Melatonin inhibits oxidative modification of human low-density lipoprotein

Authors

  • Melissa R. Kelly,

    1. Department of Food, Nutrition and Food Service Management, School of Human Environmental Sciences, University of North Carolina at Greensboro
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  • George Loo

    Corresponding author
    1. Department of Food, Nutrition and Food Service Management, School of Human Environmental Sciences, University of North Carolina at Greensboro
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Address reprint requests to Dr. George Loo, Department of Food, Nutrition and Food Service Management, School of Human Environmental Sciences, University of North Carolina at Greensboro, Greensboro, North Carolina 27412–5001.

Abstract

ABSTRACT: An important property of melatonin is that it is a free-radical scavenger or antioxidant. Since free radicals can induce oxidative modification of low-density lipoprotein (LDL), a process believed to be involved in atherogenesis, we were prompted to evaluate the capacity of melatonin to prevent oxidative modification of LDL. To induce oxidation, human LDL (0.4 mg protein/ml) was incubated at 37°C with either 10 μM cupric chloride or 10 mM 2,2′-azo-bis-(2-amidinopropane) dihydrochloride (AAPH) for 3 hr or 24 hr, respectively. Several assays were then performed to unequivocally determine the extent of LDL oxidation. Compared to native LDL, oxidized LDL had increased agarose gel electrophoretic mobility and weaker immunoreactivity with a murine monoclonal antibody to human apolipoprotein B-100. Measurement of thiobarbituric acid-reactive substances (TBARS) revealed that native LDL contained 1.8 ± 0.6 nmoles TBARS/mg protein, whereas copper-oxidized LDL contained 53 ± 4 nmoles TBARS/mg protein. However, when present during incubation, melatonin (0.125–4 mM) inhibited in a concentration-dependent manner the increase in electrophoretic mobility, decrease in immunoreactivity of LDL, and increase in formation of TBARS caused by either copper or AAPH. In a fourth assay, phospholipid analysis of LDL was performed. Native LDL contained 420 ± 9 nmoles phosphatidylcholine (PC)/mg LDL protein and 30 ± 20 nmoles lysophosphatidylcholine (LysPC)/mg LDL protein. LDL incubated with copper had a decreased PC content (276 ± 48 nmoles PC/mg LDL protein) and increased LysPC content (76 ± 22 nmoles LysPC/mg LDL protein). But when present during the incubation of LDL with copper, melatonin attenuated in a concentration-dependent manner the degradation of PC to LysPC. Therefore, we conclude that melatonin can inhibit oxidative modification of LDL in vitro.

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