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Previous studies from this laboratory employing the technique of optical rotatory dispersion (ORD) have shown that human serum high-density lipoprotein (HDL) of d 1.063-1.21 gm/ml has a high content in a-helix which is to a large extent retained in its lipid-free form, apo HDL.1 On the basis of the ORD parameters, an unordered structure was assigned to the nonhelical portion of the apoprotein, and this conclusion received corroboration from infrared spectroscopy analysis.2 The present paper describes the optical properties of HDL and its products as studied by the technique of circular dichroism (CD). This method has the advantage over ORD of more clearly resolving bands relative to the optically active chromophores in the ultraviolet and far-ultraviolet regions. In agreement with the previous findings, both HDL and apo HDL exhibited a high a-helical content. The most significant observation was that the a-helical spectra were sufficiently different to permit a clear distinction between lipid-free and lipid-bound products. Moreover, they were found to differ in their optical behavior after either chemical modification by succinic anhydride or changes in the solvent medium. The studies were carried out on the two HDL products, HDLI (d 1.063-1.125) and HDL3 (d 1.125-1.21). Since the results were similar, this report will be limited to HDL3. Materials and Methods.-Methods for the isolation and purification of human serum HDL3 have been reported previously.3 Apo HDL3 was obtained from HDL3 by extraction with 3:2 ethanol-ethanol ether mixture at -10°.4 Either HDLs or apo HDL3 was succinylated by the reaction with succinic anhydride.' Apo HDL3 was reduced and alkylated as previously described.5 Relipidation of either apo HDL3 or succinylated apo HDL3 (S-apo HDL3) was carried out with aqueous dispersions of HDL3 phospholipids and the resulting apo HDLa-phospholipid complex isolated by ultracentrifugation.6 The products studied had a protein: lipid ratio of 2: 1 by weight. All the materials were extensively dialyzed against the desired buffers before analysis. On occasion, the preparations had to be clarified by centrifugation (10,000 X g, 10 min). Measurements of circular dichroism in the spectral region between 300 and 185 m/A were carried out at 270 in a Cary spectropolarimeter using quartz cells of 0.1-mm path length (Pyrocell, N.J.) with protein concentrations ranging between 1 and 2 mg/ml. All CD spectra were run at least twice. Baseline runs were made in the same cell, usually immediately after the sample run. The values of molar ellipticity [0] in units of deg

On the structure of human serum high-density lipoprotein: studies by the technique of circular dichroism.