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K L Retsky

Publications and source records attributed to K L Retsky.

4 recordsLinked to original sources
PubMed1999

Inhibition of copper-induced LDL oxidation by vitamin C is associated with decreased copper-binding to LDL and 2-oxo-histidine formation.

Oxidatively modified low-density lipoprotein (LDL) has numerous atherogenic properties, and antioxidants that can prevent LDL oxidation may act as antiatherogens. We have previously shown that vitamin C (L-ascorbic acid, AA) and its two-electron oxidation product dehydro-L-ascorbic acid (DHA) strongly inhibit copper (Cu)-induced LDL oxidation. These findings are unusual, as AA is known to act not only as an antioxidant, but also a pro-oxidant in the presence of transition metal ions in vitro, and DHA has no known reducing capacity. Here we report that human LDL (0.4 mg protein/ml) incubated with 40 microM Cu2+ binds 28.0 +/- 3.3 Cu ions per LDL particle (mean +/- SD, n = 10). Co-incubation of LDL with AA or DHA led to the time- and concentration-dependent release of up to 70% of bound Cu, which was associated with the inhibition of LDL oxidation. Incubation of LDL with Cu and AA or DHA also led to the time-dependent formation of 2-oxo-histidine, an oxidized derivative of histidine with a low affinity for Cu. Addition of free histidine prevented the formation of the LDL-Cu complexes and inhibited LDL oxidation, despite the fact that Cu remained redox-active. Interestingly, histidine was more effective than AA or DHA at limiting Cu binding to LDL, but at low concentrations AA and DHA were more effective than histidine at inhibiting LDL oxidation. These data suggest that there are at least two types of Cu binding sites on LDL: those that bind Cu in a redox-active form critical for initiation of LDL oxidation, and those that bind Cu in a redox-inactive form not contributing to LDL oxidation. The former sites may be primarily histidine residues of apolipoprotein B-100 that are oxidized to 2-oxo-histidine in the presence of Cu and AA or DHA, thus explaining, at least in part, the unusual inhibitory effect of vitamin C on Cu-induced LDL oxidation.

Antioxidants↗
PubMed1995

Vitamin C prevents metal ion-dependent initiation and propagation of lipid peroxidation in human low-density lipoprotein.

Lipid peroxidation and oxidative modification of low-density lipoprotein (LDL) have been implicated as causal factors in the pathogenesis of atherosclerosis, and prevention of LDL oxidation by antioxidants may be an effective strategy to inhibit the progression of the disease. We investigated the effects of the reduced form of vitamin C (L-ascorbic acid, AA) and its two-electron oxidation product (dehydro-L-ascorbic acid, DHA) upon metal ion-dependent oxidative modification of human LDL. We found that low micromolar concentrations of both AA and DHA protect LDL against oxidation induced by Cu2+ or by hemin and hydrogen peroxide. In a dose-dependent manner, AA and DHA prevented the initiation of lipid peroxidation in LDL, as determined by a sensitive and selective assay for lipid hydroperoxides utilizing HPLC with chemiluminescence detection. AA and DHA also preserved the LDL-associated antioxidants alpha-tocopherol, beta-carotene, and lycopene, but not ubiquinol-10. Furthermore, AA was able to stop propagation of lipid peroxidation in LDL, whereas DHA lacked this ability. The addition of 60 microM AA to LDL containing up to 38 nmol/mg protein of pre-formed lipid hydroperoxides led to their rapid disappearance; this activity of AA was dependent on the presence of redox-active copper, but did not lead to the formation of lipid hydroxides, the reduced form of lipid hydroperoxides. Our data show that in Cu(2+)-exposed LDL (i) vitamin C primarily spares, rather than regenerates, alpha-tocopherol and other endogenous antioxidants, except for AA and DHA prevent initiation of lipid peroxidation in LDL; and (iii) AA can terminate lipid peroxidation, thereby protecting partially oxidized LDL against further oxidative modification.

Adult↗
PubMed1993

Ascorbic acid oxidation product(s) protect human low density lipoprotein against atherogenic modification. Anti- rather than prooxidant activity of vitamin C in the presence of transition metal ions.

The oxidative modification of low density lipoprotein (LDL) has been proposed as an important causative event in the development of human atherosclerosis. As a corollary of this hypothesis, antioxidants that can prevent LDL oxidation may inhibit atherosclerosis. Oxidative modification of LDL in vitro, either induced by Cu2+ or mediated by cultured arterial wall cells in media containing trace amounts of transition metal ions, is strongly inhibited by vitamin C (L-ascorbic acid (AA)). AA, however, is known to act as a prooxidant rather than an antioxidant in the presence of transition metal ions. We observed that AA is oxidized rapidly when incubated with Cu2+ and LDL, leading to transient formation of dehydro-L-ascorbic acid (DHA). Although AA and DHA can no longer be detected after 3.5 h of incubation, LDL resists oxidative modification for at least 20 h, as assessed by anodic gel electrophoretic mobility. Remarkably, DHA protects LDL more effectively against both Cu(2+)-induced lipid peroxidation and shifts in electrophoretic mobility than does AA; indeed, AA per se, without oxidation to DHA, offers no protection. By inhibiting oxidative modification of LDL, AA and DHA prevent uptake of LDL by macrophages via the scavenger receptor pathway. When LDL is incubated with DHA followed by gel filtration, LDL remains protected against subsequent Cu(2+)-induced oxidative modification, suggestive of stable modification of LDL in the presence of DHA. In contrast, DHA is ineffective against a metal ion-independent type of oxidative stress, viz. aqueous peroxyl radicals; under these conditions, only AA is able to inhibit lipid peroxidation in LDL. Our data indicate that vitamin C protects LDL against atherogenic modification by two different mechanisms that may act in concert: (i) free radical scavenging by AA prevents aqueous oxidants from attacking and oxidizing LDL, and (ii) stable modification of LDL by DHA or decomposition product(s) thereof imparts increased resistance to metal ion-dependent oxidation.

Adult↗