Conformational changes of the alpha 1-proteinase inhibitor affecting its cholesterol binding ability

The effect of conformational changes of the alpha 1-proteinase inhibitor (alpha 1PI) on alpha 1PI-cholesterol complex (1:2 mol/mol) formation in vitro was studied with electrophoretic and gel chromatographic methods. Native alpha 1PI was modified by adding free thiol agents such as glutathione, cysteine HCl, or DL-homocysteine, by heating, or by cleavage with pancreatic elastase or trypsin. Conformational changes of the alpha 1PI molecule induced by these procedures were all accompanied by a loss of its ability to bind cholesterol in vitro under standard experimental conditions. The data suggest alpha 1PI-cholesterol binding to be affected by both direct and indirect modifications of the alpha 1PI-reactive center, that is situated on a mobile peptide loop.