Characterization of a cocaine binding protein in human placenta

[³H]-Cocaine binding sites are identified in human placental villus tissue plasma membranes. These binding sites are associated with a protein and show saturable and specific binding of [³H]-cocaine with a high affinity site of 170 fmole/mg protein (K_d 16.7 nM). The binding is lost with pretreatment with trypsin or heat. The membrane bound protein is solubilized with the detergent 3-(3-cholamidopropyl)dimethyl-ammonio-1-propane sulphonate (CHAPS) with retention of its saturable and specific binding of [³H]-cocaine. The detergent-protein complex migrates on a sepharose CL-6B gel chromatography column as a protein with an apparent molecular weight of 75,900. The protein has an S_20,w value of 5.1. The binding of this protein to norcocaine, pseudococaine, nomifensine, imipramine, desipramine, amphetamine and dopamine indicates that it shares some, but not all, the properties of the brain cocaine receptor. The physiologic significance of this protein in human placenta is currently unclear.