40 II. DIGESTION AND ABSORPTION OF FATS 



earlier conclusions that cholinesterase inhibitors of the urethane group are 

 actually competitive substrates with exceedingly low turnover numbers. 



Mercuric chloride causes inactivation of cholinesterases by two inde- 

 pendent reactions. Goldstein and Doherty 234 reported that, at high con- 

 centrations, it appears to combine rapidly and reversibly at the enzyme 

 active center, causing inhibition which is competitive with respect to sub- 

 strate. This enzyme active center apparently does not contain an active 

 sulfhydryl ( — SH) group, and the mercury must be assumed to react with 

 some other group. Secondly, mercuric chloride apparently also combines 

 elsewhere on the enzyme surface (possibly through — SH), thereby initiat- 

 ing a slow and irreversible denaturation. The latter process is most obvi- 

 ous at a low concentration of mercuric chloride. 



(c') Inhibition by Diisopropyl Fluorophosphate. — Diisopropyl fluoro- 

 phosphate (DFP) is the most important synthetic inhibitor of cholin- 

 esterase; it has proved useful in the investigation of the nature of this 

 enzyme. DFP has the following formula : 



CH 3 O CH 3 



C— O- 



x? - -U- 9/ 



/ i \ 



CH 3 F CH 3 



The effectiveness of fluorophosphates as anticholinesterases was first 

 established by Mackworth 235 in 1942. Richards and Gilman 236 reported 

 that the fluorophosphates as a group have a cholinesterase-inhibiting ac- 

 tion. This property is shared by sodium fluorophosphate; however, the 

 effectiveness is greatly enhanced if alkyl groups are substituted for the 

 hydrogens. The product thus becomes lipid-soluble. DFP is one of the 

 most effective of these anticholinesterases, and it has the added character- 

 istic that the inhibition is irreversible. 236 Thus, it is only by the synthesis 

 of new enzyme that the ability to hydrolyze acetylcholine can be restored. 



Nachmansohn and Feld 237 emphasize the high specificity of the action of 

 DFP, and its powerful effect on cholinesterase in low concentration. The 

 coincidence of death and inactivation of cholinesterase, therefore, suggests 

 that the toxicity of the compound is due to its action on the enzyme. Of 

 the alkyl fluorophosphonates examined by Mackworth and Webb 230 as 

 inhibitors of horse serum cholinesterase, the diisopropyl compound was the 



234 A. Goldstein and M. E. Doherty, Arch. Biochem., S3, 35-49 (1951). 

 236 J. F. Mackworth, personal communication to A. Mazur and O. Bodansky, ./. Biol. 

 Chem., 163, 261-276 (1946), p. 261. 



236 A. N. Richards and A. Gilman, Federation Proc, 5, 285-292 (1946). 



237 D. Nachmansohn and E. A. Feld, J. Biol. Chem., 171, 715-724 (1947). 



