The effect of 0,0-Diethyl-O-p-nitrophosphate (Paraoxon) on the growth of Tetrahymena in culture was investigated. At all concentrations tested (dilutions 1: 10.000 to 1 million) growth was depressed.
After pretreatment with Paraoxon at a dilution of 1:100.000, resistence to the compound developed. Thus 5 days pretreatment increased the survival time in high concentrations of Paraoxon threefold, and 14 days ninefold, when compared with controls. Resistant and non-resistant cells were compared. After 8 days pretreatment oxygen consumption and catalase activity doubled. Acid phosphatase activity doubled following 4 weeks pretreatment and a twofold increase in cholinesterase activity was seen after 8 weeks pretreatment.
No specific esterases capable of splitting phosphate esters could be shown in resistant cells.
It may be significant that in resistant cells enzymes sensitive to Paraoxon are increased.
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