Structural requirements for the action of steroids as inhibitors of electron transfer

Abstract

A series of C 21, C 19, and C 18-steroids (including hormonal steroids and derivatives) have been tested as inhibitors of NADH-oxidase and succinate oxidase activities of heart-muscle sarcosomal fragments (Keilin-Hartree preparation). NADH oxidation is much more sensitive to steroids than succinate oxidation, in accordance with electron transfer inhibition in the vicinity of the NADH-flavoprotein site. Steroid activity on NADH-oxidase has been titrated and the 50% inhibitory concentration ( I 50) of each steroid has been determined. Among the steroids tested, the following were the more effective inhibitors, having I 50 values ≦ 1.0 μM 19-norethynyltestosterone acetate, 5-pregnen-3β,20α-diol, 5α-pregnan-3β,20α-diol, 5-pregnen-3β,20β-diol, 5-pregnen-3β,17α,20α-triol, testosterone acetate, testosterone enol diacetate, testosterone benzoate, androstanolone acetate, and 17β-ethynylestradiol benzoate. Comparison I 50 values for steroids differing in a single group (or structure) shows that the following structural features promote the inhibitory potency: (a) a trans A B junction; (b) an alcohol or ketone group at C-3 and/or C-17; (c) an acetyl chain at C-17; (d) 17α-methyl or ethynyl groups at C-17 of 17β-hydroxylated steroids; (e) acetylated or benzoylated hydroxyl groups at C-3 or C-17; (f) α configuration of 3-OH groups in 5α-androstane derivatives and β-OH configuration in 5β-pregnane derivatives; (g) α configuration of hydroxyl groups at C-17; (h) an angular β-methyl group at C-10; (i) a hydroxyl group at C-20 of 5α-pregnane and 5-pregnene derivatives. Decreased inhibitory potency is determined by the following structural features: (j) a cis A B junction; (k) Δ 1 and Δ 4 double bonds; (l) ionizable (hydrophylic) groups at C-3 or C-17; (m) hydroxyl groups at C-17 or C-21 of C 21-steroids; (n) relatively large substituents at C-3 or C-17 (hexahydrobenzoate and propionate derivatives); (o) substituents at the skeleton of the steroid molecule (C-2, C-4, C-6, C-11, C-16) irrespective of group-polarity and configuration. Accordingly, the most effective steroid structure to inhibit electron transfer is a planar and narrow hydrocarbon framework with relatively small, adequately orientated polar end-groups and a large intermediate hydrophobic area. These structural characteristics are consistent with the interaction of steroids with the phospholipid components of the electron transfer chain at the NADH-flavoprotein site. Many of the structural requirements for inhibition of electron transfer resemble those for androgenicity.