Clinical chemistry of common apolipoprotein E isoforms

Apolipoprotein E plays a central role in clearance of lipoprotein remnants by serving as a ligand for low-density lipoprotein and apolipoprotein E receptors. Three common alleles (apolipoprotein E(2), E(3) and E(4)) give rise to six phenotypes. Apolipoprotein E(3) is the ancestral form. Common apolipoprotein E isoforms derive from nucleotide substitutions in codons 112 and 158. Resulting cysteine-arginine substitutions cause differences in: affinities for low-density lipoprotein and apolipoprotein E receptors, low-density lipoprotein receptor activities, distribution of apolipoprotein E among lipoproteins, low-density lipoprotein formation rate, and cholesterol absorption. Accompanying changes in triglycerides, cholesterol and low-density lipoprotein may promote atherosclerosis development. Over 90% of patients with familial dysbetalipoproteinaemia have apolipoprotein E(2)/E(2). Apolipoprotein E(4) may promote atherosclerosis by its low-density lipoprotein raising effect. Establishment of apolipoprotein E isoforms may be important for patients with diabetes mellitus and several non-atherosclerotic diseases. Apolipoprotein E phenotyping exploits differences in isoelectric points. Isoelectric focusing uses gels that contain pH 4-7 ampholytes and urea. Serum is directly applied, or prepurified by delipidation, lipoprotein precipitation or dialysation. Isoelectric focusing is followed by immunofixation/protein staining. Another approach is electro- or diffusion blotting, followed by protein staining or immunological detection with anti-apolipoprotein E antibodies and an enzyme-conjugated second antibody. Apolipoprotein E genotyping demonstrates underlying point mutations. Analyses of polymerase chain reaction products are done by allele-specific oligonucleotide probes, restriction fragment length polymorphism, single-stranded conformational polymorphism, the primer-guided nucleotide incorporation assay, or denaturating gradient gel electrophoresis. Detection with primers that either or not initiate amplification is performed with the amplification refractory mutation system. Disparities between phenotyping and genotyping may derive from isoelectric focusing methods that do not adequately separate apolipoprotein E posttranslational variants, storage artifacts or faint isoelectric focusing bands

Patients with systemic vasculitis have increased levels of autoantibodies against oxidized LDL

Oxidation of low density lipoprotein (LDL) is considered to play an important role in the development of atherosclerosis and increased levels of autoantibodies against oxidized LDL have been found in patients with various manifestations of atherosclerosis. Patients with vasculitis are prone to the development of atherosclerosis. Since production of radical oxygen species in these patients may result in increased production of oxidized LDL (Ox-LDL), we hypothesized that antibodies against Ox-LDL are elevated during lesion development in vasculitis. Therefore we measured anti Ox-LDL antibodies in 25 patients with ANCA-associated vasculitis and in 42 healthy controls using an ezyme-linked immunosorbent assay (ELISA) in which malondialdehyde modified LDL (MDA-LDL) was coated on microtitre plates. Anti Ox-LDL antibodies were significantly higher in patients as compared to controls (P = 0.0001). Anti Ox-LDL levels were also measured in 11 patients during active disease and in these same patients during complete remission. Anti Ox-LDL levels were significantly higher in patients during active disease than during full remission (P = 0.001). Our results suggest that patients with ANCA-associated vasculitis are more susceptible to oxidation of LDL, which may contribute to accelerated atherosclerosis development

gamma-linolenic acid does not augment long-chain polyunsaturated fatty acid omega-3 status

Augmentation of long chain polyunsaturated omega 3 fatty acid (LCPUFA omega 3) status can be reached by consumption of fish oil or by improvement of the conversion of a-linolenic acid (ALA) to LCPUFA omega 3. Since gamma-linolenic acid (GLA) might activate the rate-limiting Delta-6 desaturation, we investigated whether GLA augments LCPUFA omega 3 status. Eight adults received 1.4 g GLA for 4 weeks and subsequently 2.2 g ALA+1.4 g GLA daily during another 4 weeks. Another seven adults received a daily oral dose of 2.2 g ALA for 4 weeks, and subsequently 2.2 g ALA+1.4 g GLA during another 4 weeks. ALA, or ALA+GLA, did not significantly augment EPA and DHA contents. We conclude that the LCPUFA omega 3 status can not be improved by supplementation of low dose GLA, neither by co-supplementation of ALA. Poor conversion of ALA to LCPUFA omega 3 may be caused by preferential beta-oxidation of ALA, negative feedback of arachidonic acid from the omnivorous diet, or by the low dietary ALA/LA ratio