Inhibition of HDL binding by tetranitromethane is not related to cross-linking of phospholipids to apoproteins

The authors have shown that treatment of human HDL with tetranitromethane (TNM) inhibits the lipoprotein specific binding to rat liver plasma membranes and to cultured rat hepatoma cells. In addition to the expected nitration of tyrosine residues, cross-linking of lipids to apoproteins and apoproteins to one another occurred during treatment of HDL with TNM. In order to determine the role of cross-linking of phospholipids to apoproteins they prepared a reconstituted HDL in which the native phospholipids were replaced with dimyristoylphosphatidylcholine (DMPC), which has no double bonds in the acyl chains. The reconstituted HDL (DMPC-HDL) has properties, including the ability to bind to HDL binding sites, similar to those of native HDL. On nitration with TNM, DMPC-HDL, like native HDL, lost its ability to bind to the HDL binding sites of isolated rat liver and rat testes plasma membranes as determined by competitive binding with 125I-HDL. Nitrated DMPC-HDL contained only traces of phospholipids covalently linked to apoproteins, whereas 21% of phospholipids were linked to apoproteins of nitrated native HDL; cross-linking of apoproteins to themselves was detected in both nitrated HDL's. These results show that covalent cross-linking of phospholipids to apoproteins is not responsible for the inhibition of HDL binding by TNM