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Relative binding affinities of testosterone, 19-nortestosterone and their 5α-reduced derivatives to the androgen receptor and to other androgen-binding proteins: a suggested role of 5α-reductive steroid metabolism in the dissociation of “myotropic” and “androgenic” activities of 19-nortestosterone

1982; Pergamon Press; Volume: 17; Issue: 6 Linguagem: Inglês

10.1016/0022-4731(82)90567-2

1878-2353

Miklós Tóth, Tamás Zakár,

Receptor Mechanisms and Signaling

The relative binding affinities of testosterone (T), 19-nortestosterone (N) and their 5α-reduced derivatives: 5α-dihydrotestosterone (DHT) and 5α-dihydro- 19-nortestosterone (DHN) to the androgen receptor of the rat seminal vesicle was studied using competition experiments. In cell-free extracts incubated at + 10 C for 18 h the relative binding affinities of these steroids (DHT > T = DHN = N) proved to be specific for the androgen receptor, in the sense that only prostatic extracts gave a similar result while three other androgen binding proteins (human sex steroid binding globulin, rat epididymal androgen binding protein and an antibody raised against T) exhibited quite different binding specificities. In minced seminal vesicles incubated at 37 C for 1 h the binding affinities showed marked differences (DHT > N > T > DHN) and similar patterns were observed with both the cytoplasmic and the nuclear receptors. Our findings suggest that (I) the simultaneous presence of a 4-ene double bond and 19-methyl group in T does not favor the tight binding of T to the androgen receptor; therefore, either saturation of this double bond or elimination of the 19-methyl group leads to increased binding and (II) while 5α-reduction of T increases the affinity of this steroid to the receptor, that of N does not influence or rather tends to decrease the binding affinity. The opposite changes observed in the binding affinities of T and N after their 5α-reduction may account for the lower androgenicity of N. On the other hand, the relative myotropic activity in vivo of these steroids is apparently determined by the ratio of their affinities (NT ∼ 3 at 37 C) to the androgen receptor.