The structure of human density lipoprotein: A study of the effect of phospholipase A and trypsin on its components and of the behavior of the lipid and protein moieties at the air-water interphase

Abstract

1. 1. The accessibility of the protein and lipid components of human high density lipoprotein with density between 1.064 and 1.210 g/ml (HDL) to trypsin and phospholipase A (phosphatide acyl-hydrolase) was evaluated in an effort to explore the surface structure of the lipoprotein particle. 2. 2. Results obtained by gel filtration and peptide mapping of the trypsin digests of HDL and the lipid-free protein component of HDL (apo-HDL) suggest that extensive regions of the HDL-protein in the lipoprotein particle are as accessible to trypsin as related regions in the lipid-free apo-HDL. 3. 3. Phospholipase A affected more than 95% of the choline and ethanolamine phosphatides of HDL without causing disruption of the lipoprotein. 4. 4. Apo-HDL obtained by lipid extraction of HDL showed an appreciable surface activity at the air-water interphase. Measurements of the area occupied by apo-HDL and HDL-lipids at this interphase suggest that in the HDL particle the lipid and protein components are oriented at the surface of a structure in which the polar regions of lipids and the protein are exposed to the aqueous environment.