It has been proposed that lipoxygenases, specifically 15-lipoxygenase, may play an important role in promoting the oxidation of low-density lipoprotein (LDL) in the artery wall. It is well known that peroxides are unstable in the presence of transition metals, decomposing to form the alkoxy and peroxy radicals, and so initiating lipid peroxidation. To test whether lipoxygenase-derived peroxides may promote the oxidation of LDL in the presence of copper, the lipoprotein was enriched with lipid peroxides derived from the enzymic action of 5- and 15-lipoxygenases on either linoleic or arachidonic acid. All of these products were found to promote oxidation, whereas the related hydroxy fatty acids had no effect. This suggests that lipoxygenase-derived peroxides associated with the LDL particle may promote peroxidation in the presence of a suitable transition metal catalyst. This result has implications both for the mechanism of the potential pro-oxidant action of lipoxygenases in vivo and for the ex vivo assessment of the oxidizability of LDL samples isolated from different donors.
Macrophages derived from the human monocyte cell line THP-1 or isolated from the peritoneum of C3H/HEJ mice were incubated with oxidized low-density lipoprotein (LDL) and the total glutathione content (oxidized plus reduced) was measured. An initial depletion of glutathione was followed by an increase, such that after a period of 24 h the glutathione content has approximately doubled. This response required the oxidation of the lipid phase of the LDL molecule, since both native LDL and acetylated LDL had little effect on glutathione levels. The response of the cells to oxidized LDL was dependent on the extent of oxidative modification of the protein. It was also found that 4-hydroxynonenal had a similar effect on THP-1 cells, and we suggest that this or other aldehydes present in oxidized LDL causes the induction of glutathione synthesis in response to an initial oxidative stress and consequent glutathione depletion. In addition, we found that both cell types possess transferases and peroxidases capable of detoxifying aldehydes and peroxides. However, treatment of cells with oxidized LDL or 4-hydroxynonenal for a period of 24 h had no effect on the activities of these enzymes.