Studies on Carboxyesterase in Malathion-Resistant Culex tarsalis1

The concentration of carboxyesterase, under monofactorial genetic control, was determined to be 13 times as great in the R as in the S larval homozygotes. It was purified at least 100 times, and separated from phosphatase activity, by column chromatography through DEAE cellulose. The enzyme proved to be a single protein, M. W. 16,000, in both R and S material. R and S enzymes carried the same anionic electrophoretic charge, and showed identical U-V spectra. The R enzyme differed from the S enzyme in being acid-precipitable at pH 5, and much more heat-labile at 30°C.; its dephosphorylation constant and turn-over number appeared to be slightly lower. The carboxyesterase was particularly abundant in the gut, from which phosphatase was found to be absent.