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The bryostatins are macrocyclic lactones that represent an additional structural class of potent activators of protein kinase C. These marine animal biosynthetic products are of unusual interest because they induce only a subset of the biological responses induced by the phorbol esters. We have now determined the binding affinities of naturally occurring and semisynthetic bryostatins for protein kinase C by competition analysis with [26-3Hlbryostatin 4 as the radioactive ligand. Esterification of the hydroxyl group at C26 caused dramatic loss of activity as did inversion of the asymmetric center at this position. In contrast, neither of the ester groups at C7 and C20 had a major influence on activity. Computer modeling of the phorbol esters, related diterpenes, and indole alkaloids suggested that the C20, C9, and C4 oxygens of phorbol represented critical elements of the phorbol ester pharmacophore. The C26 oxygen of the bryostatins, together with the C1 and C19 oxygens, gave an excellent spatial correlation with this model, with a root-mean-square deviation of 0.16 A (compared to 0.10-0.35 A among phorbol-related diterpenes). The extension of the phorbol ester pharmacophore model to the bryostatins and its agreement with the structureactivity relations for the bryostatin class of compounds provide additional support for the validity of the model. Protein kinase C mediates one arm of the signal-transduction pathway proceeding through inositol phospholipid breakdown (1,2). This pathway is directly involved in the action of a broad range of cellular effectors, including growth factors and oncogenes, and indirectly affects other transduction pathways such as that of the cyclic AMP second-messenger system. Protein kinase C is proposed to be activated by 1,2-diacyl-sn-glycerol compounds and by certain exogenous analogs such as the phorbol ester tumor promoters (3, 4). In addition to the phorbol esters and related diterpenes, a number of structurally distinct natural products have been identified that also bind to protein kinase C with high affinity and that resemble the phorbol esters in their actions at nanomolar concentrations in biological systems. These compounds include the indole alkaloids such as teleocidin and the polyacetates such as aplysiatoxin (5).Recently, considerable attention has focused on an unusual class of potent activators of protein kinase C, the bryostatins. These compounds are macrocyclic lactones isolated from Bugula neritina and other marine bryozoans on the basis of antineoplastic activity against the P388 leukemia cell system (6). The bryostatins at nanomolar concentrations inhibit phorbol ester binding to protein kinase C and stimulate enzymatic activity in vitro to a comparable degree as do the phorbol esters (7-9). Biologically, however, the bryostatins induce only a subset of the typical phorbol ester responses. Moreover, the bryostatins block, in an apparently noncompetitive fashion, the action of the phorbol esters on those responses that they themselves do not induce. Exampl...

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