Covalent cross-linking of photosensitive phospholipids to human serum high density apolipoproteins (apoHDL).

Human serum high density apolipoproteins were reassociated with three different lecithin species substituted with radioactively labelled photosensitive azido fatty acids, bis([3H]-16-azidopalmitoyl)-, bis([3H]12-azidooleoyl)- and bis([3H]18-azidolinoleoyl)glycerophosphocholine. The lipoprotein particles were reconstituted from a mixture of azido-labelled phosphatidylcholine and non-labelled dioleoylglycerophosphocholine (1:9). Excess lipid was separated from the homogeneous particles by Bio-Gel A-5m. The molecular weight, stoichiometry, fluorescence and circular dichroism of the reconstituted particles were determined before and after photoactivation with covalent cross-linking of the phospholipids with the apoproteins. The physical parameters of the reconstituted lipoproteins remained unperturbed by the cross-linking reaction between the generated nitrenes and apolipoprotein A-I and A-II. Thus the hydrophobic interactions of the phospholipid molecules with the apoproteins have been proved for the first time by a chemical method.