Oxidative susceptibility of apolipoprotein AI in serum

Abstract

Objective We compared the oxidative susceptibility of apolipoprotein AI (apoAI), the major protein component of high-density lipoprotein, in serum with that of low-density lipoprotein (LDL) and of thiobarbituric acid reactive substance (TBARS). Methods Serum samples were oxidized by adding AAPH (2,2′-azobis (2-amidinopropane) dihydrochloride), and exposure to fluorescent light or ultraviolet (365 nm) light for 4 days at 23 °C. Then the levels of oxidized apoAI, total apoAI, oxidized LDL and TBARS were measured. Results The apoAI in serum samples was readily oxidized on exposure to light, the amount of oxidized apoAI increasing 18.4-fold under fluorescent light and 58.8-fold under UV light compared to that in the untreated serum. On the other hand, the level of TBARS increased only 1.9- and 4.9-fold with the respective exposure to light. The amount of oxidized LDL remained unchanged with the treatment of AAPH and fluorescent light, and decreased under ultraviolet light. The oxidation of apoAI on exposure to the lights was inhibited by quercetin, an antioxidant. Conclusion ApoAI was more susceptible to oxidation than LDL and TBARS. Thus, oxidized apoAI can be used as an early marker for oxidative stress in humans.