Difference in fenitrothion-inhibited rat plasma cholinesterase activities determined by delta pH-method and...

Rat cholinesterase (ChE) activities were measured by DTNB-method after an oral administration of fenitrothion, and the following facts were observed. The inhibited plasma ChE (pseudo ChE) obtained within several hours after the administration was spontaneously reactivated at 10 degrees C or over, whereas no reactivation was observed at 1 degree C. Neither red blood cell nor brain ChE (true ChE) was spontaneously reactivated. In vitro, the spontaneous reactivation was also observed in rat plasma ChE inhibited by oxon-type of fenitrothion. In case the activity of plasma ChE obtained 30 min after administration was determined by delta pH-method, the activity was higher than the actual value, because of the spontaneous reactivation taking place during an incubation for 1 h at 37 degrees C. It is suggested from these results that an utilization of delta pH-method is unsuitable for the measurement of the activity of inhibited ChE which is spontaneously reactivated.