The serine/threonine kinase protein kinase C (PKC) is regarded as playing a key role in stimulation of cellular responses in many different cells and tissues. There are now known to be at least 10 different isoenzymes of PKC (α, βI , βII , γ, δ, ɛ,ζ, η, θ, and λ@#@)which vary in their cofactor requirement for activation and also in their substrate specificity. The conventional PKCs (cPKC) (α, βI , βII , and γ) require Ca2+ , phospholipid, and diacylglycerol (DAG) for activation. The novel PKC (nPKC) enzymes (δ, ɛ,ζ, η,and θ) are Ca2+ -independent, whereas members of the atypical PKCs (aPKC), ζ and λ, lack the binding site for DAG. Phorbol esters, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), also bind to and activate those enzyme isotypes that are sensitive to DAG and are used as exogenously applied activators of isolated enzymes or of cells to mimic the actions of DAG (1 ).
