Mechanism of Low-Density Lipoprotein Oxidation by Hemoglobin-Derived Iron

Abstract

Excellular hemoglobin is an extremely active oxidant of low-density lipoproteins (LDL), a phenomenon explained so far by different mechanisms. In this study, we analyzed the mechanism of met-hemoglobin oxidability by comparing its mode of operation with other hemoproteins, met-myoglobin and horseradish peroxidase (HRP) or with free hemin. The kinetics of met-hemoglobin activity toward LDL lipids and protein differed from that of met-myoglobin and HRP, both quantitatively and qualitatively. Those differences were further clarified by analyzing heme transfer from the above-mentioned hemoproteins to LDL. It appeared that met-hemoglobin transferred most of its hemin to LDL, and the presence of H(2)O(2) accelerated the process. In contrast, met-myoglobin partially released hemin, but only in the presence of H(2)O(2), while HRP could not transfer heme at all. The minor amount of hemin transferred from met-myoglobin to LDL sufficed to trigger ApoB oxidation, forming covalent aggregates via inter-bityrosines. This indicated that heme bound to high affinity site(s) is responsible for oxidation. LDL components providing the sites were analyzed by binding heme-CO monomers to LDL. Soret spectra revealed that the high affinity site of monomeric hemin is located on the LDL protein, ApoB. The complex heme-CO-ApoB underwent instantaneous oxidation to hemin-ApoB, and the bound hemin then slowly disintegrated in conjunction with LDL oxidation. Hemopexin prevented LDL oxidation by trapping hemoprotein transferable heme. We concluded that met-hemoglobin exerts its oxidative activity on LDL via transfer of heme, which serves as a vehicle for iron insertion into the LDL protein, leading to formation of atherogenic LDL aggregates.

Hemoglobin is Associated with Serum High Molecular Weight Adiponectin in Japanese Community-Dwelling Persons

“…Hemoglobin is the main carrier and buffer of nitric oxide 28) . Hemoglobin, which relates to iron metabolism, is an extremely active oxidant, and met-hemoglobin exerts its oxidative activity on LDL via the transfer of heme, which serves as a vehicle for iron insertion into the LDL protein 29) . In the prediabetic stage, subjects with higher hemoglobin levels exhibited lower HDL-C and higher blood pressures and elevated levels of soluble CD40L, which is a pro-inflammatory cytokine, and hemoglobin was an independent determinant of soluble CD40L levels 30) .…”

Redox Reactions of Myoglobin

“…Acidic pH may be necessary for ferriprotoporphyrin IX that dissociates from metMb to intercalate into phospholipids because apoMb effectively removed hemin from liposomes at pH 8.0 (40,41). At the same time, met(III)Mb was reported to transfer some ferriprotoporphyrin IX into LDL at pH 7.4 in the presence of H 2 O 2 , leading to apoB and LDL oxidation (21). Thus, dissociated ferriprotoporphyrin IX from metMb is a relevant oxidant in various biological systems; however, other reactive forms of Mb and iron derived from the degraded porphyrin of Mb should also be considered.…”

Methemoglobin—It's not just blue: A concise review

“…Since in methemoglobin has a reduced capacity to bind hemin (Fe(III)), it can be transferred to ApoB in the LDL [54]. There is a linear oxidation of LDL by the methemoglobin and a burst due to hemin breakdown and oxidation by free iron.…”