Radioiodination of low density lipoprotein initiates lipid peroxidation: protection by use of antioxidants.

It is now apparent that low density lipoprotein (LDL) is very susceptible to lipid peroxidation and that the resulting oxidized LDL has altered biological properties. Radia- tion, particularly of longer duration and lower intensities, initi- ates lipid peroxidation, yet radioiodination with 1251 and 1311 is a frequently used method to label LDL for biological studies. To test the possibility that this procedure alters the biological properties of LDL, native LDL was radioiodinated with 1*51/1311 using IC1 to average specific activities of approximately 300 and approximately 100 cpmhg protein, respectively. Lipid peroxida- tion was monitored by TBARS and conjugated diene formation. Biological properties were monitored by fibroblast and macro- phage uptake of LDL as well as by rate of plasma clearance (FCR) in guinea pigs. '3'I-labeled LDL showed enhanced in- dices of lipid peroxidation compared to 'z5I-labeled LDL and both were greater than native LDL. The FCR of 1311-labeled LDL was greater than that of '251-labeled LDL (by 20-40%) and both increased progressively (by > 250%) when measured at 2, 6, and 13 days after iodination. The radioiodinated LDL samples were also more susceptible to pro-oxidant conditions. Thus, after exposure to Cu2', '3'I-labeled LDL showed greatly enhanced lipid peroxidation, decreased uptake by fibroblasts, in- creased uptake by macrophages and greatly accelerated FCR in guinea pigs. Exposure of LDL to 13'I-labeled albumin produced similar changes. Protecting LDL with antioxidants such as BHT and ascorbate immediately after radioiodination generally ameliorated the adverse effects.- Khouw, A. S., S. Parthasara- thy, and J. L. Witztum. Radioiodination of low density lipoprotein initiates lipid peroxidation: protection by use of anti- oxidants. J Lipid Res. 1993. 34: 1483-1496. oxidized LDL antioxidants metabolic Many studies have shown that ionizing radiation such as X-rays, gamma rays, and high energy electrons will cause lipid peroxidation in aqueous solutions of fats and oils. Doses of X-rays and gamma rays as low as 1000 rads have been shown to produce lipid peroxidation in fat emulsions, fatty acid micelles, and phospholipid lipo- somes. These changes are manifested by increases in con- jugated diene formation, lipid hydroperoxide formation, oxygen uptake, and decreases in polyunsaturated fatty acid contents (1-5). The extent of oxidation seems to be positively correlated with substrate concentration, and in- versely correlated with dose rate. In other words, the ex- tent of oxidation at