Epidemiological studies show an inverse correlation between high-density lipoprotein (HDL) concentration and the risk of developing coronary artery disease [1]. According to a widely accepted hypothesis, HDL or its subtractions play an important role in recruiting and transporting cholesterol from peripheral tissues to the liver for excretion, a series of events known as reverse cholesterol transport [2]. Other properties of HDL link its antiatherogenic functions to its antioxidative effects. Some studies have shown that co incubation of LDL with HDL in the presence of divalent copper prevents the oxidative modification of LDL [3]. In some reports this finding could not be confirmed, and in fact it has been demonstrated that in vitro HDL is oxidized faster than other lipoproteins [4]. When HDL is oxidatively modified, it alters to a form that causes macrophages to accumulate cholesterol. [5]. It has been suggested that systemic inflammation gives rise to prooxidant and proinflammatory HDL particles [6]. Oxidatively modified HDL is found in atheromatous plaques from human aorta [7]. Oxidatively modified HDL is no longer capable of removing cholesterol from cells, and it enhances LDL oxidation [8].

The contradictory findings on the role of HDL on LDL oxidation in vitro may be due to rather small study populations, and the reported heterogeneity of oxidation kinetics between lipoprotein preparations in vitro [9] which might be due to individual intrinsic properties of the lipoproteins. In the present paper we report the results of a study of the effect of HDL subtractions and gender on ex vivo oxidation of LDL from a population of 61 healthy free-living human subjects.

The oxidation of LDL of all subjects gave rise to typical conjugated diene vs. time -curves, where the different phases of hydro peroxide formation were clearly discernible. Co incubations of LDL with either HDL₂ or HDL₃ produced biphasic profiles with faster oxidation in the beginning, followed by a slower rate and finally a faster propagation phase. The profiles looked similar in all participants.

Although the mean lag time was shorter in the presence of HDL₂ or HDL₃, there were nine subjects who had longer lag time when HDL₂ was co incubated with LDL. We analyzed, whether there were any differences between these nine subjects and the rest of the study group that could explain the increased lag time. These nine subjects had a significantly smaller peroxidizability index of HDL₂ (88.0 ± 12.3 vs. 95.4 ± 9.5, p < 0.05) than the rest of the group. Their LDL lag time was slightly shorter (55.7 ± 4.7 vs. 61.0 ± 7.7 μmol/l/min, p < 0.05) and their fasting blood glucose concentration was lower (4.1 ± 0.5 vs. 4.5 ± 0.5 mmol/l, p < 0.05).

We found that co incubation of HDL₂ or HDL₃ with LDL in the presence of Cu²⁺ resulted in shortening of the mean lag time and acceleration of the oxidation rate in comparison with that of incubation of LDL alone. If lag time or propagation rate are thought of as indices of oxidation resistance, this outcome contradicts the role of HDL as an antioxidant. Our findings are in line with the studies by Bowry et al. [4], Suzukawa et al. [10], Schnitzer et al. [11], Ohmura et al. [12] and Raveh et al. [9], who have come to the conclusion that HDL is more easily oxidized than LDL. In other studies, HDL has appeared to be less prone to oxidation and even to protect LDL against copper-induced oxidation [3, 13–15]. In the study of Kontush et al. [16] all subtractions of HDL exhibited limited capacities to protect LDL at early stages of oxidation. At later phases, small dense HDL particles were the most potent inhibitors of LDL oxidation under mildly oxidative conditions. If strongly oxidative conditions were used [5 μmol/l Cu²⁺), none of the HDL subtractions offered any protection to LDL. The results were fairly similar whether the subspecies were isolated from serum or EDTA-plasma despite their widely differing paraoxonase activities suggesting that paraoxonase may have had a smaller role in the inhibition. In our study the HDL₂ subtraction of the majority of the subjects had properties that enhanced the onset of propagation phase. However, in 9 participants this phase was delayed in the presence of a moderate concentration of Cu²⁺ emphasizing that the individual variation of intrinsic characteristics of lipoproteins can not be overlooked.

In all, kinetic analyses by different investigators of the effects of HDL on copper-induced peroxidation of LDL are difficult to compare. (a) Firstly, the concentrations of LDL and HDL have been inconsistent and their expression has been variably based on protein, total lipid, total mass, phospholipid or cholesterol concentration, molar concentrations or particle numbers. The investigations into the kinetics of lipoprotein oxidation of Raveh et al. [9] and Ziouzenkova et al. [17] showed that the lag time and the propagation rate are dependent on LDL concentration. (b) Secondly, copper concentrations have also differed between the experiments. This has profound implications, since it has been shown in kinetic experiments that the lag time and the oxidation rate are correlated with the copper concentration until a saturating concentration is reached [9, 16, 18]. However, until more data are available, there is reason to think that the number of copper binding sites of lipoproteins is not constant but varies [18]. (c) Thirdly, the ratio of copper to lipoprotein has varied between studies. It has been found that the kinetic profile of LDL oxidation changes in response to copper concentration and that the familiar monophasic, auto accelerating profile is only obtained when the copper concentration is relatively high [17]. All of the compound kinetic curves in our study had a biphasic shape. The first phase of rapid oxidation in such a profile, whether observed with HDL or LDL, has been interpreted to occur via a tocopherol-mediated mechanism [9] where vitamin E acts as a prooxidant. Because the rate of oxidation is regulated by the ratio of bound copper/lipoprotein as outlined above, the addition of HDL to LDL should, theoretically, have lengthened the lag time and oxidation rate instead of shortening it, since HDL bound part of the copper. Obviously, many factors are involved in determining the outcome of this kind of experiment. Furthermore, it is apparent that ex vivo oxidization experiments with lipoproteins require standardization.

Earlier studies have shown that there are several intrinsic properties of lipoproteins that can affect their susceptibility to oxidation. Lipoprotein antioxidant content [19, 20], fatty acid composition [21, 22], presence of various enzymes [13] and LDL and HDL size [16, 23] are among the factors that have been shown to have an impact on oxidation parameters, the former especially in supplementation studies. Also, long-term habitual diets with different fatty acid contents have been shown to influence LDL oxidation susceptibility [24]. We analyzed the fatty acid compositions of LDL, HDL₂ and HDL₃ particles. Fatty acids are highly intercorrelated, and therefore their use in multivariate analysis as predictors is problematic. We tried to overcome this difficulty by uniting the information in the fatty acid profiles into a single term – the peroxidizability index – which describes the combined reactivity of fatty acids towards reactive oxygen species [25]. The results (Table 4) show that this index was significantly larger in both HDL₂ and HDL₃ than in LDL in men as well as in women, suggesting that HDL particles might be more susceptible to oxidation than LDL. This opinion was further strengthened by the findings that in multiple regression analysis the peroxidizability index of LDL, HDL₂ or HDL₃ in combination with fasting blood glucose concentration were the best predictors of lag time when LDL was oxidized alone or in mixtures with HDL₂ or HDL₃, respectively. Furthermore, our finding of a smaller peroxidizability index in those subjects whose lag time lengthened in the presence of HDL2 is in line with the suggestion that the rate of oxidation is governed by the ratio of bound copper to oxidizable lipids [26]. The results of our experiments also confirm the findings of earlier studies and suggest that the proportions of polyunsaturated fatty acids as well as those of linoleic acid and oleic acid [27–29] are related to the oxidation rate and the amount of dienes formed during in vitro oxidation. We have no ready explanation as to why the glucose concentration had a positive correlation with lag time of oxidation especially since all our subjects were normoglycemic. It has been shown that LDL isolated from patients with poorly controlled type I diabetes is more susceptible to copper-induced oxidation than LDL from control subjects [30]. Consequently, it has been suggested that glycated LDL might be particularly prone to oxidation. Nonetheless, our result is more in line with the results obtained in well-controlled type I diabetics, where glycated LDL gave a longer lag time than that of nonglycated LDL [31].

In conclusion, we report that the lag time, the maximum propagation rate for the formation of dienes and the amount of dienes formed by Cu²⁺ -induced oxidation of LDL alone or in the presence of HDL₂ or HDL₃ do not differ between healthy men and women despite significant differences in lipid concentrations. Our findings do not support the concept that co incubation of LDL with HDL in the presence of divalent copper prevents its oxidative modification. Rather, our findings support previous results that in vitro HDL is oxidized fastest of all lipoproteins [4] partly because of its fatty acid composition, which is oxidation promoting.