Phorbol esters, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), directly activate Ca²⁺-activated, phospholipid-dependent protein kinase (protein kinase C), which normally requires unsaturated diacylglycerol. Kinetic analysis shows that TPA can substitute for diacylglycerol and significantly increases the enzyme's affinity for Ca²⁺ and phospholipid. This activation is linked to receptor-mediated phosphatidylinositol breakdown, which may lead to cellular functions or proliferation. In human platelets, TPA enhances protein kinase C-specific phosphorylation without phosphatidylinositol breakdown. Various phorbol derivatives also activate this kinase in vitro. Studies suggest that TPA acts directly on cell surface membranes. One of the earliest effects of phorbol esters is platelet aggregation, associated with release reactions. TPA activates protein kinase C without requiring phosphatidylinositol turnover, indicating a direct interaction with the enzyme. The enzyme requires Ca²⁺ and phospholipid, particularly phosphatidylserine, for activation. TPA enhances the enzyme's activity by increasing its affinity for Ca²⁺ and phospholipid. In experiments, TPA activates protein kinase C in human platelets, leading to phosphorylation of a 40-kilodalton protein, possibly involved in serotonin release. TPA does not induce diacylglycerol formation, unlike thrombin. This suggests that TPA activates protein kinase C independently of phosphatidylinositol breakdown. TPA's effects on protein kinase C are dose-dependent and occur at lower concentrations than those required for in vitro activation. TPA does not affect protein kinase A or calmodulin-dependent protein kinase in similar systems. The results indicate that TPA directly activates protein kinase C without phosphatidylinositol breakdown, suggesting a direct interaction with the enzyme. This activation may be linked to cellular functions or proliferation. The structural requirements for tumor promotion by phorbol esters appear similar to those for protein kinase C activation. The role of protein kinase C in tumor promotion remains unclear, but its activation by TPA suggests a potential link to the tumor-promoting mechanism. Further studies are needed to explore the detailed metabolic cascade of phospholipids following TPA addition.Phorbol esters, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), directly activate Ca²⁺-activated, phospholipid-dependent protein kinase (protein kinase C), which normally requires unsaturated diacylglycerol. Kinetic analysis shows that TPA can substitute for diacylglycerol and significantly increases the enzyme's affinity for Ca²⁺ and phospholipid. This activation is linked to receptor-mediated phosphatidylinositol breakdown, which may lead to cellular functions or proliferation. In human platelets, TPA enhances protein kinase C-specific phosphorylation without phosphatidylinositol breakdown. Various phorbol derivatives also activate this kinase in vitro. Studies suggest that TPA acts directly on cell surface membranes. One of the earliest effects of phorbol esters is platelet aggregation, associated with release reactions. TPA activates protein kinase C without requiring phosphatidylinositol turnover, indicating a direct interaction with the enzyme. The enzyme requires Ca²⁺ and phospholipid, particularly phosphatidylserine, for activation. TPA enhances the enzyme's activity by increasing its affinity for Ca²⁺ and phospholipid. In experiments, TPA activates protein kinase C in human platelets, leading to phosphorylation of a 40-kilodalton protein, possibly involved in serotonin release. TPA does not induce diacylglycerol formation, unlike thrombin. This suggests that TPA activates protein kinase C independently of phosphatidylinositol breakdown. TPA's effects on protein kinase C are dose-dependent and occur at lower concentrations than those required for in vitro activation. TPA does not affect protein kinase A or calmodulin-dependent protein kinase in similar systems. The results indicate that TPA directly activates protein kinase C without phosphatidylinositol breakdown, suggesting a direct interaction with the enzyme. This activation may be linked to cellular functions or proliferation. The structural requirements for tumor promotion by phorbol esters appear similar to those for protein kinase C activation. The role of protein kinase C in tumor promotion remains unclear, but its activation by TPA suggests a potential link to the tumor-promoting mechanism. Further studies are needed to explore the detailed metabolic cascade of phospholipids following TPA addition.