The effects of phorbol esters with different biological activities on protein kinase C

The sapintoxins are a series of naturally occurring fluorescent phorbol esters with a range of selective biological activities (e.g. pro‐inflammatory but non‐tumour promoting). Their ability to activate protein kinase C (PKC) in vitro has been studied. Both tumour promoting and non‐promoting phorbol derivatives activate the enzyme in vitro at low concentrations. 12‐deoxyphorbol‐13‐phenylacetate‐20 acetate (DOPPA) acts as a partial agonist in the activation of protein kinase C. Structurally distinct phorbol esters may therefore preferentially activate different forms of protein kinase C. α‐sapinine, a biologically inactive compound, binds to protein kinase C without stimulating the enzyme and prevents subsequent activation by phorbol esters such as 12‐ O ‐tetradecanoyl phorbol‐13‐acetate (TPA).