Irreversible loss of [3H]forskolin binding sites in human platelets by alpha-haloacetyl analogs of forskolin

The 7-bromoacetyl-7-desacetyl (BrAcFsk) and 7-chloroacetyl-7-desacetyl (CIAcFsk) analogs of forskolin were synthesized as alkylating agents to study the high affinity binding sites for forskolin. BrAcFsk and CIAcFsk activated adenylate cyclase in human platelet membranes with EC50 values of about 20 and 12 microM, respectively. Both analogs increased cyclic AMP in human platelets; however, they were less potent that forskolin. Forskolin inhibited [3H]forskolin binding to human platelet membranes with an IC50 of 20 nM, whereas BrAcFsk and CIAcFsk inhibited [3H] forskolin binding with IC50 values of 0.1 microM. Pretreatment of intact platelets with 10 microM BrAcFsk caused a 90% irreversible loss in [3H]forskolin binding sites, whereas pretreatment with 10 microM CIAcFsk led to a loss of 55% of the binding sites. The loss of binding sites occurred within 5 min for BrAcFsk and within 30 min for CIAcFsk. The time required for the loss of binding sites produced by either alkylating agent was increased by the inclusion of 200 microM forskolin in the pretreatment buffer. The inactive bromoacetyl analog of forskolin 7-bromoacetyl-7-desacetyl-1.9-dideoxyforskolin (1,9-dideoxy-BrAcFsk) did not activate adenylate cyclase, inhibit [3H]forskolin binding, or cause an irreversible loss of [3H]forskolin binding sites. Adenylate cyclase was assayed in membranes from platelets treated with either 10 microM BrAcFsk or 10 microM 1,9-dideoxy-BrAcFsk. The stimulation of adenylate cyclase by prostaglandin E1, guanosine-5'-O-(3-thio)triphosphate, and AIF4 was inhibited by about 50% in membranes from platelets treated with BrAcFsk. However, the stimulation of adenylate cyclase by forskolin was unaffected by preincubation with BrAcFsk. Pretreatment of human platelets with 1,9-dideoxy-BrAcFsk had no effect on the stimulation of adenylate cyclase by prostaglandin E1, AIF4, or forskolin.