Problem
Let's return to that mystery drug we discussed in the first unit. And consider that we can develop drugs using target based drug delivery and phenotypic drug discovery.
You have recently cloned a cDNA from RNA isolated from human brain tissue. The cDNA may encode a putative adrenergic receptor, based on its putative DNA and amino acid sequence homologies to known adrenergic receptors. You can assume it will be a new member of the beta adrenergic receptor family, called beta 5 receptor. You are also given a drug X123 which has potent action (EC50=1 x 10-11nM) in vivo similar to beta adrenergic agonists, but which does NOT bind any of the known alpha or beta receptors. You may assume an active, radiolabelled form of X123 ([125I]-X123) is available.
You now have a putative beta5 receptor-expressing CHO cell line which binds [125I]-X123 specifically in a receptor binding assay. Discuss what it means for the receptor to bind a ligand specifically??(What are the features of specific binding?) How would you experimentally demonstrate, using whole cells, that X-123 specficially binds the beta 5 receptor expressed on the cell surface of these CHO cells? (note that this question is worth more than a few points and this is a molecular pharmacology course which requires that you leave the course understanding how specific binding is measured!!)