Preparation and Properties of Porcine Parotid Butyrylcholinesterase

A butyrylcholinesterase, purified from porcine parotid glands, behaved as a single component in sedimentation velocity and sedimentation equilibrium centrifugation and migrated as a single esterase in starch gel electrophoresis. The molecular weight of the enzyme was estimated to be 368,000. Amino acid analysis showed a high proline content (7.6%) and the presence of two unidentified components. Substrate specificity and inhibitor studies indicated that the enzyme is a butyrylcholinesterase of the type described in porcine serum and colostrum. Over a broad range of acetylcholine concentrations, the enzyme follows MichaelisMenten kinetics. A study of product inhibition indicated choline to be a competitive inhibitor with a Ki of 2.4 x 10-3 m; no inhibition by acetate could be shown. Maximum activity and lowest Km with acetylcholine were observed at pH values 8 to 9. The effect of pH on Vmax suggested that a group at the active site of the enzyme has a pK' of 6.3 to 6.5. Tetramethylammonium ion was a competitive inhibitor, the dissociation constant of which decreased as the pH was increased.