Cell surface receptors

qtochrome5 was the observation by Ryan and Engel’ that tbc oxidation of estrogen was inhibited b; carbon monoxide. in 1964 Omura 2ti Sate described the presence of rytochfome P-I50 tn microsomes and about [be same t mc Ccx~per.Estahrook and coworkers demnnslraled that this pigment was responsible for the ontdative metabol-

ism of drugs 2nd steroids’@. These discoveries led tcr a surge of inve~~~at~ons on the microromal cytochro~ P-450 enzymes and the metabolism of drugs. \tcmids and mar:y foretpn compounds. FolInwtnp the dircclvery of microsomal P-43) tt u a+found that eJectrons flow from TPNH to a cytochrome C reductase and there to cyqtrhrome P,4 H;Jwhich acts as the terminal nnidar fornidny substrates. After years ol mtcwive investigations in many laborator& the ~yt~hr~ P-4.9 drugmetabolizing er,zymes were purified and fotmd to exisl rn many forms’V. In a recent publication u5i::g mice l~enetically respok *ive to mduct:-; agent*,. it was rqorted

Cell srrtface receptors Uhcn I+ a binding sue a nxeptor site? This want or;e of tfx key questions raised by

Kouerfitz fAtrer&en) and tin up by other quaker5 at a Workshop of Cell Surfac,e Rccepor, kid recently in Jesus COG Jege. ~Cambr+e. U.K. I%-30 March I%?. organized on beblf of the Neuro&mr:al Gro.rp of the 12 K. Biochemical Soctet, J ft uw apeed rha: htndin~ sites can be equated with receptor 5ites only when the brndiy sites can ;-S shown to be a?lsociated with 2 pharm~Jo&cal or physiological event This i4 important as the widespread Jse of ligand-binding assays. in pan owing ‘2 their awent drnplicity. has led to tk &tin of biding. 5ites uhiih may have M ph?sio~icaJ reJevanse. Kosterlitz went on to r&ew the current state of knowledge of the bios:nth3is of the endogenous opi~~~~~h~~~t~~~ multipJe opiate recep37rs lii.~,~). The receJL&orsub-cJassesrnaybed&&m&&dby uy’ of 5eBrclive lizand and he; a fur&r pitfall in ligand-binding assays was empha&XXJ, rtarrielr the iof &fintng the free radroli~nd c5ncentration m sianration analyses. The clinical sipifi came ofti muIt@ sites is not clear at pre se@tbutasitesmayhassociatedwith anaJ@sia and =+atoV em ~~~~S~~~~~~~~ sy$?em and in cotxroiling intestinal ion

microinjected into Xenopus oocytes lead-

ing to synthesis and assembly of the multisubunit receptor which is in.sened into the transport. Knowledge about these muhiple membrane. The receptors inserted into the sites may help in produciri~ more sefective ooeytc surface membrane are buttons in drugs. that nicotinic receptor linked ion channels Our knowledge of nicotinic acetyl. resembling those in other systems may be choline receptors has been greatly demonstrated in the oocytes. He went on to advanced by the availability of Torpedo describe how, by use of the techniques of fish and two ~e~ntations emphasized molecular genetics. it has been pas:ible to prwess in this area. Fuchs (Rehovot ) iout- obtain an amino-acid sequence for part of lrned the role of nicotinic receptors as aiuto. the nicotinic receptor which is in very good antigens in the autoimmune disease ~JXL.~- agreement with that obtained convention;&en&rgrovir and described the discovery ally. Using these meth& it shouldbe possible to define the funct~~al significance of ni the la‘boratory correlate of this disease. experimental autoimmtme rrr~usr/re~niu each subunit of the nicotinic acetylcholine gruvLr (EAMG). She outlined her studies receptor and these exciting developments must soon Fmd application in other sysrm the receptor. showing that the pharterns. macotogical and myas&c& a&vi&s reside within a confo~at~j StIUCNIe in The ~phol~pid portion of the memthe mofecttL Ihat is sensiti,veto denatiufa- brane has often been neglected in favotu of tion and resisiaru IO mild enzymic dil;es the protein part where cell surface receptor lion. This has led to the possibilig of systems ate concerned. Thus, the com~~mu~~sion of EAMG by immunments of Berridge (C~b~dge) on the role ization with denatured nicotinic receflor. of phos~bolipid alterations in receptor sysEAMG cart also be regulated by the use of tems were particularly pertinent. He cnnanti-idiotypes. Studies on Xonoclonal anti- centrated on the role of phosbodies were de.sc~ibed indtiating how anti- phatidylirmsitol (PI) and the por:.ibility that its breakdown may con~b~e to a general bodies are of great use in snaJysi.5of recq?. tar smtcture and function. Of particufar transducing mechanism for a group of interest wa5 a monoclonal antibody multifun&nal receptors that gate caldirected against the receptor binding site. secwdly, Sumikawa (High Wycombe)

descriti how tJIe modem ttY&niques of m&c&r genetics eats be applied to the nicotinic receptor. Torpedo mRNA can be

cium ion, stimulatearaebidonicacid release and &m&e cyclic GNP formation. tfe descr&& his studies, ~~i~t~g the key role of Pl in controlling serotoninreceptorlinked ctdcium ion fluxes in insecl salivary

glands One model IO describe the role of PI

ctlmulated and receptor-Inhibited adcq late

PI aS an essential cofactor for open-

cycl?,,e by~.temc and po\hihly nori-cy~4;c~

pli%CtJS

ing

of calciumchannels

by agonist. PI hy-

drolysis following channel owning

leads to

linked system\ WAS carelully rcviewcd h! Kodbell (Geneva).

He upecul;ned ahou~ the

channel closing. In this model PI behaves m

quatemary structure of recePt\~r. N-protcrn

a similar manner to guanine nucleotidec in

and cyclase array% and drew an analrtp!

receptor-linked adenylate cyclase systems. In many systemseffective measurements 01

bctwccn

PI changes have been thwarted by the die appointing size of the changes. Berridge described the use of lithium ions which

antibo&

and

transmemhrane

sipnalling systems in thawhoth habe multiple recognition sites linked IO cclmmon units. The clinical role of N-protem ua\ highlighted hy Hourne (San Francisco) 111

block inositol phosphate (the initial product of PI breakdown) hydrolysis and which enable more sensitive measurements of PI

describing his \(udieb on patient\ pseudohypopararhyrotdism I. l’hls

changes to be made. In addition prolonged

metabolic effects of parrath\;roid hormone (PTH) leading to hy%%ilcemrd and it\

treatment with lithium may alter receptor sensitivity by interfering with receptorlinked PI alterations and hence tins may give a clue to the therapeutic effect of lithium in manic depressive illness. Guanine (N-protein)

nucleotide regulatory protein and its role in both receptor-

disease characterized

uith is a

h) resistance to the

sequelae. PTH is not detictent m these patients but it seems that the PI’H-Ii&cd adenylate cyclaw system is defective. The defect see:ns IO be in the N-protein uhicn can be shown IO bc rctlu
Current awareness series Key developments in pharmacology

Wanted: novel antagonists for GABAB receptors Some 2%50%

ofcentral synapses may util-

ize y-aminobutyric

acid (GABA)

as their

!

,

acid) belong IO the gn\up ~1 l/icucullineinsensitive GABA analogue$. llut another

neurotransmitter: in fact GABA appears to be the most important inhibitory transmitter

member

witbin the CNS. Although the existence of

become the focus ofrccent attet;tion. Inrerestingly. baslofen has been used clinicall)

discrete receptor subtypes for GABA

has

been considered likely for many years, this key atea of interest has received new impetus from recent evidence obtained in

of

chlorophenkl

this

group.

GABA.

bal:lotcn lilve-al).

([IIM

for almost PO years and appea’s lo be the most effecttre currentI\ available drug t;v

radioligand binding studies, isolated tissue

the treatment of spasticity asscl iated uith spinal cord lesmns’. althougl its i:~~ct

preparations and brain slice experiments. As far back as tile mid- 1970s the terminol-

mechanism of action ha* remained uncerfain. Bowery and colleague> have no\\

ogy bicucullinesensitive and -insensitive was employed to delineate two groups of

led by [:‘H]baclofen

6ABA analagues, one with inhibitory actions on the firing of central neurones which could tl: blocked by the GABA antagonist. hicuculline. We now know that benzodiazepines

and barbiturates

with specific populations of GABA

interact recep.

identified a novel populatton of bites Iabel(anJ [.‘HJG.\BA)

in

Main homogenates’. Their properties differ from classical GAB.4 receptors (d&bed GABA% by these uorkcrs) in that aponi9\ hs\e a unique rank order of potency in Ji+ placing rH]baclofen hinJing. w hilr both bicuculline and picrotoxin dre ineffectltc at

ton, and thus the pharmacology of GABA receptor subtypes has become extremely

competing for the hind@

comple.1. Indeed. it has recently been suggested fhat as many as 36 possible subtypes

present rather %ant>. hur the) .qXIr la be asxwtated with Ca” md guard? I IIUCICY~

of

tides’,

CABA

receptors could

theoreticall)

exist’. Cerrain amino acids of folded conformza c&I-aminocrotonic lion (e.p. acid, rrara-2-aminocyclohexane

carboxylic

ledge of these neu GABAH

\ite. Our kno\\receptors i\ at

and independent cnf sites affcstcd

and henzodiazepines. by barbiturales Baclofefensen4tive GAB A receptors appear to be widely distributed throughout the CNSj. and modulate catecholamine release