Figure 2.

Spontaneous decomposition of chlorpyrifos oxon (filled circles) and methyl paraoxon (open circles) in the absence of enzyme at 37°. The levels of oxon within incubations were evaluated by measuring the capacity of a sample of the incubation to inhibit 75–80 pM acetylcholinesterase by the Ellman reaction (Ellman et al, 1961). The slopes represent first order rate constants that are added to the k_i model to account for loss of each oxon as a result of spontaneous breakdown. This first order rate constant for chlorpyrifos oxon was 0.7967 h⁻¹, while that for methyl paraoxon was 1.0975 h⁻¹.