Vitamin C protects against and reverses specific hypochlorous acid- and chloramine-dependent modifications of low-density lipoprotein

Activated phagocytes produce the highly reactive oxidant hypochlorous acid (HOCl) via the myeloperoxidase-catalysed reaction of hydrogen peroxide with chloride ions. HOCl reacts readily with a number of susceptible targets on apolipoprotein B-100 of low-density lipoprotein (LDL), resulting in uncont rolled uptake of HOCl-modified LDL by macrophages. We have investigated the effects of vitamin C (ascorbate), an effective water-soluble antioxidant, on the HOCl- and chloramine-dependent modification of LDL. Co-incubation of vitamin C (25-200 mu M) with LDL resulted in concentration-dependent prote ction against HOCl (25-200 mu M)-mediated oxidation of tryptophan and lysin e residues, formation of chloramines and increases in the relative electrop horetic mobility of LDL. Vitamin C also partially protected against oxidati on of cysteine residues by HOCl, and fully protected against oxidation of t hese residues by the low-molecular-mass chloramines, N-alpha-acetyl-lysine chloramine and taurine chloramine, and to a lesser extent monochloramine (e ach at 25-200 mu M). Further, we found that HOCl (25-200 mu M)dependent for mation of chloramines on apolipoprotein B-100 was fully reversed by 200 mu M vitamin C; however, the loss of lysine residues and increase in relative electrophoretic mobility of LDL were only partially reversed, and the loss of tryptophan and cysteine residues was not reversed. Time-course experimen ts showed that the reversal by vitamin C of HOCl-dependent modifications be came less efficient as the LDL was incubated for up to 4 h at 37 degrees C. These data show that vitamin C not only protects against, but also reverse s, specific HOCl- and chloramine-dependent modifications of LDL. As HOCl-me diated LDL modifications have been strongly implicated in the pathogenesis of atherosclerosis, our data indicate that vitamin C could contribute to th e anti-atherogenic defence against HOCl.