A mixed micellar assay was used to study the in vitro binding of [H-3]
phorbol-12,13-dibutyrate ([H-3]PDBu) to pure recombinant protein kinas
e C (PKC)-alpha, -beta(1), -beta(2), -gamma, -delta, -epsilon, and -ze
ta isotypes expressed in the baculovirus/insect cell system. Scatchard
analysis revealed that all isotypes except PKC-zeta were able to spec
ifically bind PDBu, with K-d values ranging from 1.6 to 18 nM in the p
resence of calcium. In the absence of calcium PKC-alpha, -beta(1), -be
ta(2), and -delta were observed to have a 2-3-fold drop in affinity, a
lthough B-max values remained unchanged, at a stoichiometry of 1.4-2.8
mol of PDBu/mol of enzyme. Competition with specific [3H]PDBu binding
was assessed for the phorbol esters PDBu, 12-tetradecanoylphorbol-13-
O-acetate, 12-deoxyphorbol-13-O-phenylacetate, 12-deoxyphorbol-13-O-ph
enylacetate-20-acetate, thymeleatoxin, resiniferatoxin, and sapintoxin
A. Resiniferatoxin and 12-deoxyphorbol-13-O-phenylacetate-20-acetate
were found to compete effectively only with PDBu bound to the PKC-beta
(1) and -beta(2) isotypes and were the least potent of the phorbol est
ers tested (IC50, > 5 mu M). The phorbol esters sapintoxin A, 12-deoxy
phorbol-13-O-phenylacetate, 12-tetradecanoylphorbol-13-O-acetate, and
PDBu (in order of potency) competed for binding to all isotypes (IC50
values ranging from 2 to 70 nM), with unchanged or slightly decreased
potency when calcium was replaced by ethylene glycol bis(beta-aminoeth
yl ether)-N,N,N',N'-tetraacetic acid. Thymeleatoxin, which was similar
in other respects to these potent phorbol esters, was found to be les
s able to compete with binding to PKC-alpha and -epsilon isotypes (IC5
0, 3-5 mu M). It appears that, whereas the binding of phorbol esters t
o PKC depends primarily on the C20 substituent, other areas of the mol
ecule have an influence on this interaction and the PKC isotypes thems
elves display heterogeneity in their phorbol ester-binding characteris
tics.