Abstract
A large amount of specific high affinity binding sites for tumor promoting phorbol esters as well as of a Ca 2+- and phospholipid-dependent protein kinase is present in cytosol of chick oviduct. 12- O-Tetradecanoyl-phorbol-13-acetate (TPA) is able to replace either Ca 2+ or the phospholipid phosphatidylserine as activators of the kinase to some extent. The maximum activity of the enzyme in the presence of Ca 2+ 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.
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