The nature of an “anionic” site in butyrylcholinesterase compared with that of a similar site in acetylcholinesterase

Abstract

• 1.

  1. The nature of the active site of the butyrycholinesterase (EC 3.1.1.8) of human blood serum was compared with that of the acetylcholinesterase (EC 3.1.1.7) of Torpedo marmorata electric organ using a series of carbinol acetates of pyridine and N-methylpyridine. The activities of the two cholinesterases were affected in opposite directions by quaternization of the pyridine N atom, as well as by changing the position of the carbinol substituent in the ring.

• 2.

  2. The effects of certain pyridine derivatives on the enzymatic activity differed, particularly as regards the effect of pyridyl-2- and 3-carbinols, by which acetylcholinesterase was activated and butyrylcholinesterase inhibited.

• 3.

  3. A comparison was made between the rates of hydrolysis of pyridyl-2,6-dicarbinol diacetate and its N-methyl derivative by butyrylcholinesterase.

• 4.

  4. The pH dependence of the enzymatic activity of the two esterases revealed that the charge on the pyridine N atom played a much more important role in complex formation between the enzymes and the compounds studied in the case of acetylcholinesterase than for butyrylcholinesterase.

• 5.

  5. The results presented support the view that butyrylcholinesterase contains a second “non-esteratic” site which differs from the anionic site of acetylcholinesterase, and constitutes the main difference between the two cholinesterases. The dominant type of force involved in reactions with the second site of butyrylcholinesterase are Van der Waals forces, in contrast to the Coulombic attractions which favour complex formation between the anionic site of acetylcholinesterase and substrates and inhibitors of the onium type. In addition, some evidence is presented suggesting that the esteratic sites of the two esterases also differ.