Isolation of acetylcholine receptors.

Abstract

When ACh acts in an excitatory way, it increases the conductance of a post-synaptic membrane for Na+ and K+, leading to membrane depolariza­ tion. We shall use the term acetylcholine receptor (AChR) for those ma­ cromolecules involved in this transduction that bear the recognition site( s) for ACh and other cholinergic ligands. It is now widely believed that cho­ linergic receptors can be isolated, using techniques that parallel those em­ ployed by enzymologists. Early attempts (1-4) failed because it was not recognized that isolated receptors must possess rather precisely specified properties. These earlier attempts were thoroughly reviewed (5-9), there­ fore we shall restrict our treatment of the subject to current attempts. The key to the problem is the discovery of the right indices to follow as purification proceeds. Enzymologists can follow the catalytic activity in the various fractions they isolate. Receptologists have no such single index. In­ stead they must search for macromolecules that bind (but not incor­ rect) ligands with the affinity and reversibility, and that are present only in the appropriate amount in appropriate tissues. By correct we mean corresponding to the physiological response. The question of what quantity of AChR one should expect to find is an important and difficult one. Waser (10) used autoradiography of dried dia­ phragm (1 mm thick) of mice after exposure to curare and found it to ac­ cumulate in the end-plates. Its concentration was estimated to be 4 X 106 molecules/end plate. This number was employed by several workers to esti­ mate the concentration of AChR in d ifferent tissues. For example, Trams ( 11) estimated that 1.1 nmole/ g of A ChR should be found in eel electro­ plax. We combined the Waser data with Nachmansohn's estimate of 50,000 synapses per el ec troplax and calculated a concentration of 0.01 nmole/g for