Structural requirements for oxidation of low‐density lipoprotein by thiols

Abstract

Oxidation of low-density iipoprotein (LDL) by macrophages, endothelial cells and smooth muscle cells, may be mediated by production of free thiols in the presence of transition metals. We examined the structural requirements, within a series of cysteinyl derivatives, for oxidation of thiols and of LDL in Hams FI0 medium. The primary mechanism by which such thiols mediate oxidation of LDL is largely independent of superoxide production, but strongly correlated with the susceptibility of each thiol to iron-catalysed auto-oxidation. These effects are compared and contrasted with thiol-dependent oxidation of LDL by stimulated human monocytes and macrophages.

Impaired Homocysteine Metabolism and Atherothrombotic Disease

“…Although cytotoxicity was not specific to homocysteine, endothelial cell detachment is also observed with cysteine (Dudman et al, 1991), an increase in the hydrogen peroxidesensitive cellular fluorescent probe, 2',7'-dichlorofluorescein, was observed in cultured endothelial cells exposed to homocysteine (Toborek and Hennig, 1996). Besides the initiation of lipid peroxidation at the cell surface, homocysteine auto-oxidation with trace metal ions, generating reactive oxygen species such as superoxide anion, hydrogen peroxide, hydroxyl, and thiol free radicals (Munday, 1989;Schöneich et al, 1989), would directly oxidize low-density lipoprotein (LDL) (Heinecke et al, 1987;Hirano et al, 1994;Wood and Graham, 1995). Homocysteine would also contribute to the LDL oxidative modifications mediated by endothelial cells, macrophages, and smooth muscle cells (Heinecke et al, 1987;Sparrow and Olszewski, 1993;Wood and Graham, 1995), which are deeply involved in the initial steps of atherogenesis.…”

“…Besides the initiation of lipid peroxidation at the cell surface, homocysteine auto-oxidation with trace metal ions, generating reactive oxygen species such as superoxide anion, hydrogen peroxide, hydroxyl, and thiol free radicals (Munday, 1989;Schöneich et al, 1989), would directly oxidize low-density lipoprotein (LDL) (Heinecke et al, 1987;Hirano et al, 1994;Wood and Graham, 1995). Homocysteine would also contribute to the LDL oxidative modifications mediated by endothelial cells, macrophages, and smooth muscle cells (Heinecke et al, 1987;Sparrow and Olszewski, 1993;Wood and Graham, 1995), which are deeply involved in the initial steps of atherogenesis. In agreement with these observations, an increase in the reduced form of plasma homocysteine associated with an elevation of the oxidation rate of homocysteine and cysteine (expressed as a decreased ratio of plasma reduced to total aminothiols) was reported in experimental hyperhomocysteinemia (Guttormsen et al, 1993;Mansoor et al, 1992Mansoor et al, , 1993a) and in hyperhomocysteinemic patients (Andersson et al, 1995;Mansoor et al, 1993bMansoor et al, , 1994Mansoor et al, , 1995.…”

Biochemical evidence for a link between elevated levels of homocysteine and lipid peroxidation in vivo

“…The thiol then autooxidizes, generating superoxide, which in turn oxidizes LDL if metal ions are present. The ability of thiols to oxidize LDL in the absence of cells strongly supports this proposal [23][24][25]. Moreover, cultured macrophages and endothelial cells use an L-cystine-dependent pathway to generate extracellular thiol that oxidizes LDL if the medium contains metal ions [26][27][28].…”

Reduction of transition metals by human (THP-1) monocytes is enhanced by activators of protein kinase C

“…Importantly, this does not involve initiation of lipid peroxidation within the LDL particle; I291 rather, the thiol-dependent reduction of Fe 3+ to Fe2+(Equation (1)) catalyses the decomposition of low "seeding' levels of lipid peroxides and thus is essentially propagative in nature (Equation (2)). This process involves the active uptake of L-cystine, via the x~-transporter, I26"3H cellular reduction, and subsequent release of free thiol (predominantly L-cysteine) into the culture medium, t26-28ml Thiol 'recycling' occurs in activated human monocyte-derived macrophages (primary, and THP-1), [27,28] murine peritoneal macrophages t311 and rabbit endothelial ceils; [26'31] cell types, like rat A10 smooth muscle cells, which do not exhibit this process, oxidize LDL at a much slower rate. ~28~ KS-+M "+ --+ M (n-l)+ (1) LOOH + M (n-l)+ --+ LO" + OH-+ M n+ (2) M n+ + e-(cell derived) --* M (n-l)+…”