Scavenger receptor-mediated recognition of chemically modified lipoproteins by macrophages.

Scavenger receptor-mediated endocytosis of chemically modified-low density lipoprotein (LDL) by macrophages or macrophage-derived cells has been intensively studied from the potential link to atherosclerosis. Taking the advantage of the fact that maleyl albumin is also endocytosed via the same receptor, we have investigated the ligand specificity of this receptor by using rat peritoneal macarophages and sinusoidal liver cells. Maleylation of albumin of >50% of its lysine residues resulted in a threshold increase in the ligand activity, whereas demaleylation of maleyl-albumin to 20% did not affect the ligand activity, suggesting that maleylation of selective lysine amino groups of albumin may lead to the formation of the domain structure required for the receptor recognition. To further minimize the ligand domain, five albumin peptides isolated from cyanogen bromide-cleaved albumin were maleylated and determined for their ligand activity. The result demonstrated that albumin peptides with 102 amino acids but not with 37 amino acids gained the ligand activity upon maleylation, suggesting that generation of the ligand activity does not require a whole molecule, but rather, the domain itself might be regional. We next compared maleylalbumin with demaleyl-albumin in their cellular binding and endocytic degradation. Amounts of intracellular degradation of maleyl-albumin were proportionally increased up to a certain level. However, a further increase in cell surface-bound ligands did not affect the subsequent intracellular degradation. Based on these data, we would propose the ‘two binding sites’ model, where two binding sites on surface membranes are involved in the scavenger receptor-mediated ligand recognition; one is specific for the ligand domain and coupled to subsequent intracellular degradation and the other serves a binding site for polyanionic compounds.