An altered form of apolipoprotein H binds hepatitis B virus surface antigen most efficiently

Using recombinant (r)HBsAg as a ligand, we previously found a 46-kDa human plasma protein capable of specific binding, and identified this protein as apolipoprotein H (apo H). Apo H is able to bind to rHBsAg containing only the small S protein, in both ligand blot and enzyme immunoassay systems (H. Mehdi, M.J. Kaplan, F.Y. Anlar, X. Yang, R. Bayer, K. Sutherland, and M.E. Peeples, J. Virol. 68, 2415-2424, 1994). Apo H is a plasma glycoprotein, some of which is associated with lipoproteins, particularly chylomicrons and high-density lipoproteins (HDL). During normal lipid trafficking in the bloodstream, chylomicrons and HDL are targeted to the hepatocyte, the primary host cell for HBV, for degradation. In this report the method of apo H presentation was examined. rHBsAg bound to apo H very poorly if the apo H was coated directly on a microtiter well, or if it was presented in a soluble form. Binding was 100-fold more efficient when apo H was presented as a complex with monoclonal antibody (MAb) P2D4. These results suggest that binding to this MAb alters apo H, making it highly reactive with rHBsAg. Apo H binding to rHBsAg is not dependent on divalent cations and is optimal at pH 6.5-8.0. Removal of lipids from rHBsAg resulted in denaturation, preventing analysis of binding activity. Removal of sialic acid or complete removal of N-linked carbohydrates from apo H did not change its ability to bind rHBsAg, indicating that apo H carbohydrates are not involved in rHBsAg binding. Likewise, chemical modification of the arginine residues of apo H had no effect on binding. However, chemical modification of as few as three of the 29 lysine residues of apo H destroyed binding, indicating that one or a few lysines in apo H are involved in rHBsAg binding.