that converts testosterone to estradiol, playing a pivotal role in estrogen biosynthesis and hormonal balance crucial for CSIR NET Life Sciences aspirants studying steroidogenesis.
Aromatase Mechanism
Aromatase (CYP19A1) executes three sequential oxidations on testosterone’s angular methyl group at C19, leading to aromatization of the A-ring and production of estradiol. This process requires NADPH and oxygen, occurring mainly in ovarian granulosa cells under FSH stimulation.
Role in Steroidogenesis Pathway
In the two-cell model of estrogen production, theca cells supply androstenedione/testosterone via CYP17 and 3β-HSD, while granulosa cells use aromatase for final conversion to estradiol, regulating reproductive cycles.
Clinical Relevance
Aromatase inhibitors like anastrozole treat estrogen-dependent breast cancers by blocking testosterone-to-estradiol conversion, highlighting its therapeutic importance. Excess activity links to gynecomastia in males.
