Prooxidant and antioxidant effects of hemin-mediated LDL oxidation
ผลของฮีมินในการเป็นสารเร่งออกซิเดชั่นและสารต้านออกซิเดชั่น ในปฏิกิริยาออกซิเดชั่น
ของไลโปโปรตีนชนิดความหนาแน่นต่ำ
Kwanta Na-Thalang, D. of Clinical Pharmacy, F. Of Pharmaceutical Sci., PSU.
Birgit Mayer, Inst. of Biochemistry and Food Chemistry, Graz Technical U., Graz, Austria
Alexandra Zirngast, Inst. of Molecular Biology, Biochemistry and Microbiology, U. of Graz, Austria
Albin Hermetter, Inst. of Biochemistry and Food Chemistry, Graz Technical U., Graz, Austria
Peter M. Abuja, Inst. of Molecular Biology, Biochemistry and Microbiology, U. of Graz, Austria
Corresponding e-mail : nkwanta@ratree.psu.ac.th
Grant : North-South-Dialogue-Scholarship-Programme, Austria
Presented : Satellite Symposium of the XIIth International Symposium on Atherosclerosis - Oxidative Stress & Atherosclerosis. Oslo, Norway, 22-24 June 2000.
Key words : hemin, low density lipoprotein, LDL oxidation, prooxidant, antioxidant
An accumulating body of information testifies to the crucial role played by the oxidation of
low-density lipoprotein (LDL) in the pathogenesis of atherosclerosis. Much attention has been paid searching for possible in vivo initiators of LDL oxidation. Hemin derived from degradation of hemo-globin has been suggested as an effective in vivo inducer of LDL oxidation.
This present study was aimed at elucidating the mechanistic aspects of hemin-mediated LDL oxidation in vitro. We monitored in vitro LDL oxidation by following low-level chemiluminescence (LL-CL). At low concentrations of hemin (0.1-5 mM), the maximum amplitude of CL intensity and
the length of lag phase were directly dependent on hemin concentration, whereas at higher concentrations of hemin (> 5 mM), no light emission was observed within 12-15 hours. In agreement with LL-CL, conjugated diene formation (CD) at 234 nm showed a prolongation of the length of lag-phase
with increasing concentrations of hemin (0.1-2.5 mM), while at higher concentration CD formation
was inhibited. Complete consumption of a-tocopherol (TocOH) was observed after 180 minutes of incubation at concentrations of hemin ranging from 0.5 to 5 mM. This implies a different mechanistim regarding the role of TocOH in hemin-induced LDL oxidation compared to Cu-induced LDL oxi-dation. In contrast to native LDL, hemin led to TocOH consumption in a concentration-dependent manner in LDL loaded with TocOH.
Interestingly, co-oxidation of LDL with hemin and Cu showed a synergistic effect whenever hemin concentrations were lower than the concentration of Cu used. When hemin concentration was higher than Cu, a prolongation of lag phase in Cu-induced LDL oxidation was observed. A possible mechanistic explanation for this is that hemin might compete with the better oxidant, i.e., Cu, for LOOH. Studies of the effect of hemin on the compartmentation of LDL by labelling LDL with a fluorescent analog of phosphatidyl choline containing diphenylhexatriene (DPHPC) showed that
there was only minimal oxidation of surface lipids of LDL at low concentrations of hemin (0.1-1 mM) compared to a very fast oxidation at higher concentrations of hemin (2.5-10 mM).
Hemin in the absence of peroxide can mediate the oxidation of LDL and was likely to exert
both roles as oxidant at low concentration and as antioxidant at high concentrations. The antioxidative effect was likely to occur in the presence of an other stronger oxidant, such as Cu. We assume that the reduction of heme iron by a-TocOH cannot be considered as the important step in the initiation of
LDL oxidation by hemin. Rather the compartmentation of LDL, surface and core lipids, and the ability of hemin to insert differently into each compartment might play a significant role in hemin-induced oxidation of LDL.
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