A novel high-density lipoprotein particle and associated protein in rat plasma

This report describes the characterization of a novel rat apolipoprotein, which, as partial sequencing suggests, does not correspond to any described protein. The protein (termed PX) has an estimated molecular mass of 19.5 kDa and pI in the range 5.5-5.8. Monoclonal antibodies were obtained against protein PX and results on distribution among rat lipoproteins show it to be associated mainly with high-density lipoproteins (HDL), but also with VLDL. Immunoaffinity chromatography of total HDL shows protein PX to be included in a distinct lipoprotein particle, particularly enriched in free cholesterol, with which only traces of other apolipoproteins are associated. Immunologically crossreacting entities are found in the plasma of several species, including man. Retention of the epitope carried by the protein PX would suggest that it is of particular structural or functional importance. It remains to be established whether its function is associated with lipid metabolism.