Further consideration of esterase activity

Having reviewed the properties of a variety of organo-phos-phorus compounds, we are now in a position to consider in greater detail their action towards enzymes and to utilize the information given on pp. 35 et seq. and 61 et seq. 

We must stress that organo-phosphorus compounds are not specific inhibitors for the cholinesterases, but are rather inhibitors for enzymes possessing carboxyhc esterase activity. All the enzymes mentioned below will hydrolyse carboxyhc esters. However, not all esterases are inhibited, for example,. d-esterase which hydrolyses phenyl acetate is not inhibited by organo-phosphorus compounds. 

Cholinesterase (true and pseudo-) Liver esterase Milk esterase 

Thus using inhibitors such as D.F.P. containing P, it has been shown that the inhibited cholinesterase contains phosphorus which is very tightly bound. 

Using erythrocyte chohnesterase, Aldridge has studied the kinetics of its reaction with inhibitors. With one compound in excess, he has shown that the reaction is bimolecular, and the energy of activation is 10-11 kcal./mol. Such a value is not in agreement with a simple absorptive process, and it is assumed that a chemical change has taken place, e.g. phosphorylation of the enzyme. 

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