---,

Thelackofpersistenceofprophylacticeffectsofsurfactantonquartzandthe consequentrestorationofparticletoxicityhavebeenmeasuredincell-freesystem modelsofenzymaticdigestionprocesses,forcellularprocessesinvitro,andin limitedinvivoanimalmodelstudies.Incell-freeenzymaticdigestionexperiments, phospholipaseA2(PLA2)(Fig.3)digestionofDPPC-treatedquartzdustindicated thatenzymecouldhydrolyzethefattyacidsfromtheadsorbedDPPC,withsubsequentlossoflysolecithindigestionproductfromtheparticles.Analysisofthe digestionratedatausedakineticsmodelforenzymaticdigestionofanadsorbed bilayerconsideringsterichindrancebothoftheoutermolecularlayertotheinner particle-surface-associatedDPPClayer,andofhindrancebytheenzyme--substrate complextoenzymeaccesstoadjacentDPPCmolecules.Theanalysessuggested thatDPPCisadsorbedasatightlyheldbilayer,withtheouterpartofthebilayer rapidlydigestedbyPLA2,andtheinnerpartofthebilayerdigestedmuchmore slowly,andwithmineralspecificrates.Hemolyticactivitiesofparallelsamples showedacorrelationofrestorationofmembranolyticactivitywithremovalof theparticle-surface-adsorbedlayerofsurfactant(Wallaceetal.,1992).These cell-freeanalysesindicatedthatphospholipasecoulddigestivelyremoveDPPC fromsilicaandconsequentlyrestoreparticlecytotoxicity;butcell-freeexperiments cannotpredictabsolutevaluesofratesofsuchrestorationforcellularorinvivo processes(Wallace,etal.,1994).