BOD AN SKY: CHOLINESTERASE 



533 



in vitro and in vivo inactivation of cholinesterase by DFP has been 

 studied by Mazur and Bodansky,^® and the results of these studies will 

 now be briefly described. 



In Vitro Inhibition of Cholinesterase Activity by Diisopropyl-Fluoro- 

 phosphate and by Physostigmine. The inhibition of the activities of 

 serum, red cell, muscle, and brain cholinesterases of the rabbit, monkey, 

 and man were determined, at various concentrations of DFP and 

 physostigmine. In order to obtain a general measure of the extent of 

 inhibition of the different cholinesterases, the relative velocities were 

 plotted against the negative logarithm of the molar concentration of 

 DFP or physostigmine. The negative log molar concentration at 

 which 50 per cent inhibition occurred, was termed the pCi value. 

 These values for various enzyme preparations are shown in table 6. 



Table 6 

 Sensitivity of Various Cholinesterases to Inhibition by Diisopropyl-Fluoro- 



PHOSPHATE and PhYSOSTIGMINE 



The values are expressed in terms of the negative log of the concentration of in- 

 hibitor required to produce a oO per cent inhibition of cholinesterase activity (pCi). 



Serum 



Red cells 



Muscle 



Brain 



Diisopropyl-fl uorophosphate 



Physostigmine 



* Horse serum cholinesterase (purified), prepared by Drs. Northrop and Kunitz, according to 

 directions of Mendel and Rudney. 



It may be seen that, of the various serum cholinesterases studied, 

 that of the rabbit was least sensitive to inhibition by DFP. Thus, a 

 negative log molar concentration of 4.1 of DFP was necessary for 50 

 per cent inhibition of rabbit serum cholinesterase, whereas concentra- 

 tions of about one ten thousandth as much (negative log molar values 

 of 7.7 to 8.3) gave 50 per cent inhibition of monkey, human, and horse 

 serum cholinesterase activity. The various red cell cholinesterases 



