8 Questions
Which cell type is primarily responsible for synthesizing testicular androgens?
Leydig cells
What is the rate-limiting step in testicular steroidogenesis?
Delivery of free cholesterol to mitochondria by StAR
Which hormone promotes the conversion of cholesterol to pregnenolone in the testes?
Luteinizing hormone (LH)
In the conversion of pregnenolone to testosterone via the Δ5 pathway, which enzyme converts DHEA to androstenediol?
17β-hydroxysteroid dehydrogenase (17β-OHSD)
Which enzyme is responsible for converting androstenediol to testosterone in the Δ5 pathway?
Δ5'4-isomerase
Which of the following enzymes converts pregnenolone to 17-hydroxypregnenolone?
17α-Hydroxylase
Which intermediate in the Δ4 pathway is responsible for the conversion to testosterone?
Androstenedione
What is the final product of the Δ5 pathway in testicular steroidogenesis?
Testosterone
Study Notes
Testicular Steroidogenesis
- Testicular androgens are synthesized by Leydig cells.
- Cholesterol is the precursor of testicular steroids.
- The rate-limiting step is the delivery of free cholesterol to mitochondria by StAR.
- The conversion of cholesterol to pregnenolone is identical to adrenal cortex.
- LH (Luteinizing hormone) promotes the conversion of cholesterol to pregnenolone, unlike ACTH or angiotensins II and III in adrenal cortex.
5-Pathway
- The 5-pathway is the more common pathway in testicular steroidogenesis.
- Pregnenolone is converted into 17-hydroxypregnenolone and DHEA.
- DHEA is converted into androstenediol by 17β-hydroxysteroid dehydrogenase (17β-OHSD).
- Androstenediol is converted to testosterone by 3β-hydroxysteroid dehydrogenase and Δ5'4-isomerase.
4-Pathway
- The 4-pathway involves the conversion of progesterone into 17-hydroxyprogesterone and then into androstenedione.
- Androstenedione is converted to testosterone by 17β-hydroxysteroid dehydrogenase (17β-OHSD).
Key Enzymes and Steps
- 17α-Hydroxylase: converts pregnenolone to 17-hydroxypregnenolone.
- 17,20-Lyase: converts 17-hydroxypregnenolone to Dehydroepiandrosterone.
- 3β-hydroxysteroid dehydrogenase: converts Dehydroepiandrosterone to Androstenediol.
- Δ5'4-isomerase: converts Androstenediol to Testosterone.
Learn about the synthesis of androgens in the testes, including the role of Leydig cells, cholesterol, and LH in steroidogenesis.
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