Effect of pH on the activity of nervous cholinesterases of the rat towards different biochemical and histochemical substrates and inhibitors

Summary Cholinesterase activities of homogenates of rat brain and superior cervical ganglion were determined by automatic titration using several biochemical and histochemical substrates. High hydrolysis rates were observed when acetylcholine, acetylthiocholine, propionylcholine or propionylthiocholine was used as substrate; -naphthyl acetate and acetyl--methylcholine were hydrolyzed at a moderate rate, and activities were low towards butyrylcholine, butyrylthiocholine and benzoylcholine. With most substrates, the enzyme activity increased from pH 5 to pH 10 and decreased at pH 11. Acetylcholine and acetyl--methylcholine showed an activity maximum at pH 7 or 8. Inhibition by the selective inhibitor of specific cholinesterase 284 C 51 was not markedly affected by pH. On the other hand, the inhibiting power of the selective inhibitor of non-specific cholinesterase iso-OMPA markedly decreased when the pH was lowered. The inhibitor data at different pHs and with different concentrations of eserine, 284 C 51 or iso-OMPA at pH 6 indicated that acetylcholine, propionylcholine and propionylthiocholine are readily hydrolyzed by both specific and non-specific cholinesterase, while acetyl--methylcholine is mainly split by specific cholinesterase and butyrylcholine mainly by non-specific cholinesterase. The significance of propionylcholine and propionylthiocholine as substrates of specific cholinesterase is emphasized.