The effect of bryostatin 1 on [26-3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding to a washed paniculate preparation from rat brain was examined. Bryostatin 1 inhibited phorbol ester binding at concentrations considerably lower than previously reported. As would be expected for a ligand of high affinity, the apparent displacing potency of bryostatin 1 was dependent on the concentration of tissue/binding sites included in the assay. Decreasing the concentration of [3H]PDBu binding sites to the picomolar detection limit resulted in apparent bryostatin displacing potencies in the picomolar range with these values representing an upper estimate of the true affinity. When included in saturation studies with [3H]PDBu, bryostatin 1 displayed mixed competitive/non-competitive inhibition. Using either repetitive washing or dialysis of the membrane preparation, it was not possible to reverse the inhibition produced by bryostatin 1. The greater affinity of bryostatin 1 compared to other classes of agents that act directly on protein kinase C and the stability of its association may contribute to the unique biological properties of the bryostatins.
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