Table 8.

Effect of enzyme inhibition on esterase efficacy in removing permethrin-associated toxicity to Ceriodaphnia dubia^a

Esterase inhibitor	Permethrin (ng/L)b	No enzymec	Active enzymed	Inhibited enzymee
Chlorpyrifos-oxonf	0	0	0	0
	500	95 ± 10	5 ± 10	5 ± 10
	750	100	0	0
	1,000	100	0	0
OTFPg	0	0	0	0
	500	100	0	60 ± 0
	750	100	0	90 ± 12
	1,000	100	0	95 ± 10

^aData are shown as the mean percentage mortality ± standard deviation for 48-h C. dubia toxicity using five neonates with four replicates.

^bPermethrin concentrations are nominal water concentrations.

^cToxicity assays were performed in the absence of esterase, or 0x, where 1x dilution = 2.3 × 10⁻³ U/ml. A unit of activity is the amount of enzyme that hydrolyzes 1.0 μmol of butyrate to butyric acid and ethanol per minute at pH 8.0 at 25°C (as defined by Sigma Chemical, St. Louis, MO, USA).

^dUninhibited enzyme (10x) was added to each container (liquid preparation, lot number 102K7062; Sigma Chemical).

^eThe indicated inhibitor (10x) was added to the esterase at a final concentration of 0.5 mM.

^fThe specific activity of the porcine esterase preparation before inhibition was 112.9 μmol/min/mg as determined by p-nitrophenyl acetate (PNPA) hydrolysis. Following inhibition with 0.5 mM chlorpyrifos-oxon and subsequent filtration-based inhibitor removal, no PNPA hydrolysis activity was detected. However, 72 h after toxicity assay initiation, the esterase specific activity had increased to 81.2 μmol/min/mg, indicating reactivation of the enzyme.

^gOTFP = 1,1,1-trifluoro-3-octylthiol-propan-2-one.