Date of Award:

5-1-1989

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Biology (Entomology)

Committee Chair(s)

William A. Brindley

Committee

William A. Brindley

Committee

Wilford J. Hanson

Committee

Howard M. Deer

Abstract

Properties of esterases from Lygus hesperus Knight and the roles of the esterases in insecticide resistance were studied with field populations. The esterases are 8-type esterases, and carboxyl esterases are predominant in both susceptible (S) and resistant (R) populations. The optimal pH and temperature were 7.0-8.0 and 40-45° C, respectively, for general esterases, and 8.5 and 35-40° C, respectively, for acetylcholinesterases. For general esterases, Km determined with 1-naphthyl acetate at 43° C was 8.8x10-6 M for the males and 1.3x10-5 M for the females, while Vmax was 27.0 and 136.1 umol/min/mg protein for the S and R populations, respectively. There was no significant difference in Km between the two populations (P > 0.05), but Vmax was 5- fold as great in the R population as in the S population. For acetylcholinesterases, Km determined with acetylthiocholine at 38° C was 1. 2xl0-4 M for the S population and 1.1x10-4 M for the R population, while Vmax was 43.4 and 45.3 nmol/min/mg protein for the S and R populations, respectively. There was no significant difference in either Km or Vmax between the two populatians (P > 0.05). The general esterases appear to be water soluble, while the acetylcholinesterases seem to be membrane bound. The esterases in the R population did not differ from the esterases in the S population either in sensitivity to inhibition by paraoxon, trichlorfon or eserine nor in affinity to substrate (1-naphthyl acetate). However, the esterase activity was 4.2- to 6.1-fold as great in the R population as in the S population. These results suggest that the differences in esterases between the two populations are quantitative rather than qualitative. The greatly increased activity in the R population is considered to be brought about by the production of more carboxylesterases, which may play an important role in insecticide detoxification. Acetylcholinesterases that are insensitive to inhibition by paraoxon were found in the R population. The enzymes were homogeneous in the S population, but not in the R population. The acetylcholinesterases in the R population appear to have different insensitivity levels rather than two distinct forms. Accordingly, their contribution to resistance may greatly enlarge as the concentration of insecticide decreases. The decreased sensitivity of acetylcholinesterases to the inhibitor without changes in Km and Vmax to the substrate in the R population may provide an extra advantage for insect survival.

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