Apparent saturability of a 4S dihydrotestosterone-binding protein in rat muscle cytosol: role of 3α-hydroxysteroid dehydrogenase and albumin

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Abstract
The existence of a saturable 4S dihydrotestosterone-binding protein of low affinity (Kd = 1.16 .times. 10-6 M) was found in rat skeletal muscle. The binding of dihydrotestosterone (DHT) was due to 3.alpha.-hydroxysteroid dehydrogenase (3.alpha.-OHSD) enzyme responsible for the conversion of DHT to androstanediol (Adiol). Both the binding and the enzymatic activity were inhibited by various 3-keto steroids. The Kd and the Km of the enzyme were similar. Experiments with ammonium sulfate fractions of the cytosol showed a partial separation of the binding and of the enzymatic activity. DHT (3 .mu.M) was almost completely metabolized to Adiol during a 2 h incubation at 0.degree. C even in the absence of added coenzymes. The 4S protein bound Adiol more strongly than DHT and this binding was not saturable. The binding behavior of both DHT and Adiol with either muscle cytosol or rat albumin was similar when subjected to ammonium sulfate fractionation and sucrose density gradient centrifugation. The skeletal muscle 3.alpha.-OHSD rapidly metabolized DHT into Adiol which then bound strongly to a nonspecific binding protein, presumable rat serum albumin. The observed saturability of DHT binding was only apparent.

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