Soluble phorbol ester binding sites and phospholipid- and calcium-dependent protein kinase activity in cytosol of chick oviduct.

A large amount of specific high affinity binding sites for tumor promoting phorbol esters as well as of a Ca2+- and phospholipid-dependent protein kinase is present in cytosol of chick oviduct. 12-O-Tetradecanoyl-phorbol-13-acetate (TPA) is able to replace either Ca2+ or the phospholipid phosphatidylserine as activators of the kinase to some extent. The maximum activity of the enzyme in the presence of Ca2+ and phosphatidylserine, however, cannot be increased further by TPA. Various second stage tumor promoters also exhibit the capacity to stimulate the protein kinase, whereas the non-promoting phorbol ester 4-O-methyl-TPA, as well as the non-promoting, but with respect to other responses TPA-like, calcium ionophore A23187, do not affect the kinase.