Table 1.

Biochemical composition, size and secondary structure of SAA-POPC complexes obtained by thermal remodeling. Parent lipoproteins, which were prepared by cholate dialysis using 1:10 mol:mol SAA:POPC, were incubated at 60 °C for 1 h. Peak fractions I, II and III were isolated by SEC and analyzed as described in Figs. 5 and 6. Protein and lipid content in each fraction was determined as described in Methods. The biochemical analysis was repeated twice using two independent lipoprotein preparations, and the results agreed within better than 10%. Hydrodynamic size range of lipoproteins was determined by non-denaturing gel electrophoresis and confirmed by negative-stain EM (Fig. 5). Helical content was determined from far-UV CD spectra at 25 °C (Fig. 6A) based on the molar residue ellipticity at 222 nm (Mao and Wallace, 1984) with ±5% accuracy. These results were used to estimate the stoichiometry in the HDL-size particles (peak I). Since SAA in these particles is ~50% folded, the unfolded region is expected to increase the apparent Stokes diameter, leading to a potential overestimate in the particle size. This effect is probably not large since the particle size assessed from the native PAGE agrees with that observed by EM (Figs. 1, 5). Further, the biochemical analysis provides an average protein:lipid ratio that may not represent any particular sub-population of particles from the discrete bands seen on the native PAGE (Figs. 4B, 5B). With these caveats, the measured size and stoichiometry of the HDL-size particles suggest that, on average, each particle contains about 12 SAA and 72 POPC molecules.

Lipoprotein sample	SAA:POPC, mol:mol	Size, nm	α-helical content
Intact	1:10	8–12	53%
Heated
peak I	1:6	8–11	55%
peak II	1:25	>20	25%
peak III	1:1	7.5–8.5	41%