[Interaction of peptides with cholesterol].

Interaction of cholesterol with apoproteins A-I and E was studied in absence of phospholipids in vitro. As shown by two methods - ultracentrifugation in density gradient of KBr and an extraction technique - an apoprotein E molecule bound 30-35 molecules of cholesterol and a molecule of A-I bound 17-22 molecules of the sterol. If the primary structure of apoprotein A-I and the fragments of apoprotein E are involved in consideration, the hydrophilic reaction appears to occur between the hydroxyl group of cholesterol and a guanidine group of the protein arginine residues as well as hydrophobic reaction - between side aliphatic chain of the sterol and the branched-chain amino acid, present in the protein at a distance of 4 amino acid residues from the arginine residue. A molecular model, illustrating the cholesterol reaction with polypeptide fragments, is developed. Considering high alpha-spirality of apoproteins E and A-I with specific localization of the charged residues, reaction of the sterol and apoproteins, described in the terms of this model, was possible only after a decrease in order of the protein structure. Dispersion spectra and optical rotation of apoprotein E showed that beta-structures were destroyed and alpha-spiralization was decreased in the protein in presence of cholesterol. The data obtained suggest that amino acid composition and structure of polypeptides are important for their reaction with cholesterol.