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Two types of alpha adrenoceptors in liver cells
Current awareness series Key developments in pltarmacologl
Conformationalanalysisand drug design
verified by an S-ray crysfal structure def~m~i~f~on. Thus although I has the saffie pharttfacophoric groups as molindone. if appears to be mom than seven-fold mom potent than molindone in the avoidance bkxkade (AB) test. The impeffanceof the dim&on of the nitrogen lone pair is strongly supponed by the enantiomeric potency ratio of I which is mom than 12.fold in fhe AB test. The presentstudyrepm!+ents an exeellenf example of how present-day knowledge beyond the classical 2-D mpfesentafion (see Figures) cun be pieced together to anive at a hypothetical receptor model which may offer a guide in the synthetic &on of a medicinal chemist. In fact. using well-known crystallographic data on neurolepticaand straightforwardphysicochemical principles stafine.that a profonated nitrogen atom interacts with an anionicsite, that aromatic rings interactvia stacking interactions with other aromatic ring systems and using computer-assisfcd visualizationof the sfructuresinvolved. this article showsquite c~~vi~ingly the value of confomfational analysis as a powerful aid in rationa\ drug design. J. P.TDLLENAERE
It is generally aceoptedthat fhc an ot’dmg designcoufd be greatly improved if recep tars were known in pm&e molecular detail. This info~~ati~~nlacking, a logical step to fake is to look at the structural andfomv of a series of drug molecules interacting with a given receptor. This is exactly what Olson et crf.’ have done in the case of the d~~~rnine recepfor. Over the last decadeconsiderdblcpmgresshad been (2) R :- tort-butyl (butaclamol) made in fhe derailed characterizationoffhe ( -1 R = propyl (dexclamol) sfru~tural anafonry of dop;lminc:agonists and antagonists. Based on well-establis~d strucfurdI information (X-ray crystallographicdata) on dexclamol (the crystal structureof bufaclamol is not precise enough) and R-apomorphine. the authors propose a H hy~t~t~cal molecular model of fhe recep moiindone for. They note that dopaminc agonistsand antagonistsall possess a basfc nitrogen atomseparatedby a S-7 A chain or molecuIar fmmework from an aromatic ring. The secondring B of dexclamol is consideredto be the third binding mire.A fourth binding site is a relatively large lipophilic cavity which may bind bulky alkyl or aryl groups and a variety of sprropiperidineor hen& imidazolone groups encountered in some bufyrophenonetype neurolepfics. The Reference model not only enabledthe predictionoffhe I Olwn. C. L.. CheunP. Hi. C.. Ment.an. K. D.. receptormodel itself is constructedby posi4r. tafrans oonfigurafion but also the Eloum.J. F , Todw,, L.. Ber@t I... Davidson. tioningan anionic site ah>ve the protonafed absolute confieuration of the most potent A. B. andBon. E. (1981)j. iWed. Cfwm. 24. basic nifrogen atom at a normal linear hyenantiomerof I as ( - )&R. 8aR which was in2&tcfZ4 dfnKenbond distanceof 2.9 A. A phenyl ring is positionedabove the convex face 01 fhc A nng of dexclamol at a distanceequal to twice the vandc Waals thickness(3.6 A) of a phenyl ring with fhe ring planes parallel. Studiesch~e~zin~ the alpha adrenocep ceil. C&e grouphassuggestedthatthey rep Using this hypotheticalreceptormodel, a tars present in liver cell memhmnes have resent alphai (pnzosin binding sites) and series of 4a. Xa-rrflns-pyrrolo [ 7._3-gjiso. been n:porcd by at leastthree pmups’ -7. In alphas(yohitnbine and epinephfinebinding quinoline derivatives. incorporating some these studies the ligand dihydroergoqp sites) adrenoceptors.Evidence suppofling fun~fionaljtiesof molindone(cfystal stfue- tine seemsto label all the alpha adtenergic the preferfee of alpha, and alpiun ture defemrined by the aufhurs). were binding siresof the liver cell plasma mem admnoceptorsin liver felts already exists. designed in conformity to this receptor brane (6f)(f-I20 fmol mg I pmtein)i~H~T Actions of cpinephrine. such as the model. The 4a. Ra-rrorrs ring fusion of whereas the natural catccholamincs sfimulafions of glycogenolysis, glucrr fhesecompoundsassuresthat tile nitrogen epinephrinr or norepinephrinelabel only a neogenesis and ureogenesis am medilone pair. the aromafic pyrrole ring and the sntall percentageof the alphaud~n~e$fo~ af~xi fhroffgh alpha, ~~n~e~{~~, and cathonyl group are fixed in an ~~t~entation ( I2fKMO fnrol mg- ’ protein)’ Ct. Razosin involve p~~atidyli~sit~ turnover and u hich maximizes a producfive interaction and yohimbine binding represent4OCi-Ml0 calcium mobilizations. Alpha? admnocep with the first. secondand third balding site and I00 fnfol mg’f pmfein respecfivelyH~7, tots are linked to adenylafe cyclase in an of the receptor. These groupsconcludedthat them are two inhibitory fashion”,their physiologicalsigThe th~edi~nsi~alify of the receptor typas of alpha adrenneeptorsin the liver nificanrr is nt the presentunknown.
Two types of alpha adrenoceptors in liver cells
On Ihe other h;utd, it has been suggested that both alphaadnmergicbinding silrs uzp rckn( varieties of the alpha1 adrc~ crptor’ 3,‘f. Ae~~i~ lo ~he.seaudmrs. ol%? of lhe forms repWselllS the physiologicallyactive fmm of the mepor tcpinephrine or nolepinephrine binding sites)’ +J whereasthe o&ercould be a p CUIWW of the active acceptor(pramsin bind. mg sites)‘. The displacement of bound epinephriae by adrrnergic antagonists seems (0 be closely cornzlated with their ability to antagonize epinephrhte effects and the displacement by agonists seerm also to -late well with their metaboliic effects~. However. it is stupfising that prazosin is only 20-30 times mofe potent than yohimbine in inhibiting epinephrine binding since in most ~~~gkat studies pra2osin is about duee orders of magnitude more potent than yohimbine in inhibiting epinephrineeffectsR7.*. Treatment of plasma membranes with low concentrations of trypsin markedly incnxses the binding of epinephrinc or n~pi~’ and it bas been suggested that pioteasesmight alter the balance be twecn thfz two forms of the alpha& adrenocepmrby transformingthe precursor into the active form* or a form of the recep tar with high affinity for agonists, which represents a fmt step towards receptor internalization or degradation’. Although this is a very attractive hypothesisno sup pun from pharmacologicaldata is available yet. Another contmveniaf point is the e&t of guanine nuckotides on the binding of alpha1adrenergicagonists’.**‘. This point has a great impxtancc from a mechanistic pointof view and its clarifiiaGon surelywill help to etucidatethe processof signaltrans duction for alpha1adnmergicamines. In summary. although a considerable amountof effolt has heen devoted towards the studyof alphaadrrnuceplorsin the liver cell. data aboutsome crucial pointsremain controversial. The i~f~at~n available should be eonside~d as a signif=ant advance rather than a final answer in our understanding of Ihe hepatic alpha ~nergic phenf,~,tlm*. 1. AD0LF0GARCtASAINZ
In many tissuesa recqtor medii rise m CAMP concentrationthroughactrvationof adenylate cyclic ttiggen the crllular responseby ~irnu~ing c~P-~~m protein kinases. Simuhaneour stimulation of other receptor3 in the same iicsue can inhibit this rir inrA&fPcoacentration. For example. &adrcnergic stmmlafion of tlx parotid leads to incrcascdkvels of CAMP which CXUI be sued by stimulationof Ihc muscarinic cfiofinergx txxeptor or the cr-adrenerglcreceptor. Inter-ipceprormod. UhiOR has also been dem~f~~ltated fOT fib. mblaW, neuroblastoma cell Ime=. and thymi# where muscarinii chohncrgic receptors inhibit tigand stimulated cAMP accumulation. The expnxsion of the biologicalfunction of the Iigand-sensitive xknylate cyclax involves at least lhrpe dlstintq major corn.. ponenrs: the catalytic protein. the CTPbinding regulatoryproteinaud rhe tuxmtw receptor. Since al1 tbcsc componentsxc associatedin the membraneII seemsl&cl? thatthe lipidenvimnmcnt ma) bc rmpunant ~&~g~~~~~~n~~~ ing of the enzyme a&v*:: of fhc adenl\late cyclase. Studies U&L a bacterial phosph0hpa.W C specific for phosphaudyt inositol have ~~ns~t~ J spcc#fK requitrment for this phospholipid. TRWmcnt of membranes of the turtq rrythi ~ucyk’ cx of rat hean urcukmma* w~h this enzyme leads 8o a marked hns al the adenylatecyclaseactiviiy. Hydrolysis of phosphatd)linchitol proh asly catalysedby a cyWs0ltcenr)mc 16alwt un ter hormonal control and hx. been den onstratedfor a wide r;mgcof nxcpturs incltding the muscariniccholincqr txcep Ior’ One might ask thenzforcif rhezrrISan! tW.conship between ~hc mu~anmc cbJhnergic stimulalron of pho,sfh+ tidyhnositol hydroly& and of the mht. bitmn of cAhlP ~curnut~~~~n‘? One dttractive possibility is that the hydrolysisof the inositd lipid following muscarinic choliaergic stirn~u~i~~n nsuks in inhibition of the adenylate cyclase. .4 possible mechanism;LSto how phosphatidylinoritol
Conformationalanalysisand drug design
verified by an S-ray crysfal structure def~m~i~f~on. Thus although I has the saffie pharttfacophoric groups as molindone. if appears to be mom than seven-fold mom potent than molindone in the avoidance bkxkade (AB) test. The impeffanceof the dim&on of the nitrogen lone pair is strongly supponed by the enantiomeric potency ratio of I which is mom than 12.fold in fhe AB test. The presentstudyrepm!+ents an exeellenf example of how present-day knowledge beyond the classical 2-D mpfesentafion (see Figures) cun be pieced together to anive at a hypothetical receptor model which may offer a guide in the synthetic &on of a medicinal chemist. In fact. using well-known crystallographic data on neurolepticaand straightforwardphysicochemical principles stafine.that a profonated nitrogen atom interacts with an anionicsite, that aromatic rings interactvia stacking interactions with other aromatic ring systems and using computer-assisfcd visualizationof the sfructuresinvolved. this article showsquite c~~vi~ingly the value of confomfational analysis as a powerful aid in rationa\ drug design. J. P.TDLLENAERE
It is generally aceoptedthat fhc an ot’dmg designcoufd be greatly improved if recep tars were known in pm&e molecular detail. This info~~ati~~nlacking, a logical step to fake is to look at the structural andfomv of a series of drug molecules interacting with a given receptor. This is exactly what Olson et crf.’ have done in the case of the d~~~rnine recepfor. Over the last decadeconsiderdblcpmgresshad been (2) R :- tort-butyl (butaclamol) made in fhe derailed characterizationoffhe ( -1 R = propyl (dexclamol) sfru~tural anafonry of dop;lminc:agonists and antagonists. Based on well-establis~d strucfurdI information (X-ray crystallographicdata) on dexclamol (the crystal structureof bufaclamol is not precise enough) and R-apomorphine. the authors propose a H hy~t~t~cal molecular model of fhe recep moiindone for. They note that dopaminc agonistsand antagonistsall possess a basfc nitrogen atomseparatedby a S-7 A chain or molecuIar fmmework from an aromatic ring. The secondring B of dexclamol is consideredto be the third binding mire.A fourth binding site is a relatively large lipophilic cavity which may bind bulky alkyl or aryl groups and a variety of sprropiperidineor hen& imidazolone groups encountered in some bufyrophenonetype neurolepfics. The Reference model not only enabledthe predictionoffhe I Olwn. C. L.. CheunP. Hi. C.. Ment.an. K. D.. receptormodel itself is constructedby posi4r. tafrans oonfigurafion but also the Eloum.J. F , Todw,, L.. Ber@t I... Davidson. tioningan anionic site ah>ve the protonafed absolute confieuration of the most potent A. B. andBon. E. (1981)j. iWed. Cfwm. 24. basic nifrogen atom at a normal linear hyenantiomerof I as ( - )&R. 8aR which was in2&tcfZ4 dfnKenbond distanceof 2.9 A. A phenyl ring is positionedabove the convex face 01 fhc A nng of dexclamol at a distanceequal to twice the vandc Waals thickness(3.6 A) of a phenyl ring with fhe ring planes parallel. Studiesch~e~zin~ the alpha adrenocep ceil. C&e grouphassuggestedthatthey rep Using this hypotheticalreceptormodel, a tars present in liver cell memhmnes have resent alphai (pnzosin binding sites) and series of 4a. Xa-rrflns-pyrrolo [ 7._3-gjiso. been n:porcd by at leastthree pmups’ -7. In alphas(yohitnbine and epinephfinebinding quinoline derivatives. incorporating some these studies the ligand dihydroergoqp sites) adrenoceptors.Evidence suppofling fun~fionaljtiesof molindone(cfystal stfue- tine seemsto label all the alpha adtenergic the preferfee of alpha, and alpiun ture defemrined by the aufhurs). were binding siresof the liver cell plasma mem admnoceptorsin liver felts already exists. designed in conformity to this receptor brane (6f)(f-I20 fmol mg I pmtein)i~H~T Actions of cpinephrine. such as the model. The 4a. Ra-rrorrs ring fusion of whereas the natural catccholamincs sfimulafions of glycogenolysis, glucrr fhesecompoundsassuresthat tile nitrogen epinephrinr or norepinephrinelabel only a neogenesis and ureogenesis am medilone pair. the aromafic pyrrole ring and the sntall percentageof the alphaud~n~e$fo~ af~xi fhroffgh alpha, ~~n~e~{~~, and cathonyl group are fixed in an ~~t~entation ( I2fKMO fnrol mg- ’ protein)’ Ct. Razosin involve p~~atidyli~sit~ turnover and u hich maximizes a producfive interaction and yohimbine binding represent4OCi-Ml0 calcium mobilizations. Alpha? admnocep with the first. secondand third balding site and I00 fnfol mg’f pmfein respecfivelyH~7, tots are linked to adenylafe cyclase in an of the receptor. These groupsconcludedthat them are two inhibitory fashion”,their physiologicalsigThe th~edi~nsi~alify of the receptor typas of alpha adrenneeptorsin the liver nificanrr is nt the presentunknown.
Two types of alpha adrenoceptors in liver cells
On Ihe other h;utd, it has been suggested that both alphaadnmergicbinding silrs uzp rckn( varieties of the alpha1 adrc~ crptor’ 3,‘f. Ae~~i~ lo ~he.seaudmrs. ol%? of lhe forms repWselllS the physiologicallyactive fmm of the mepor tcpinephrine or nolepinephrine binding sites)’ +J whereasthe o&ercould be a p CUIWW of the active acceptor(pramsin bind. mg sites)‘. The displacement of bound epinephriae by adrrnergic antagonists seems (0 be closely cornzlated with their ability to antagonize epinephrhte effects and the displacement by agonists seerm also to -late well with their metaboliic effects~. However. it is stupfising that prazosin is only 20-30 times mofe potent than yohimbine in inhibiting epinephrine binding since in most ~~~gkat studies pra2osin is about duee orders of magnitude more potent than yohimbine in inhibiting epinephrineeffectsR7.*. Treatment of plasma membranes with low concentrations of trypsin markedly incnxses the binding of epinephrinc or n~pi~’ and it bas been suggested that pioteasesmight alter the balance be twecn thfz two forms of the alpha& adrenocepmrby transformingthe precursor into the active form* or a form of the recep tar with high affinity for agonists, which represents a fmt step towards receptor internalization or degradation’. Although this is a very attractive hypothesisno sup pun from pharmacologicaldata is available yet. Another contmveniaf point is the e&t of guanine nuckotides on the binding of alpha1adrenergicagonists’.**‘. This point has a great impxtancc from a mechanistic pointof view and its clarifiiaGon surelywill help to etucidatethe processof signaltrans duction for alpha1adnmergicamines. In summary. although a considerable amountof effolt has heen devoted towards the studyof alphaadrrnuceplorsin the liver cell. data aboutsome crucial pointsremain controversial. The i~f~at~n available should be eonside~d as a signif=ant advance rather than a final answer in our understanding of Ihe hepatic alpha ~nergic phenf,~,tlm*. 1. AD0LF0GARCtASAINZ
In many tissuesa recqtor medii rise m CAMP concentrationthroughactrvationof adenylate cyclic ttiggen the crllular responseby ~irnu~ing c~P-~~m protein kinases. Simuhaneour stimulation of other receptor3 in the same iicsue can inhibit this rir inrA&fPcoacentration. For example. &adrcnergic stmmlafion of tlx parotid leads to incrcascdkvels of CAMP which CXUI be sued by stimulationof Ihc muscarinic cfiofinergx txxeptor or the cr-adrenerglcreceptor. Inter-ipceprormod. UhiOR has also been dem~f~~ltated fOT fib. mblaW, neuroblastoma cell Ime=. and thymi# where muscarinii chohncrgic receptors inhibit tigand stimulated cAMP accumulation. The expnxsion of the biologicalfunction of the Iigand-sensitive xknylate cyclax involves at least lhrpe dlstintq major corn.. ponenrs: the catalytic protein. the CTPbinding regulatoryproteinaud rhe tuxmtw receptor. Since al1 tbcsc componentsxc associatedin the membraneII seemsl&cl? thatthe lipidenvimnmcnt ma) bc rmpunant ~&~g~~~~~~n~~~ ing of the enzyme a&v*:: of fhc adenl\late cyclase. Studies U&L a bacterial phosph0hpa.W C specific for phosphaudyt inositol have ~~ns~t~ J spcc#fK requitrment for this phospholipid. TRWmcnt of membranes of the turtq rrythi ~ucyk’ cx of rat hean urcukmma* w~h this enzyme leads 8o a marked hns al the adenylatecyclaseactiviiy. Hydrolysis of phosphatd)linchitol proh asly catalysedby a cyWs0ltcenr)mc 16alwt un ter hormonal control and hx. been den onstratedfor a wide r;mgcof nxcpturs incltding the muscariniccholincqr txcep Ior’ One might ask thenzforcif rhezrrISan! tW.conship between ~hc mu~anmc cbJhnergic stimulalron of pho,sfh+ tidyhnositol hydroly& and of the mht. bitmn of cAhlP ~curnut~~~~n‘? One dttractive possibility is that the hydrolysisof the inositd lipid following muscarinic choliaergic stirn~u~i~~n nsuks in inhibition of the adenylate cyclase. .4 possible mechanism;LSto how phosphatidylinoritol