alpha-Tocopherol (alpha-TOH), generally regarded as the most important
lipid-soluble, chain-breaking
antioxidant in human plasma, can also be a
pro-oxidant in isolated
low-density lipoprotein (
LDL) (Bowry V. W.; Stocker R. J. Am. Chem. Soc. 115:6029-6044; 1993). Here we examined whether this
pro-oxidant activity of alpha-TOH is of more general relevance. We compared the oxidizability of
lipid hydroperoxide-free, in vivo or in vitro alpha-TOH-depleted
LDL and
high-density lipoprotein (HDL), as well as plasma reconstituted with alpha-TOH-depleted
lipoproteins, with that of the corresponding native and alpha-TOH-supplemented samples, using water- and
lipid-soluble peroxyl radicals (ROO.),
hydroxyl radicals (.
OH), Cu2+, the transition
metal-containing Ham's F-10 medium, soybean
15-lipoxygenase, and
horseradish peroxidase as
oxidants.
Lipoprotein and plasma oxidizability was assessed by the loss of
cholesteryl esters and alpha-TOH and the accumulation of hydroperoxides of
cholesteryl esters and
phospholipids. Compared to native
LDL, HDL, and plasma, the in vivo and in vitro alpha-TOH-depleted counterparts were highly resistant to peroxidation initiation by all
oxidants when used at mild radical flux conditions. Wherever tested, the oxidizability of isolated
LDL decreased proportionally with decreasing alpha-TOH content. Initiation of
LDL lipid oxidation by
lipoxygenase and Cu2+ (even up to Cu2+:
LDL ratio of 20:1) had an absolute requirement for alpha-TOH. Oxidation of reconstituted plasma with ROO. showed that in the absence of the
vitamin, plasma
lipids were largely resistant to oxidation, whereas
bilirubin and
urate oxidized more rapidly. Replenishing the in vitro depleted
LDL with alpha-TOH, but not with
alpha-tocopherol acetate, fully restored its original content of
vitamin E and its oxidizability. Similarly, dietary supplementation with alpha-TOH restored the
vitamin content and oxidizability of the in vivo alpha-TOH-depleted
lipoproteins and plasma obtained from a patient with
familial isolated vitamin E deficiency. Under high fluxes of ROO. and .
OH, the activity of alpha-TOH in
LDL switched from pro- to
anti-oxidant, with the switching point for .
OH observed at a lower radical flux than that for ROO.. Together, our results show that alpha-TOH generally makes
lipoproteins more reactive towards radical
oxidants; this can result in a
pro-oxidant activity depending on the specific oxidation conditions.