In this experiment, 1% cholesterol load to the animals generated hypercholesterolemia in the model used. Studies both in animals and human have clearly demonstrated that prolonged high cholesterol concentration in the circulating blood have a positive correlation on developing atherosclerosis [4
]. Results from the plasma lipid profiles of 1% high cholesterol fed animals for 10 weeks showed concentration of plasma TC and LDL respectively has increased 4.5 and 8.3 times higher in w = 5 and further increased 8.7 and 17.3 times in w = 10 compared to the level before the experiment, whereas HDL was decreased. These finding were similar to findings from previous studies [7
]. These changes are associated with a phenomenon that excessive load of cholesterol to the liver above the acceptable level of its normal process causes the system to be unable in metabolising the lipids resulting in high cholesterol return in the circulating blood [24
The value of TC: HDL ratio has been used as an indicator of lipid lowering property. In this experiment, the ALA group showed significantly (p<0.5) lower TC: HDL ratio at both w = 5 and w = 10 with 4 and 2 times reduction respectively in the values when compared to the HCD group. The 1% cholesterol load to NZW rabbits was find to increase the TC level significantly in both HCD and ALA groups but the low TC: HDL ratio observed in the ALA group indicating that supplementation of alpha-lipoic acid may have possibly facilitated the hepatic HDL biosynthesis in vivo.
In the present study, when Atherogenic Index in Plasma (AIP) was calculated (log TG/HDL), it was found that there were no significant differences in the value of AIP between ALA (0.41 and 0.46) and N (0.47 and 0.49) at both w = 5 and w = 10 respectively, which suggest the antiatherogenic property of alpha-lipoic acid to the animals. Interestingly, the value of AIP in ALA supplemented group also remained relatively stable throughout the study period (0.41–0.46). The use of AIP is a useful predictor of coronary risk potentials as it gives indirect measure of lipoproteins (LDL and HDL) particle size. The significantly lower AIP in both N and ALA groups relative to the HCD group (0.5–0.6 at w = 5 and w = 10 respectively) observed in this study correspond well to the inverse correlation of the LDL size and AIP values analyzed from 35 cohorts of 1433 subjects with various risk of atherosclerosis and a cohort of 35 normal subjects [26
In the present study, we also found that supplementation of alpha lipoic acid in high cholesterol fed animals could inhibit the progression of atherosclerosis. The formation of atheromatous plaque in the alpha lipoic acid treated group was found to be significantly lower compare to that of the non-treated group (HCD). This data may provide to a new lipoate activity in vivo
. The inhibition effect by alpha lipoic acid in atherogenesis might be attributable partly to its hypocholesterolemic property. Previous study indicated that the related metabolic functions of alpha lipoic acid is on its role in blood glucose disposal [13
] through the glucose-metabolizing enzymes, prostaglandin dehydrogenase and alpha keto-glutarate dehydrogenase eventhough some researchers suspects a more direct role in cellular glucose uptake at the cellular membrane [27
]. However, the involvement of this substance in cardiovasular related diseases and hypercholesterolemia have not been studied. Nevertheless, the prophylaxis of atherosclerosis using antioxidant therapy has been extensively evaluated in animal experiments.
Oxidative stress is one of the causative factors that link hypercholesterolemia with the pathogenesis of atherosclerosis [4
]. Alpha lipoic acid through its reduced form DHLA exhibits a protective measure against free radicals activity eventhough its precise mechanism is still yet to be discovered. In our study, the TBARS level indicated by MDA concentration at w = 5 was significantly lower in the alpha lipoic acid treated group compared to that of the untreated group (HCD) indicating that apart of its hypocholesterolemic effect, alpha lipoic acid also may act as antioxidant. This findings was parallel to previous reports suggesting that alpha lipoic acid would be a potent metabolic antioxidant source to quench free radicals in vitro
and in vivo
]. Sen and coworkers [27
] reported that alpha lipoic acid analogue have a positively charged in physiologic pH. DHLA is a potent sulfhydryl reductant; redox potential of the DHLA: ALA couple is −0.32 V (compared to −0.24 V for the glutatione:oxidized glutatione couple) [31
]. This low redox potential may enable DHLA to easily donate its electron to electron-deprived molecules subsequently neutralized free radical activity. Studies with human Jurkat T cells have shown that when added to the culture medium, lipoate readily enters the cell where it is reduced to DHLA and rapidly efflux into the culture medium [16
]. The concentration of MDA increased in w = 10. This phenemenon was speculated due to high retention of cholesterol, particularly the LDL in the plasma that in turn exposing them to free radical attacks. It is well known that high cholesterol diet induces the overproduction of reactive oxygen species (ROS) which could initiate lipid peroxidation (LPO) [14
]. Supplementation of alpha lipoic acid has shown to have favourable effect for the first five week of treatment, however, excessive cholesterol level in blood due to continual cholesterol load may possibly have overshadowed the existing effect in the subsequent period. This study also reveals that prolonged intake of high cholesterol diet may cause retention of TC and LDL therefore exposing them to free radical attacks even with supplementation of exogenus antioxidant. The extra high concentration of cholesterol may accumulated in the hepato-biliary system resulting in relative high blood LDL. Reduction of TC and LDL would heavily depend on the cholesterol concentration in the diet.
This experiment suggest that alpha lipoic acid supplementation may increase lipid and lipoprotein regulation. The mechanism on how alpha lipoic acid able to reduce TC and LDL concentration is unknown, but probably via lipoprotein lipase (LPL) activity or through cholesterol metabolisme by the liver [32
]. Chiba and co-worker reported LPL activity and HDL level is increased in cholesterol fed NZW rabbit after administration of NO-1886 [33
]. Alpha lipoic acid probably capable to initiate LDL receptor synthesis in the liver which in turn increase the uptake of cholesterol back to the hepatic system and increase synthesis of apoprotein A component (moeity of HDL particles) for reversed cholesterol transport [14
]. With consideration to the dose of alpha lipoic acid, progression of atherosclerotic disease probably could be reduced to some extent. Although the present data do not allow the conclusion that alpha lipoic acid supplementation could prevent atherosclerosis-free radical activity, the data are in agreement with a model in which antioxidant supplementation may contributes to a reduction of bad cholesterol in the circulatory system.