Characterization of the Apolipoprotein B Polypeptide of Human Plasma Low Density Lipoprotein in Detergent and Denaturant Solutions*

Apolipoprotein B, the polypeptide moiety of human serum low density lipoprotein, is subject to degradation (as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) both in the intact particle and after delipidation. Protease inhibitors, sodium azide, and nitrogen saturation did not influence the rate or degree of degradation. Lipid-free apolipoprotein B prepared by gel exclusion chromatography in sodium dodecyl sulfate bound a limited number of detergent molecules (up to 300) in monomeric sodium dodecyl sulfate solutions; circular dichroic spectra of this complex were similar to spectra of the intact lipoprotein. Near the critical micelle concentrations, a large, cooperative increase in detergent binding occurred, accompanied by circular dichroic changes indicating increased a helicity. By sucrose density centrifugation, lysopalmitoyl phosphatidylcholine could be substituted for the anionic detergent; about 300 mol of lysolipid were bound to the polypeptide. Replacement of detergent with guanidine hydrochloride by dialysis produced a soluble polypeptide with no ordered structure at denaturant concentrations above i’ M. At lower guanidine hydrochloride concentrations, structural elements were regained in a broad, reversible transition. It appears that apolipoprotein B is an easily degraded polypeptide with regions resembling water-soluble proteins but other regions which interact with lipid (or synthetic amphiphiles) and produce an overall insolubility in aqueous solution in the absence of amphiphilic ligands.