[The active structure of tumor promoters]

Diterpene esters containing 12-0-tetradecanoylphorbol-13-acetate (TPA) and the alkaloid teleocidins are structurally unrelated natural products that exhibit similar potent skin tumor-promoting activity. These promoters are classified as TPA-type promoters because they bind equally to the phorbol ester receptor. TPA can be considered as an amphiphilic compound, with a hydrophilic domain spanning the C-3 to C-20 region of the molecule and a lipophilic domain consisting of the acyl substituents on C-12 and C-13. Teleocidins can also be considered as amphiphilic compounds, with the hydrophilic domain spanning the C-11 to C-14 region of the molecule and the lipophilic domain consisting of the alkyl substituents on C-6, C-7 and C-12. Teleocidins exist in two conformational states, the TWIST form and the SOFA form, in solution. From the ratio of the two conformations in solution, the free-energy difference between them was calculated to be 0-1.5 kcal/mol. Therefore a possible role of one of the two conformations should be considered in the modeling of receptor mapping. Computer modeling of the SOFA form of teleocidins and TPA showed a marked similarity with regards to the hydrogen bonding sites of the hydrophilic substituents. In this case, good superposition of the lipophilic regions of both types of compounds was obtained.