5ñ-Reductases, also known as 3-oxo-5ñ-steroid 4-dehydrogenases, are enzymes involved in steroid metabolism. They participate in three metabolic pathways: bile acid biosynthesis, androgen and estrogen metabolism. There are three isozymes of 5ñ-reductase encoded by the genes SRD5A1, SRD5A2, and SRD5A3.
5ñ-Reductases catalyze the following generalized chemical reaction:
Where a 3-oxo-5ñ-steroid and acceptor are substrates, and a corresponding 3-oxo-ÃÂ4-steroid and the reduced acceptor are products. An instance of this generalized reaction that 5ñ-reductase type 2 catalyzes is:
where dihydrotestosterone is the 3-oxo-5ñ-steroid, NADP<sup>+</sup> is the acceptor and testosterone is the 3-oxo-ÃÂ4-steroid and NADPH the reduced acceptor.
The enzyme is produced in many tissues in both males and females, in the reproductive tract, testes and ovaries, skin, seminal vesicles, prostate, epididymis and many organs, including the nervous system. There are three isoenzymes of 5ñ-reductase: steroid 5ñ-reductase 1, 2, and 3 (SRD5A1, SRD5A2 and SRD5A3).
5ñ-Reductases act on 3-oxo (3-keto), ÃÂ<sup>4,5</sup> C19/C21 steroids as its substrates; "3-keto" refers to the double bond of the third carbon to oxygen. Carbons 4 and 5 also have a double bond, represented by 'ÃÂ<sup>4,5</sup>'. The reaction involves a stereospecific and permanent break of the ÃÂ<sup>4,5</sup> with the help of NADPH as a cofactor. A hydride anion (HâÂÂ) is also placed on the ñ face at the fifth carbon, and a proton on the ò face at carbon 4.
5ñ-R1 is expressed in fetal scalp and nongenital skin of the back, anywhere from 5 to 50 times less than in the adult. 5ñ-R2 is expressed in fetal prostates similar to adults. 5ñ-R1 is expressed mainly in the epithelium and 5ñ-R2 the stroma of the fetal prostate. Scientists looked for 5ñ-R2 expression in fetal liver, adrenal, testis, ovary, brain, scalp, chest, and genital skin, using immunoblotting, and were only able to find it in genital skin.
After birth, the 5ñ-R1 is expressed in more locations, including the liver, skin, scalp and prostate. 5ñ-R2 is expressed in prostate, seminal vesicles, epididymis, liver, and to a lesser extent the scalp and skin. Hepatic expression of both 5ñ-R1 and 2 is immediate, but disappears in the skin and scalp at month 18. Then, at puberty, only 5ñ-R2 is reexpressed in the skin and scalp.
5ñ-R1 and 5ñ-R2 appear to be expressed in the prostate in male fetuses and throughout postnatal life. 5ñ-R1 and 5ñ-R2 are also expressed, although to different degrees in liver, genital and nongenital skin, prostate, epididymis, seminal vesicle, testis, ovary, uterus, kidney, exocrine pancreas, and the brain.
5ñ-R3 is ubiquitously expressed in most tissues but no apparent role in androgen processing or signaling. Instead, 5ñ-R3 functions in reduction of polyphenol substrates and N-linked glycosylation pathways.
Specific substrates include testosterone, progesterone, androstenedione, epitestosterone, cortisol, aldosterone, and deoxycorticosterone. Outside of dihydrotestosterone, much of the physiological role of 5ñ-reduced steroids is unknown. Beyond reducing testosterone to dihydrotestosterone, 5alpha-reductase enzyme isoforms I and II reduce progesterone to dihydroprogesterone (DHP) and deoxycorticosterone to dihydrodeoxycorticosterone (DHDOC). In vitro and animal models suggest subsequent 3alpha-reduction of DHT, DHP and DHDOC lead to steroid metabolites with effects on cerebral function achieved by enhancing GABA<nowiki/>ergic inhibition. These neuroactive steroid derivatives enhance GABA via allosteric modulation at GABA(A) receptors and have anticonvulsant, antidepressant and anxiolytic effects, and also alter sexual and alcohol related behavior. 5ñ-dihydrocortisol is present in the aqueous humor of the eye, is synthesized in the lens, and might help make the aqueous humor itself. Allopregnanolone and THDOC are neurosteroids, with the latter having effects on the susceptibility of animals to seizures. In socially isolated mice, 5ñ-R1 is specifically down-regulated in glutamatergic pyramidal neurons that converge on the amygdala from cortical and hippocampal regions. This down-regulation may account for the appearance of behavioral disorders such as anxiety, aggression, and cognitive dysfunction. 5ñ-dihydroaldosterone is a potent antinatriuretic agent, although different from aldosterone. Its formation in the kidney is enhanced by restriction of dietary salt, suggesting it may help retain sodium as follows:
5ñ-DHP is a major hormone in circulation of normal cycling and pregnant women.
5ñ-Reductase is most known for converting testosterone, the male sex hormone, into the more potent dihydrotestosterone:
This removes the ÃÂ<sup>4,5</sup> double-bond on the A (leftmost) ring.
The following reactions are known to be catalyzed by 5ñ-reductase:
5ñ-Reductase is a membrane bound enzyme that catalyzes the NADPH dependent reduction of double bonds in steroid substrates to increase potency. The crystal structure of a homolog of 5ñ-reductase isoenzymes 1 and 2 has been found in Proteobacteria (proteobacteria 5ñ-reductase). This exists as a monomer with a seven alpha-helix transmembrane structure housing a hydrophobic pocket that holds cofactor NADPH and monoolein which occupies the steroid substrate binding pocket. In insect cells monoolein is not found, but is subbed out for other androgens and inhibitors. The integral seven transmembrane topology is likely conserved across species, with the N terminus in the endoplasmic reticulum lumen and the C terminus facing the cytosol. High conformational dynamics of the cytosolic region likely regulate NADPH/NADP+ exchange. Sequence conservation across known crystal structures has corroborated high conservation in enzyme structure.
In the 5ñ-reductase from Proteobacteria bacterium, PbSRD5A, NADPH is bound by an extensive hydrogen bonding network, including residues Arg34, which hydrogen bonds to the nicotinamide group, Arg170, which hydrogen bonds to the 2'-phosphate on the ribose group bound to nicotinamide, Asn192 and His230, which hydrogen bond to the nicotinamide nucleotide phosphate group, Tyr32 and Tyr193, which hydrogen bond to the adenine nucleotide phosphate group, and Asn159, Glu196, and Thr219, which hydrogen bond to the adenine group of NADPH. The steroid-binding pocket contains a motif of Gln, Glu, and Tyr residues, which form a triad of hydrogen-bonds, which coordinate the C3 ketone of steroids into close proximity to the nicotinamide of NADPH, which allows a hydride transfer and ÃÂ<sup>4</sup> double-bond reduction. These residues are Gln53, Glu54, and Tyr87 in PbSRD5A.
The mechanism of 5ñ reductase inhibition is complex, but involves the binding of NADPH to the enzyme followed by the substrate. 5ñ-Reductase inhibitor drugs are used in benign prostatic hyperplasia, prostate cancer, pattern hair loss (androgenetic alopecia), and hormone replacement therapy for transgender women.
Inhibition of the enzyme can be classified into two categories: steroidal, which are irreversible, and nonsteroidal. There are more steroidal inhibitors, with examples including finasteride (MK-906), dutasteride (GG745), 4-MA, turosteride, MK-386, MK-434, and MK-963. Researchers have pursued synthesis of nonsteroidals to inhibit 5ñ-reductase due to the undesired side effects of steroidals. The most potent and selective inhibitors of 5ñ-R1 are found in this class, and include benzoquinolones, nonsteroidal aryl acids, butanoic acid derivatives, and more recognizably, polyunsaturated fatty acids (especially linolenic acid), zinc, and green tea. Riboflavin was also identified as a 5ñ-reductase inhibitor .
Additionally, it has been claimed that alfatradiol works through this mechanism of activity (5ñ-reductase), as well as the Ganoderic acids in lingzhi mushroom, and the Saw Palmetto.
Inhibition of 5ñ-reductase results in decreased conversion of testosterone to DHT, leading to increased testosterone and estradiol. Other enzymes compensate to a degree for the absent conversion, specifically with local expression at the skin of reductive 17ò-hydroxysteroid dehydrogenase, oxidative 3ñ-hydroxysteroid dehydrogenase, and 3ò-hydroxysteroid dehydrogenase enzymes.
Gynecomastia, erectile dysfunction, impaired cognitive function, fatigue, hypoglycemia, impaired liver function, constipation, and depression, are only a few of the possible side-effects of 5ñ-reductase inhibition. Long term side effects, that continued even after discontinuation of the drug have been reported.
Finasteride inhibits two 5ñ-reductase isoenzymes (II and III), while dutasteride inhibits all three. Finasteride potently inhibits 5ñ-R2 at a mean inhibitory concentration IC<sub>50</sub> of 69 nM, but is less effective with 5ñ-R1 with an IC<sub>50</sub> of 360 nM. Finasteride decreases mean serum level of DHT by 71% after 6 months, and was shown in vitro to inhibit 5ñ-R3 at a similar potency to 5ñ-R2 in transfected cell lines.
Dutasteride inhibits 5ñ-reductase isoenzymes type 1 and 2 better than finasteride, leading to a more complete reduction in DHT at 24 weeks (94.7% versus 70.8%). It also reduces intraprostatic DHT 97% in men with prostate cancer at 5 mg/day over three months. A second study with 3.5 mg/day for 4 months decreased intraprostatic DHT even further by 99%. The suppression of DHT in vivo, and the report that dutasteride inhibits 5ñ-R3 in vitro suggest that dutasteride may be a triple 5ñ reductase inhibitor.
5ñ-Reductase type 1 inactivated male mice have reduced bone mass and forelimb muscle grip strength, which has been proposed to be due to lack of 5ñ-reductase type 1 expression in bone and muscle. In 5 alpha reductase type 2 deficient males, the type 1 isoenzyme is thought to be responsible for their virilization at puberty.
Impaired 5ñ-reductase 2 activity can result from mutations in the underlying SRD5A2 gene. The condition, known as 5ñ-reductase 2 deficiency, has a range of presentations as atypical appearances of the external genitalia in males. This is because 5ñ-reductase 2 catalyzes the transformation of testosterone to the potent androgen dihydrotestosterone, which is required for the proper masculinization of male genitalia.
When small interfering RNA is used to knock down the expression of 5ñ-R3 isozyme in cell lines, there is decreased cell growth, viability, and a decrease in DHT/T ratios. It has also shown the ability to reduce testosterone, androstenedione, and progesterone in androgen stimulated prostate cell lines by adenovirus vectors.
Congenital deficiency of 5ñ-R3 at the gene SRD5A3 has been linked to a rare, autosomal recessive condition in which patients are born with severe intellectual dysfunction and cerebellar and ocular defects. The presumed deficiency is reduction of the terminal bond of polyprenol to dolichol, an important step in N-glycosylation of proteins, which in turn is important for proper folding of asparagine residues on nascent protein in the endoplasmic reticulum.
Isolation rearing has been shown to lower protein expression of 5ñ-reductase isoenzymes 1 and 2 in cortical and subcortical brain regions of rat models. However, the amount of 5ñ-reduced metabolite remained unaffected. This means isolation rearing likely leads to changes in the expression and activity of 5ñ-reductase in the brain, leading to dysregulation of dopamine neurotransmission, resulting in early chronic stress Treatment with finasteride, a 5ñ-reductase inhibitor, has been shown to mimic the effects of SSRI's causing sexual dysfunction. Research has shown that 5ñ-reductase is the rate-limiting enzyme in neurosteroid synthesis, specifically in the conversion of progesterone to allopregnanolone, low levels of allopregnanolone has been tied to depression, anxiety and schizophrenia. Sleep deprivation can enhance 5ñ-reductase expression and activity in the prefrontal cortex, leading to mania-related symptoms in rats. It is also contested whether the use of 5ñ-reductase inhibitors is associated with suicidal ideation and depression in patient populations who use them for benign prostatic hyperplasia. These symptoms have been found during active use of inhibitors and in immediate followup. However, it is unknown if these symptoms arise naturally from benign prostatic hyperplasia.
An alternative mechanism of cortisol regulation is regulated via 5ñ-reductase which catalyzes an A-ring reduction of cortisol, metabolizing the compound. Type 1 and 2 of 5ñ-reductase are the principal enzymes involved in cortisol clearance through the liver. Excess cortisol has been tied to metabolic dysfunctionâÂÂassociated steatotic liver disease (MASLD),àbut in-vitro studies have found that an over expression of 5ñ-reductase type 2 can suppress lipogenesis. The key role of 5ñ-reductase in cortisol breakdown and fat buildup has elucidated some of the side effects of 5ñ-reductase inhibitors. In randomized studies on human volunteers it was found that 5ñ-reductase inhibition through the use of dutasteride and finasteride can lead to hepatic lipid accumulation in men. In critical illness, overstimulation of cortisol as part of a stress response can lead to decreased clearance of cortisol through the liver via 5ñ-reductase and kidneys via 11ò-hydroxysteroid dehydrogenase type 2, longterm elevation of cortisol can lead to Cushing's syndrome.
This enzyme belongs to the family of oxidoreductases, to be specific, those acting on the CH-CH group of donor with other acceptors. The systematic name of this enzyme class is 3-oxo-5ñ-steroid:acceptor ÃÂ<sup>4</sup>-oxidoreductase. Other names in common use include: