a Mnreel L ruPPlemeN n1 l Zl 2oo3 tecro n e Cnlcul RewNt in Bioche ktn Md uotecularBiolagv l5 l ri5 70 2000 and Degradation Induction Regulation of Mammalian BiologicalSignificance Metallothioneins A T Mites l G M Hawkswarth lJ H Beattie 2and V Radilla3 Departrnent PolwathBuilding Unvefsityof Aberdeen and Therapeutics oi Medicine The lnsti tute Besearch Rowett ForesterhillAb rdeen AB25 2ZD ScoUand UK rschoo of Applied tlK AberdeenAB219SB Scotland Road Bucksburn Greenburn UK Scotland 1HG AB25 Aberdeen The RoberlGordonUniversity 5q1 anq 5 The RobertGordon aulhor Dr V Bodilla Schoolof AppiiedSciences Corresponding AB25 1HG Scotland UK Aberdeen University Rel6ree Or Curlis O Klaassen Departmenl ot Pharmacologv Toxicologv and Therapeutics Universilv ol Kansas Medic l Center Kan s City KS TABLEOF CONTENTS t xiII IV 36 METALLOTH ONEN 36 OF NIT DEFINITIONAND CLASSIFICATION OF BIOCIIEMICAI AND PHYSICOCT EMICAIPROPERTTES M T 3 7 MANTMALIANVIT ISOFOtu lS N F M T 3 8 b t D U c T I O NA N D R E G U L A T I OO A Ti I t u r s u r r il J L Ur I d 1 9 40 Control Posttransciptional Cell SpecificandMetal lnducedDifferentialTranscriptional 4 1 R e g u l a l i oonf M T 42 OF MT VI DEGRADATION 4 3 vn B I O L O G I C ARI O L E SO F M T 43 andApoptosis Cell Proliferation Merallothionein A 5 l letrls of Essenuxl Homeostasis B 46 MT asFreeRadicalScavengel C 16 D MetalDetoKiJication vm MT i AND OTIrERBRAIN METAILOTHIONEINS 48 x ARE TI ERE SEPARATEFLINCTIONSFOR THE DIFFERENT 49 MT ISOFORMS 50 XCONCLUSIONS 5l REFERENCES B C x n r 10 1G9218 00 50 O 2000by CRCPessLLC ABSTRACT MTs aresmall cvsteine richmet l binding pro its found in many species and although there are differences betweelr them il is of note that they ha e a great deal of sequenceand sructuralhomology MammalianMIs are 6l or 62 aminoacrdpolypePtides containing 20 conser r edcysteine residues that underyin the bindirg of metals The existenceof MT acrossspeciesis indicativeof i15biologicaldemand while the conseraation of cysteinesindicatesthat L eseareundoubtedlyce ral to trhefunctionof thisprotein Four MI isoformshavebeenfound so far MT 1 MT 2 MT 3 andMT 4 but thesealso havesubtypeswirh 17MT genesideniiiLedin man of which 10 arektown to be funcdonal Diff rent cells expressdifferent Mr isoforms ith va ing levels of expressionperhapsas a rcsult of the different funciion of each isofoml Elen dfferent metals induce and bind to MTs to differentextents Over40 yearsofresearchinto MT haveyieldedmuchinfo matron on rhis protein but have failed o assiSn lo it a deflrutive biological rcle The fact that multiple MT isoforms exist and the greal vadery of substancesand agenis that act as the searchfor the biologicalrole of MIs This nic1erevielvs induc rs tunher complicates and degradationofthis the currentknoqledgeon the biocherlxsrry irduclion regLrlation prolein in man1mals with a padcular emphasison human MTs It also consldersthe possibiebiologicalrolesofdis prolein which includeparticiPationin cell proliferatronand apoplosis homeoslasisof essentialmetals ceilular free radjcal scavengrngand melal detoxification I METALLOTHIONEIN Meallothionern lMT aas ftrst ldert proteinin equine fied asa cadfiuum binding was cuosequenLly puri lcdneyin t957rand by Kagi andVallee l fied andcharacterised by irs low molecular MT is characlenzed weighr 6 ro TlDa hrghmetJ content characteristicamino acid composition high cysteireresiduesand contentof conser ved absenceof aJomaliczmino acids spectoscopicfeanrresindicalingtetiaiedEl tbrolate MT complexesand metalthiolatecluslers a is a ubiquitous melai binding protern rr ith sfong alfirrry for group lb and b LransidoD metals MT is a major zinc binding inlracellular thiol and in many casesrepreqenLs inudcellul r Lhesrnglemos abundant proteinthiol However despitethe accumulation of delailedinformationon bolb the biochemicaland molecularaspec of MT its bioiogicalrole stucture and expression is srill not clearly undeBloodmore than40 yearsafter its discovery MT is thoughl o of essential be involved in the homeostasis rr 16as melalslr0and melal deloxification 6 r3 by Piscatorin 1964 r7 originallysuggested aitiough it also apPearsto act as a polent rr ftee radicalscavenger re I I D E F I N I T I OANN D OF MT CLASSIFICATION MTs were originallyclassifiedaccord ing to the deflnition rha they should be polypepridesresemblingequine renal metallothionein in several of their featues l MTs werei tially dividedinto three classesaccordingto their structulzlcharacren uccClas I Mfs aredehnedasPol pep ofcystelneconserude with a n ghdegree vation comparedto thosein equinekidney Ctass II MTs are polypeptideswith less well conseryedcysEineresidues oniy drstant lyrelated to equine MT Class fiI MTs are defined as atypical nonffanslationa Ily rr metalthiolatepolyPeptides r synthesized has ow beensuperseded This classification by a complexbut moresmngentevollluonary classificadoninto familjes subfa nilies suberoupsand isoforms o There is considerf blesequetcehomology emongstlvlTs from differenl species Fol erampLe i mammals567oof all lIT residuesale conserved Thesecompdseall 20 cysteineresiduesard mostofthe lysines sennes aod arginines rsThese conser ed residuesare thoughtlo play a role in metal binding r6Lysines mey be itrvolvedin the rs deroxrficaionfunctionof lvlTT andthose in rhe alpha domam appearlo be lmpoflant n mamtainingthe conformationalintegnty by interactioowith lhe metal ofthe protem23 SnrdiesusingmutMt MTs thiolateclusters e suggest lat the consefl edseamesin the lvlT sequencecouldplay a role in thestabilro Lryof metal bindi tgligands There is also similaity acrossPhyla NIT ir Neurospora crasrd contajnsonly 25 amlno acids and while it is signilicantly smaller than mammaliallMT its pnmary culcrure is very sim derIo thexmino rer Ti nal half of mammalianMTs rr Thus lhe class I MTs display divergent evolution sincethedifferencesin aminoacldsequence increasewith increasingtaxononicdistance In contrast the existenceof lhe cys x cys in tie otherwlseun and cys cys sequences rela dclassI and classI MTs is evidence for convergentevolutionprobablyasa consequenceof metal complexationrequue ments 5 quelces where x and y are non cysleine anino acids The stoichiome ryis suchthat thereaJe7 bivalenl iofls for every 20 cysteines whichfo m melaltbiolarecomplexes lherebyelrablingthe MT lo btnd beween 7 10 g aromsof metaYmoLMT in a f lo r The inLrmoLecular mertl domainstructure Linkagesstabilise lhe protein secondary sm rctureand heoce loss of meul causes