Life Sciences Vol. 16, pp . 1821-1822 Printed in the D .S .A .

Pergamoa press

PT,A4NA CHOLIlH?STF.RARR ATin THF t~RPHINR RECF,PTQR Alexander Gem and Rohprt ,T, Cagetola Iteparttaent of Pharmacology, Hahnen~ann Mc"cli.ral College, Philadelphia, pPnnsyl,vania igln7. (Received is final form Nay 24, 1975) ölten thinking of narcotic analgesics, roost pharenacologists are concerned with them only as narcotics or as analgesics . That_, howevßr, is â clinical attitude which disrngarda the many other. actions that opiates also have ; neither their analgesic action nor the developwent of dependence may be beat suited to hAlp understand the interaction of opiates with their receptors. Over the past 12 years we have studied one of these other opiate actions, naatsly, their effect on the nonspecific esterase in human blood (psetxlocholineaterase, acylcholine aryl-hydrolase, R,C.3 .1,1 .8,), This enzyme is can mercially available, if not in a pare, at leant in a reproducible state in taftich its enzymatic activity is tneasureahle with exquisite accuracy (1,2) . The enzycee obeys Nichaelis-Menten kinetics (i,4) ; and it contains a morphine receptor, that ie, a site separate fmm its active site, !or which both natural and synthetic opioida anA their specific antagonists have affintyy some also have efficacy or intrinsic activity, manifested in an increase of the rate constant of enzymatic activity, Because of the great accuracy with whidt the rata of this enzymatic reaction can be measured, we have obtained exact ntmterical values for the affinity constants and efficaciea of a number oP such eoupounds (5,5), something not easily clone with other opiate actions . 1qe have also been able to propose a hypothesis on the interaction of opiates with this receptor (7) which fits all ~sxperimental results and which makes both their affinity for the receptor and their efficacy intelligible . The hypothesis considers affinity to the receptor to depend on an N-alkyl group and a benzene ring at about b~ fran each other, with other groups supplying assistance by hydrogen bonds and hydrophobic binding, while efficacy results when a drug molecule rigidly holds the N-methyl and phenyl group at about 4~ from each other, distorting the receptor and forcing it to conforei to the drug rather than the reverse. It is the stress of this enforced conformational change in the receptor which wattifeets itself in accelerated enzymatic activity . we do not claim that the morphine receptor in plasma esterase is the morphine receptor . For lroars it has been known (R,9) that there are several narphine receptors which map differ in Txtth their affinity and their responsive nsss to opiates, and the esterase receptor does differ in various ways fr+dte the analgesic receptor(s)t for instance, methadone and meperidine have only affinity for the receptor in the enzyme bttt no efficacy, that is, they caapetitively antageniza sorphine, as does nalorphine, On the other hand, naloxona does have sflicaap in the enzyme, in fact, it is one of the asst potent agoniats we have blend . At the save tiwie, we also find stony parallels between the nations of drugs en the ensytee and on the analgesic receptors for instance, the sequence of decreasing affinities to the enzymatic receptor of some oonseoatly treed drugs is methadone - dihydrontorphinctne - levorphanol morphine - codeine, which is also the sequence of their analgesic polenciast so that it would ba wrong to disetisa the para;netera obtained with the enzyme as


Cholinesterase and Morphine Receptor

Vol . 16, No . 12

irrelav+nt for understanding analgesic uniate action . The fact has parhana not received adeduate attention that the opiate receptor is an oddity in that it is the only rPCentor that playa no known role in the physiology of the organism but rASTxmda to an exogenous chemical struc ture . That this structure has such a great variety of actions in the organism suggests to us that it can complex pith a gmuping that moat be a rather axmnonplace feature of many proteina~ and if such proteins happen to have a physiological function, the attachment of an opiate molecatla may have a pharmacological effect . This view implies acme correlation between the relative affinities of different opiates for the various organs in which they find sites of attachment, but not neceasarily any correlation between their efficaciea . if there ie any merit in this view, then rnir studies on the interaction of opioid drugs with human plasma esterase may give us sane insight into the interactions of these drugs with other receptor sites, including the analgesic receptor . Furthe>_more, since pure plasma cholinesterase has been prepared (10), we may hope eventunll~ to he able to establish its complete atnicture~ and, combined with our findings on structure-activity relationships in the binding o! dn~gs to the receptor site on the An~~me and in their agonistic efficacy, we may hope that we shal?. he able to identify that portion of the protein structure which functions as thw morphine receptor, and nerhara also to understand the details of the allosteric change in the enz~nne which we aoetulated for efficacy (7) . References 1. 2, 3. 4, 5, 5. 7. B. 9, 10 .

M.J . R1TIN(~ER and A. hRRO, Arch . int. Phartnacodyn, 164 96-110 (1QF,6) . M.J . ET'fINfJ:R and A. GRâ, Arch . int. Pharwiacodyn . 164 111-11Q (1966) . A, GKRn, Enz_vmologia 3A 283-307 (1970), A. f;RRl1, Fn~ymologia 43 7.61-269 (1g7T.) . A,P, FRRl00 and A, GRfä), Arch . int, Pharmacodyn. lA7 213-235 (1970) . A. GPRO and R,J, CAPRYY)hA, Arch, int, Pharmacodyn. 213 274-2A3 (1974) . A. GEAQ, Arch, int. Pharmacodyn. 20fi 41-46 (1973) . T.1C, A1~?.RR, J, pharmacol. Rxp. Ther . 140 155-161 (1963) . P.3 . PORTOf~SR, J. Med. Cbew . A 609-616 (1965) . P .K . DA.S and J. LIDi7Q~.L, Aiochem, J, 116 A75-AP1 (1970) .

Plasma cholinesterase and the morphine receptor.

Life Sciences Vol. 16, pp . 1821-1822 Printed in the D .S .A . Pergamoa press PT,A4NA CHOLIlH?STF.RARR ATin THF t~RPHINR RECF,PTQR Alexander Gem and...
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