Use of cyclodextrins to monitor transbilayer movement and differential lipid affinities of cholesterol

In view of the demonstrated cholesterol-binding capabilities of certain cyc lodextrins, we have examined whether these agents can also catalyze efficie nt transfer of cholesterol between lipid vesicles. We here demonstrate that beta- and gamma -cyclodextrins can dramatically accelerate the rate of cho lesterol transfer between lipid vesicles under conditions where a negligibl e fraction of the sterol is bound to cyclodextrin in steady state. beta- an d gamma -cyclodextrin enhance the rate of transfer of cholesterol between v esicles by a larger factor than they accelerate the transfer of phospholipi d, whereas, for alpha- and methyl-p-cyclodextrin, the opposite is true. Ana lysis of the kinetics of cyclodextrin-mediated cholesterol transfer between large unilamellar vesicles composed mainly of 1-stearoyl-2-oleoyl phosphat idylcholine (SOPC) or SOPC/cholesterol indicates that transbilayer flip-flo p of cholesterol is very rapid (halftime < 1-2 min at 37 degreesC). Using b eta -cyclodextrin to accelerate cholesterol transfer, we have measured the relative affinities of cholesterol for a variety of different lipid species . Our results show strong variations in cholesterol affinity for phospholip ids bearing different degrees of chain unsaturation and lesser, albeit sign ificant, effects of phospholipid headgroup structure on cholesterol-binding affinity. Our findings also confirm previous suggestions that cholesterol interacts with markedly higher affinity with sphingolipids than with common membrane phospholipids.