Chemical studies on the structure of human serum high-density lipoprotein (HDL). Photochemical crosslinking of azido-labelled lipids in HDL.

The lipid classes of native human serum high-density lipoprotein (HDL) were exchanged against phosphatidylcholine, sphingomyelin and cholesteryl ester species substituted with photosensitive fatty acyl residues and against 25-azido-27-norcholesterol. The photosensitive fatty acyl residues were 5-and 16-azidopalmitic, 12-azidooleic and 18-azidolinoleic acid, all labelled with high tritium radioactivity. The lipid exchange method previously described was used. After UV irradiation and delipidation the apoproteins AI and AII, photocrosslinked with the radioactive lipids, were separated. The yield of covalently crosslinked lipid molecules amounted to 30% of the photosensitive lipid molecules incorporated into the HDL particle. ApoAI and apoAII were labelled by each of the photosensitive lipid classes, intergrated in the HDL particle although in very different stoichiometry. The regiospecific photochemical labelling of the lipopolypeptides AI and AII was established by cyanogen bromide cleavage and the separation of the four apoAI and two apoAII CNBr fragments. The analytical data prove the close steric relation and interaction of defined protein regions with hydrophobic regions of the lipid molecules by chemical means. These results are discussed with respect of the HDL structure.