Distribution and substrate specificity of esterases in the housefly, Musca domestica L.

Abstract

Housefly homogenates perform high cholinesterase and ali-esterase activity. Warburg-manometric studies show that acetylcholine, acetyl-β-methylcholine, butyrylcholine, and benzoylcholine are exclusively hydrolysed by a cholinesterase, the properties of which are more or less comparable to those of true vertebrate cholinesterase. Methylbutyrate and tributyrin are hydrolysed by one or possibly several ali-esterases but not by the cholinesterase. The cholinesterase activity is mainly present in the head and the ali-esterase activity in the thorax and the abdomen. Various other substrates, e.g. methylacetate, ethylacetate, amylacetate, triacetin, phenylacetate, and o-nitrophenylacetate are hydrolysed by both the cholinesterase and the ali-esterases. There is some evidence that o-nitrophenylacetate hydrolysis is partly due to an organophosphate-resistant enzyme distinct from the cholinesterase and ali-esterases, which are both inhibited by low concentrations of organophosphates. The ali-esterase activity is rapidly destroyed at 37°C in slightly alkaline media, whereas the cholinesterase is not affected by these conditions.