Pregnenolone (P5), or pregn-5-en-3β-ol-20-one, is an endogenous steroid and precursor/metabolic intermediate in the biosynthesis of most of the steroid hormones, including the progestogens, androgens, estrogens, glucocorticoids, and mineralocorticoids. In addition, pregnenolone is biologically active in its own right, acting as a neurosteroid. In addition, they may have protective effects against schizophrenia.

Biological activity

Neurosteroid activity

Pregnenolone is an allosteric endocannabinoid, as it is a negative allosteric modulator of the CB<sub>1</sub> receptor. Pregnenolone is involved in a natural negative feedback loop against CB<sub>1</sub> receptor activation in animals. It prevents CB<sub>1</sub> receptor agonists like tetrahydrocannabinol, the main active constituent in cannabis, from fully activating the CB<sub>1</sub>. A related compound AEF0117 has been derived from pregnenolone and is more specific for this type of activity.

Pregnenolone has been found to bind with high, nanomolar affinity to microtubule-associated protein 2 (MAP2) in the brain. In contrast to pregnenolone, pregnenolone sulfate did not bind to microtubules. as well as a positive allosteric modulator of the NMDA receptor. In addition, pregnenolone sulfate has been shown to activate the transient receptor potential M3 (TRPM3) ion channel in hepatocytes and pancreatic islets causing calcium entry and subsequent insulin release.

Nuclear receptor activity

Pregnenolone has been found to act as an agonist of the pregnane X receptor.

Pregnenolone has no progestogenic, corticosteroid, estrogenic, androgenic, or antiandrogenic activity. and controlled by anterior pituitary trophic hormones, such as adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormone, in the adrenal glands and gonads. There are two intermediates in the transformation of cholesterol into pregnenolone, 22R-hydroxycholesterol and 20α,22R-dihydroxycholesterol, and all three steps in the transformation are catalyzed by P450scc. Pregnenolone is produced mainly in the adrenal glands, the gonads, and the brain. Although pregnenolone is also produced in the gonads and brain, most circulating pregnenolone is derived from the adrenal cortex.

To assay conversion of cholesterol to pregnenolone, radiolabeled cholesterol has been used. Pregnenolone product can be separated from cholesterol substrate using Sephadex LH-20 minicolumns. This is in contrast to pregnenolone sulfate, which does not cross the blood–brain barrier.

Metabolism

Pregnenolone undergoes further steroid metabolism in one of several ways:

  • Pregnenolone can be converted into progesterone. The critical enzyme step is two-fold using a 3β-hydroxysteroid dehydrogenase and a Δ<sup>5-4</sup> isomerase. The latter transfers the double bond from C5 to C4 on the A ring. Progesterone is the entry into the Δ<sup>4</sup> pathway, resulting in production of 17α-hydroxyprogesterone and androstenedione, precursor to testosterone and estrone. Aldosterone and corticosteroids are also derived from progesterone or its derivatives.
  • Pregnenolone can be converted to 17α-hydroxypregnenolone by the enzyme 17α-hydroxylase (CYP17A1). Using this pathway, termed Δ<sup>5</sup> pathway, the next step is conversion to dehydroepiandrosterone (DHEA) via 17,20-lyase (CYP17A1). DHEA is the precursor of androstenedione.
  • Pregnenolone can be converted to androstadienol by 16-ene synthase (CYP17A1).
  • Pregnenolone can be converted to pregnenolone sulfate by steroid sulfotransferase, and this conversion can be reversed by steroid sulfatase.

Levels

Normal circulating levels of pregnenolone are as follows:

Studies have found that pregnenolone levels are not significantly changed after surgical or medical castration in men, which is in accordance with the fact that pregnenolone is mainly derived from the adrenal glands. Conversely, medical castration has been found to partially suppress pregnenolone levels in premenopausal women. Similarly, an adrenalectomized premenopausal woman showed incompletely diminished circulating pregnenolone levels.

Chemistry

Pregnenolone is also known chemically as pregn-5-en-3β-ol-20-one. Like other steroids, it consists of four interconnected cyclic hydrocarbons.

References