A crude cell extract from a mixed bacterial culture growing on parathion, an organophosphate insecticide, hydrolyzed parathion (21 C) at a rate of 416 nmol/ min per mg of protein. This rate of enzymatic hydrolysis, when compared with chemical hydrolysis by 0.1 N sodium hydroxide at 40 C, was 2,450 times faster. Eight of 12 commonly used organophosphate insecticides were enzymatically hydrolyzed with this enzyme preparation at rates ranging from 12 to 1,360 nmol/ min per mg of protein. Seven pesticides were hydrolyzed at rates significantly higher (40 to 1,005 times faster) than chemical hydrolysis. The pH optimum for enzymatic hydrolysis of the eight pesticides ranged from 8.5 to 9.5, with less than 50%o of maximal activity expressed at pH 7.0. Maximal enzyme activity occurred at 35 C. The crude extract lost its activity at the rate of only 0.75%/day when stored at 6 C. Eight organic solvents, ranging from methanol to hexane, at low concentrations stimulated enzymatic hydrolysis by 3 to 20%, whereas at higher concentrations (1,000 mg/liter) they inhibited the reaction (9 to 50%). Parathion metabolites p-nitrophenol, hydroquinone, and diethylthiophosphoric acid, at up to 100-mg/liter concentrations, did not significantly influence enzyme activity.

An organism identified as Pseudomonas diminuta was found to hydrolyze parathion. Cells grown for 48 h contained 3,400 U of parathion hydrolase activity per liter of broth. Expression of enzymatic activity was lost at a high frequency (9 to 12%) after treatment with mitomycin C. Hydrolase-negative derivatives were missing a plasmid present in the wild-type organism. The molecular mass of this plasmid (pCS1), as determined by electron microscopy, was about 44 x 106 daltons.

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A mixed microbial culture was adapted to growth on parathion to determine the feasibility of using microorganisms to detoxify concentrated parathion in agricultural wastes. In a 600-ml chemostat, the culture was able to degrade 50 mg of parathion per liter per h. Para-nitrophenol, produced by enzymatic hydrolysis of parathion, caused delays in exponential growth which were directly proportional to its concentration. A pseudomonad, isolated from the mixed culture, exhibited optimal growth at 0.21 mM p-nitrophenol and grew in concentrations up to 3.5 mM. In metabolic studies using [14C]p-nitrophenol, the nitro group was removed ini stoichiometric quantities as nitrite and hydroquinone was tentatively identified as a metabolite.