Inorganic ion effects on the kinetic parameters of acetylcholinesterase

EARLY work on the effects of inorganic ions on the activity of acetylcholine acetyl‐hydrolase (EC 3.1.1.7; AChE) from various sources has been summarized by COHEN and OOSTERBAAN (1963) and many other reports have been published subsequently (CHANGEUX, 1966; CRONE, 1973; HELLER and HANAHAN, 1972; IVANOVA, 1967; KITZ, BRASWELL and GINSBURG, 1970; ROUF‐CALIS and QUIST, 1972; ROUFOGALIS and THOMAS, 1968; WINS, SCHOFFENIELS and FOIDART, 1970). Despite this work, no comprehensive study has yet been made to determine whether the observed effects are specific to particular ions or dependent only on the ionic strength of the medium (CHANGEUX, 1966). In some cases, specific ion effects have been observed (CHANGEUX, 1966; HELLER and HANAHAN, 1972; ROUFOGALIS and QUIST, 1972; ROUFOGALIS and THOMAS, 1968) at salt concentrations from 600 mM to below 1 mM, but the studies were not detailed enough and in some cases the total ionic strength was not rigidly controlled, so that no general deductions can be drawn. We have studied the hydrolysis of acetylcholine (ACh) by bovine erythrocyte AChE at subinhibitory substrate concentrations, and now present our results on the effect of inorganic salts at varying ionic strength on the kinetic parameters Km, and Vmax. The present work shows that this hydrolysis follows simple Michaelis kinetics very closely, and therefore these two constants suffice to define the complete pattern of initial reaction velocity as a function of substrate concentration (ATKINSON, 1966).