MEZEREIN AND 12-DEOXYPHORBOL 13-ISOBUTYRATE, PROTEIN-KINASE-C LIGANDSWITH DIFFERENTIAL BIOLOGICAL-ACTIVITIES, DO NOT DISTINGUISH PKC-ISOTYPES ALPHA, BETA-1, BETA-2 AND GAMMA

Previous in vitro experiments have shown that the phorbol-like diterpe nes 12-deoxyphorbol 13-isobutyrate (dPB), and possibly mezerein, have multiple biological target sites which differ from one another in appa rent affinity for dPB by 12.5-780 fold and for mezerein by 25-fold. Th ese two compounds are thus very important ligands because of their pot ential PKC isotype-selectivity. In the present study they were found t o have binding affinities differing by a maximum of only 1.6-fold amon g recombinant protein kinase C (PKC) isotypes alpha, beta1, beta, and gamma (the ''A-group'') in a [H-]phorbol dibutyrate binding assay. The apparent K(i)'s were 99-140 nM for dPB and 68-92 nM for mezerein. Our results are consistent with short-term 12-deoxyphorbol ester-induced mouse skin inflammation being mediated at least in part by one or more A-group PKC isotypes. The data also indicate that the pharmacological ly distinguishable target sites previously established for mezerein an d dPB must include one or more binding sites not found in the A-group of PKC isotypes and that mezerein has a high-affinity, non-A-group tar get site in brain.