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L. Wang, C.-M. Li, M. Rudolf, O. V. Belyaeva, N. Kedishvili, B. Chung, C. A. Curcio; Comprehensive Lipid Profiling of Lipoprotein-Like Particles (Llp) Isolated From Human Bruch’s Membrane (Brm). Invest. Ophthalmol. Vis. Sci. 2008;49(13):1755. doi: https://doi.org/.
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An apolipoprotein B-containing large LLP of intraocular origin may provide the neutral lipid that accumulates with normal aging in BrM and in drusen 1,2. It is unclear whether LLP are triglyceride (TG)-rich, as implied by their 60-80 nm diameter. Also unknown are the phospholipid and retinoid content of LLP.
From 7 pairs of donor eyes (51-90 yr), BrM/ choroid was dissected and homogenized. LLP were isolated by density gradient ultracentrifugation (d≤1.21 g/ml LLP fraction). LLP and post-prandial plasma lipoproteins (LP, dietary chylomicrons (CM), very low density lipoprotein (VLDL), and low density lipoprotein (LDL)) pooled from 4 normolipemic donors underwent comprehensive lipid profiling (www.Lipomics.com; preparative thin layer chromatography, gas chromatography, and flame ionization detection). Retinyl esters were detected (as retinol) in all samples by reverse phase high performance liquid chromatography following hydrolysis. Quantities are expressed in nano- or picomoles (nmol, pmol).
1) The LLP fraction contained 368.7 nmole ± 222.0 of total lipid per eye. Esterified cholesterol (EC) was the most abundant (32.4% ± 7.9 of total). EC mass varied 10-fold among donors. 2) Mean EC/ TG was 11.3 ± 6.7 vs 0.11, 0.13, and 8.5 for CM, VLDL, and LDL, respectively. 3) Of EC fatty acids (FA), linoleic (18:2n6) was the most abundant (41.7% ± 4.7), as in plasma LP. Also prominent were oleic (18:1, 18.4% ± 1.3) and palmitic (18:1n9, 15.6% ± 1.6). Docosahexaenoic acid (DHA, 22:6n3) was 0.4%. 4) Phosphatidylcholine / phosphatidylethanolamine for LLP was 1.10-1.58 vs 6.0-14.9 for plasma-LP. Cardiolipin and phosphatidylserine were detected in LLP, not plasma-LP. 5) Retinyl ester (without free retinol) was detected in the LLP fraction of 6/7 donors (39.5 pmol ± 36.0 per eye), at expected levels in CM and VLDL, and not in the d>1.21 g/ml fraction of BrM.
BrM LLP lipid composition differs from that of large and/or atherogenic plasma LP, giving credence to the hypothesis 3 that LLP are produced within the eye. A TG-poor large LP is unique. The paucity of DHA, abundant in photoreceptors, supports the principle of DHA recycling to the retina 4. BrM LLP may participate in the visual cycle, perhaps in a retinoid efflux pathway inferred from murine radiolabeled tracer studies 5.
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