Competitive ion binding to low density lipoproteins: an electron spin resonance study

A fluorescence double-quenching study of native lipoproteins in an animal model of manganese deficiency

Iodide and acrylamide were applied simultaneously in a double-quenching experiment to compare acrylamide quenching constants for internal and external fluorophores of high-density lipoproteins (HDL1 and HDL2) from manganese-adequate (MnA) and deficient (MnD) rats, free of the electrostatic effects associated with iodide. In MnA HDL1 compared to MnD HDL1, the acrylamide quenching constant for external fluorophores was different (P < 0.1). In MnA HDL2, there were two populations of fluorophores accessible to acrylamide, whereas in MnD HDL2, all fluorophores were accessible to both quenchers. We concluded that there were structural (local environmental) differences, possibly charge-related, around the external fluorophores, and a slightly larger population of buried fluorophores in the MnD HDL1 compared with MnA HDL1. In MnA HDL2, one-third of the fluorophores were accessible to iodide, and all external and internal fluorophores were accessible to acrylamide, whereas in MnD HDL2, all fluorophores were accessible to both quenchers.

Chemical modification of low-density lipoprotein enhances the number of binding sites for divalent cations

The EPR technique with paramagnetic Mn(II) ions has been used to probe the negatively charged sites on the surface of modified low-density lipoprotein (LDL). LDL modified in five different ways exhibited increased binding capacity for divalent cations. Enhanced binding is caused by the increase in the number of 'strong' binding sites. The 'strong' sites have been identified to be the aspartic acid and/or glutamic acid carboxyl residues and the 'weak' sites are zwitter-ionic phospholipids. In native LDL the negative groups make 'bonds' with the positive lysyl residues, thus stabilizing the structure. Any deprotonation or modification of the lysine amino groups makes the LDL structure more loose and the amino acid carboxyl groups accessible to divalent cations.