PROBING OF THE SURFACE OF HIGH-DENSITY-LIPOPROTEINS WITH MN(II) IONS AND EPR SPECTROSCOPY

Negatively charged sites on the surface of two human high-density lipo proteins, HDL2 and HDL3, have been studied using EPR spectroscopy. Lik e for LDL, there are two types of binding sites for the Mn(II) ions on each HDL class, characterized by essentially the same binding constan ts as those for LDL. The small number of >>strong<< sites (n = 2), att ributed to the negatively charged amino-acid residues, indicates that only a small fraction of these residues is available for cation bindin g. The binding constants for >>weak<< sites and the number of these si tes in LDL and in the two HDL subfractions studied are consistent with the idea that these sites relate to the phospholipid domains on the l ipoprotein surface.