Glutathione S-transferase and hydrolytic activity in a tetrachlorvinphos-resistant strain of housefly and their influence on resistance

The high glutathione S-transferase activity found in a tetrachlorvinphos-resistant strain CH was studied. A substrain CH3 was bred in which the gene for high transferase activity was introduced into an otherwise susceptible genome. This strain had only part of the activity of the original strain, probably through the removal of enhancing modifying genes. The resistance in strain CH3 was 56-fold for tetrachlorvinphos, but much lower for paraoxon, methylparaoxon, and parathion. This was in accordance with the much higher activity in vitro of the transferase towards tetrachlorvinphos (ca. 200 nmol per fly per hr at pH 7), more than 100-fold that for the other organosphosphorus (OP) compounds. At least four different transferases could be separated on carboxymethylcellulose columns. The ratio of activity toward tetrachlorvinphos and 1-chloro-2,4-dinitrobenzene (CDNB) was not the same but was higher for the more firmly bound enzymes. This was different from the situation in a malathion-resistant strain G, in which the only active enzyme with low activity toward CDNB and tetrachlorvinphos obtained from the column was not adsorbed. Tetrachlorvinphos was practically exclusively demethylated by the transferases. The enzyme in larvae occurred predominantly in the fat body, with smaller activities in the intestine and cuticle-muscle part. An enzyme hydrolyzing several OP compounds present in some of the CH flies was studied in another substrain CH4, where it was present together with the GSH S-transferase. This did not contribute significantly to tetrachlorvinphos resistance, but was significant for resistance to other OP compounds.