5alpha

5alpha is a term used in chemistry and biochemistry with two related senses. In organic chemistry, 5α denotes the alpha orientation at the fifth carbon in certain ring systems, especially steroids, and is used to distinguish 5α- from other stereoisomers such as 5β. In biochemistry and endocrinology, 5α commonly refers to 5α-reduction, a reaction that saturates the double bond between carbons 4 and 5 of steroid precursors, producing 5α-reduced derivatives such as 5α-dihydro steroids.

The most prominent biological context for 5α is the enzyme family known as 5α-reductases. The principal isoforms, typically labeled type 1 and type 2, catalyze the conversion of testosterone to dihydrotestosterone (DHT), a more potent androgen. A third isoform has been identified in later research, and different tissues express different combinations of these enzymes, influencing local androgen levels and biological effects.

Clinical and physiological relevance arises from the role of DHT in development and disease. DHT is essential for normal development of male genitalia in utero and contributes to secondary sexual characteristics after puberty. Abnormal 5α-reductase activity is associated with conditions such as androgenic alopecia and benign prostatic hyperplasia. Pharmacological inhibitors of 5α-reductases, such as finasteride (primarily type 2) and dutasteride (types 1 and 2), are used to treat hair loss and prostate enlargement, though they can have sexual side effects.

In scientific literature, 5α is used to denote 5α-stereochemistry or the product of 5α-reductive metabolism. It is distinct from other descriptors like 5β and from alpha or beta designations at other positions.