tion is that these are normal structures that are secreted on apo B-100 in addition to di-sialylated complex-type
oligosacccharides. Several studies report a decrease in the sialic acid
content of LDL following LDL oxidation [85 – 87]. There is no accompanying increase in free sialic acid
suggesting that sialic acid is oxidized to a product that is not detectable using conventional assays for sialic
acid [86,87]. Van Lenten and Ashwell [88] showed that oxidized sialic acid has a molar extinction coefficient
that is 45 lower than unmodified sialic acid when measured using the Warren method [89]. Thus, oxidized
sialic acid is underestimated using this method of mea- surement. The resorcinol method of Svennerholm is less
sensitive to this modification, oxidized sialic acid having a molar extinction coefficient that is 10 greater than
unmodified sialic acid [90]. It is not clear if product of sialic acid oxidation remains bound to the oligosaccha-
ride chain of its parent glycolipid or apolipoprotein [86,87]. Tertov et al. [86] showed that oxidatively
modified sialic acid is not found in the free form i.e. remains apolipoprotein- or glycolipid-bound while re-
sults from Tanaka et al. [87] were inconclusive.
Tertov et al. [86] have raised the possibility of trans- fer of sialic acid from lipoproteins to other plasma
constituents. Although Trypanosoma cruzi has been re- ported to express a trans-sialidase that transfers
protein-bound donor sialic acid to an acceptor glyco- protein, there is no evidence of this occurring in hu-
mans [2]. While sialyltransferases are demonstrable in plasma these are, as with neuraminidase, likely intracel-
lular enzymes that serve no function in plasma but have entered plasma in response to tissue injury [91]. Fur-
thermore, human sialyltransferases do not transfer protein-bound sialic acid to an acceptor but instead
transfer nucleotide-activated sialic acid CMP-neu- raminic acid, which is not found in plasma, to galacto-
sylated acceptors [61].
5. VLDL sialic acid
Apo B-100, C-II, C-III, E and gangliosides Table 3 are the main contributors to the sialic acid content of
VLDL. Sialic acid makes a major addition of charge to VLDL, its electrophoretic mobility decreasing following
neuraminidase treatment [92]. VLDL from hyper- triglyceridemic subjects was found to be a relatively
poor substrate for bovine milk lipoprotein lipase whereas incubation of hypertriglyceridemic VLDL with
neuraminidase normalized its lipolysis by this enzyme. Stoline et al. [43] also noted an increased proportion of
apo C-III
2
in hypertriglyceridemic VLDL although this had no effect on lipolysis of VLDL [44]. Lindbohm et
al. [93] reported a decreased content of sialic acid on VLDL from subjects with combined hyperlipidemia
Table 3 An example of the molar contribution of each sialic acid-containing
constituent to the total sialic acid content of the major apo B-100- containing lipoprotein fractions based on the average normal fasting
apolipoprotein [52,71,106] and ganglioside [58] content
VLDL Constituent
Lpa LDL
Apo A-II 13.0
13.0 Apo B-100
13.0 Apo C-II
7.3 110.8
Apo C-III 1.2
Apo D 12.1
Apo E 0.1
Apo J Apo a
a
119.7–290.7 19.5
5.3 Gangliosides
5.3 162.7
Total sialic acid molmol 19.6
138.0–309.0
a
Range of sialic acid contents for apo a containing from 12 to 41 kringle IV repeats [107].
when compared to those with primary hypercholes- terolemia. This may reflect enrichment of VLDL with
protein constituents that are relatively poor in sialic acid or possibly a depletion of sialylated proteins from
VLDL since sialylation was expressed on a per protein basis.
6. LDL sialic acid
Sialic acid seems to have multiple functions on LDL. As with VLDL, sialic acid contributes to the charge of
LDL [94,95]. It has been reported that partial desialyla- tion of LDL results in its precipitation from plasma or
physiological saline [96]. Orekhov et al. [12] have de- scribed a sialic acid-poor LDL fraction in patients with
CHD. Presumably this fraction contains either a rela- tively low amount of sialic acid-containing constituents
or contains constituents with a low degree of sialylation or a combination of these. It has been noted that
sialic acid-poor LDL isolated from plasma has in- creased propensity to aggregate [97]. This would sug-
gest that sialic acid enhances the solubility of LDL in an aqueous environment. Sialic acid may also be in-
volved in cellular recognition of LDL as it has been shown to be an antigenic determinant [98].
The clearance of LDL apo B-100 from plasma has been described as having rapid and slow components.
Malmendier et al. [81] noted that desialylation of LDL resulted in an enhanced rapid rate of clearance of the
LDL apo B-100 with no change in the slow clearance component in humans. In contrast, Attie et al. [82] saw
no differences in the clearance of normal and desialy- lated LDL in pigs. This discrepancy may be at-
tributable to differences in the degree of desialylation of the LDL preparations used in these studies. A prepara-
tion that is prone to aggregate may, in part, be rapidly
removed by the reticuloendothelial system. Lindbohm et al. [93] studied the clearance of native LDL contain-
ing varying amounts of sialic acid and found that there was no effect of sialic acid content on LDL clearance.
Desialylation of LDL had no effect on its binding and uptake by fibroblasts or hepatocytes [82,99]. How-
ever desialylated LDL and sialic acid-poor LDL both show an enhanced uptake by aortic smooth muscle cells
[12,62]. Enrichment of LDL with ganglioside to in- crease its sialic acid content decreased LDL uptake by
aortic smooth muscle cells [62]. The reasons for the differences in uptake by the different cell types may be
due to the differential expression of lipoprotein recep- tors. Grewal et al. [85] have hypothesized the clearance
of desialylated LDL via a lectin receptor. Yoshida et al. [100] recently described a lipoprotein receptor, LOX-1,
that recognizes oxidized LDL. This receptor is ho- mologous to the natural killer cell antigen NKR-P1,
which has lectin-like qualities [101]. Since sialic acid- poor LDL have many of the qualities of oxidized LDL
the possibility that the enhanced uptake of desialylated and sialic acid-poor LDL is due to oxidation rather
than decreased sialylation or desialylation must be con- sidered [102]. It may be worthwhile to investigate if
differences in binding and uptake are found with LDL that has been desialylated and subsequently resialy-
lated. This would enable oxidative effects to be distin- guished from sialic acid-specific effects.
Camejo et al. [94] first reported that desialylation of LDL increased its interaction with chondroitin-6-sul-
fate-rich proteoglycans isolated from arterial wall ma- trix. Enrichment of LDL with gangliosides decreased
the interaction of LDL with chondroitin-6-sulfate-rich proteoglycans [63]. However there is no difference in
the interaction of LDL with relatively low and high sialic acid content isolated from separate individuals
[63]. This suggests that individual sialic acid-containing components on LDL rather than total LDL sialic acid
can affect the interaction of LDL with chondroitin-6- sulfate-rich proteoglycans.
7. Lpa sialic acid
