Cholesterol-induced alteration of apolipoprotein A-I conformation in reassembled high density lipoprotein.

Recent reports have shown that apolipoprotein A-I (apo A-I), the major protein of high density lipoprotein (HDL) may exist in different conformational states. We studied the effects of apolipoprotein A-II and/or cholesterol on the conformation of apo A-I in reassembled HDL. Analysis of tryptophan fluorescence quenching in the presence of iodine suggested that cholesterol increased the number of apo A-I tryptophan residues accessible to the aqueous phase, but decreased their mean degree of hydration. These observations cannot be totally explained on the basis of the effect of cholesterol on phospholipid viscosity as determined by fluorescence anisotropy of diphenyl hexatriene. We did not observe any effect of apo A-II on the conformation of apo A-I.