Revised Manuscript Received October 26, 1993® abstract: The isolation of apolipoprotein AI-containing lipoproteins [Lp (AI)] by selected-affinity immunosorption minimizes the loss of associated proteins that occurs during the isolation of high-density lipoproteins (HDL) by sequential ultracentrifugation. We have used two-dimensional gel electrophoretic analysis to separate the proteins associated with Lp (AI). Using a combination of amino acid sequencing of transblotted proteins and Western blotting with specific antisera, we have identified a number of associated proteins. The positions of the apolipoproteins (apo) AI, A-II, A-IV, C-III, D, and E were located on the gels. Lecithin-cholesterol acyltransferase and cholesteryl ester transfer protein were identified inassociation with Lp (AI) to a greater extent than found associated with HDL after centrifugation. In addition to those proteins previously identified in association with HDL, we detected a number of plasma proteins associated with Lp (AI), namely, fibrinogen, haptoglobin, proline-rich protein (C4b-binding protein), and apolipoprotein J (SP40, 40 sulfated glycoprotein). The coisolation of these proteins with Lp (AI) does notappear to be an artifact in that they have very low affinity for a sham column containing covalently bound preimmune goat IgG inplace of the anti-apoA-I IgG. These findings suggest that in addition to apolipoproteinsthat exist largely in association with lipoproteins there is another class of proteins which exist both inlipoprotein-associated form and in the dispersed state. Detection and identification of these lipoprotein-associated proteins may aid in the mechanistic determination of a number of observed functions attributed to HDL.

High-density lipoproteins (HDL) 1 are thought to participate in a wide variety of biological functions. Prominent among these appears to be their participation in reverse cholesterol transport (Fielding & Fielding, 1982; Glomset, 1968). Additionally, they are also implicated in a number of other processes such as blood coagulation (Bareowcliffe et al., 1982; Carson, 1981), inhibition of viral infection (Kane etal., 1979; Levy et al., 1982), inhibition of infection by protozoal parasites (Ormerod & Venkatesan, 1982), delivery of cholesterol to steroidogenic cells (Chen etal., 1980; McNamara et al., 1981; Ohashi et al., 1981), degradation of bacterialendotoxins (Ulevitch et al., 1981), inhibition of LDL oxidation (Klimov, 1987; Kunitake et al., 1992; Ohta et al., 1989; Parthasarathy et al., 1990), and perhaps stimulation of vascular endothelium (Tauber et al., 1981).