The C4 hydroxyl group of phorbol esters is not necessary for protein kinase C binding.

To investigate the role of the hydroxyl group at position 4 of the phorbol esters in protein kinase C (PKC) binding and function, 4β-deoxy-phorbol-12,13-dibutyrate (4β-deoxy-PDBu, 5a) and 4β-deoxy-phorbol-13-acetate (6a) were synthesized from phorbol (1). The binding affinities of these 4β-deoxy compounds (5a, 6a) to the 13 PKC isozyme C1 domains were quite similar to those of the corresponding 4β-hydroxy compounds (4a, 4b), suggesting that the C4 hydroxyl group of phorbol esters is not necessary for PKC binding. Moreover, functional assays showed that 4β-deoxy-PDBu (5a) exhibited biological activities (Epstein-Barr virus induction and superoxide generation) equally potent to those of PDBu (4a). These solution phase results differ from expectations based on the previously reported solid-phase structure of the complex of PKCδ-C1B and phorbol-13-acetate (4b).