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5 alpha-reductase

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5 alpha-reductase is an enzyme that is present in highest concentration in the male reproductive tissues, the skin, especially that overlying the genitalia, and the liver. It converts testosterone to dihydrotestosterone, which has a greater affinity for androgen receptors than testosterone, and thus enhances the androgen action of testosterone.

Two isoenzymes of 5 alpha-reductase enzyme are recognised (1):

  • type 1 - which is located on the short arm of chromosome 7 and occurs in all skin tissues
  • type 2 - which is located on the short arm of chromosome 2 and occurs mainly in the genital skin

In 5 alpha-reductase deficiency, the type 2 isoenzyme is that which is lacking. Men with this condition do not develop male-pattern baldness despite evidence that the type 1 isoenzyme is the dominant form in the adult male scalp, and in 5 alpha-reductase deficiency, this isoenzyme is present at normal levels. Thus, one must conclude that the type 2 isoenzyme is necessary to the development of male pattern baldness (1,2).


  • Andersson, S. et al. (1991), Deletion of steroid 5 alpha-reductase 2 gene in male pseudohermaphroditism, Nature, 354, 159-61.
  • Rittmaster RS (1994). Finasteride. NEJM, 330, 121-6.


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