The mode of recovery of cholinesterase activity in vivo after organophosphorus poisoning. 2. Brain cholinesterase.

Previous investigators (Frawley, Hagan & Fitzhugh, 1952; Davison, 1955; Kewitz & Nachmansohn, 1957) have observed that the rate of recovery in vivo of inhibited brain true cholinesterase depended on which inhibitor was used. With diiwopropyl phosphorofluoridate (Frawley et al. 1952; Davison, 1955; Kewitz & Nachmansobn, 1957) recovery was slow, whereas with tetraethyl pyrophosphate (Davison, 1955) and diethyl pnitrophenyl phosphate (Davison, 1955; Kewitz & Nachmansohn, 1957) recovery was considerably more rapid and occurred in two stages, the first being faster than the second. Davison (1955) suggested that the two-stage recovery could be explained by the existence of two kinds of inhibited cholinesterase with differing stabilities. In the previous paper (Blaber & Creasey, 1960) it was shown that the changes in vivo in sheep-erythrocyte cholinesterase activity after inhibition by a variety of organophosphorus compounds could be explained in terms of (i) spontaneous reactivation of the inhibited enzyme by hydrolysis, (ii) aging, i.e. the gradual conversion of the inhibited enzyme from a form which can be reactivated by oxiimes into one which cannot, and (iii) new enzyme synthesis. In the present paper a similar analysis has been made of the behaviour of inhibited rat-brain cholinesterase; this is a more complicated system since both true cholinesterase and pseudocholinesterase are present and cannot easily be separated. The combined effects of spontaneous reactivation, aging, and new enzyme synthesis on the recovery of inhibited brain cholinesterase have been investigated by comparing the behaviour in vivo and in vitro of the inhibited enzyme. The inhibitors were those used previously in the study of inhibited erythrocyte cholinesterase activity (Blaber & Creasey, 1960), namely dimethyl p-nitrophenyl phosphate, diiwopropyl phosphorofluoridate, tetraethyl pyrophosphate and i8opropyl methylphosphonofluoridate. Although aging of inhibited * Present address: School of Pharmacy, Department of Pharmacology, P.O. Box 125, 29/39 Brunswick Square, London, W.C. 1. brain cholinesterase in vivo is known to occur (Hobbiger, 1957), it has not previously been investigated in relation to the recovery in vivo of inhibited cholinesterase in the brain. Whilst this work was in progress suspicion was cast upon conventional methods of cholinesterase assay by Kewitz & Nachmansohn (1957), who claimed that, unless extracellular inhibitor was removed from poisoned braSns before they were homogenized, falsely low cholinesterase levels would be obtained. The results of the present investigation do not support the idea that the inhibitors used are held at extracellular sites. The cholinesterase activities, which are reported here and which were measured by conventional methods of assay, are therefore considered to be reliable.