Substrate Preferences of Insect Cholinesterases1,2

Cholinesterase (ChE) probably has been investigated as extensively as any enzyme system in the Invertebrata, yet little is known of its properties and almost nothing of its preferred substrates. Metcalf and March (1950) and Babers and Pratt (1951) have studied the substrate- activity relationships for the cholinesterases of the house fly, Musca domestica L., honey bee, Apis mellifera L., and American cockroach, Periplaneta americana (L.). With acetylcholine (ACh) as substrate these three enzymes showed the typical bell-shaped curve, with a sharply defined substrate optimum and pronounced inhibition by excess substrate, which is characteristic of true acetylcholinesterase (AChE) in vertebrates (Nachmansohn and Wilson, 1951). However, with acetyl-/3-methylcholine (AMeCh) the fly ChE showed very low activity and a tendency toward a humped curve, whereas the bee and cockroach ChE showed high activity, no inhibition with excess substrate up to 0.3M, and a sigmoid substrate-activity curve. At concentrations of 0.03M and greater, the rate of hydrolysis of AMeCh exceeds that of ACh for the bee and cockroach ChE. The substrate-activity curves for triacetin with fly and bee ChE were also sigmoid, while with benzoylcholine (BzCh) very little activity was observed. ChE in the head of Dacus dorsalis Hendel was found to behave almost identically to that of the house fly toward ACh, AMeCh, and BzCh (Roan and Maeda, 1953).