Lipolysis-induced iron release from diferric transferrin: possible role oflipoprotein lipase in LDL oxidation

Conditions leading to oxidation of LDL in vivo are still unknown. While the occurrence of oxidized lipoproteins and catalytic free iron in advanced at herosclerotic lesions has been demonstrated, the origin of both is unclear. In vivo, iron metabolism is tightly regulated by iron-binding proteins tha t ensure that virtually no free iron exists. We examined whether physiologi cal events such as lipolysis might reduce pH, facilitate iron release from transferrin (Tf), and promote low density lipoprotein (LDL) oxidation. Lipo lysis is brought about by lipoprotein lipase (LpL), a triglyceride hydrolas e present on endothelial cell surfaces and in atherosclerotic lesions, LpL hydrolysis of Intralipid lowered pH from 7.40 to 7.00 in 10% human serum an d from 7.40 to 6.88 in phosphate-buffered saline. Similar decreases in pH w ere also observed when very low density lipoproteins were hydrolyzed by LpL . Lipolysis was accompanied by a 2-fold increase in the release of Fe-59 fr om Tf, Tf binding to subendothelial matrix (SEM), a site of key events in a therosclerosis, increased 2-fold as the pH decreased from 7.40 to 6.00. Mor e free iron also bound to SEM as the pH decreased below 7.40. We next teste d whether a reduction in pH promotes LDL oxidation. More oxidation products were found in LDL incubated at low pH for 24 h in 10% human serum, Malonal dehyde contents (nmol/mg protein), measured as TBARS, were 7.11 +/- 0.34 at pH 7.40, 7.65 +/- 0.49 at pH 7.00, 9.00 +/- 1.18 at pH 6.50, and 11.54 +/- 0.63 at pH 6.00. Based on these results, we hypothesize that lipolysis-ind uced acidic conditions enhance iron release from Tf and increase formation of oxidized LDL.