Apolipoprotein B-48: problems related to quantification

Apolipoprotein B-48: comparison of fasting concentrations measured in normolipidaemic individuals using SDS–PAGE, immunoblotting and ELISA

Raised levels of chylomicrons and chylomicron remnants, which circulate following a meal, have been implicated in the development of atherosclerosis. Apolipoprotein (apo) B-48 is exclusively associated with chylomicron particles and provides a specific direct measurement of the number of intestinally derived lipoproteins in the circulation. The quantification of apo B-48 in biological samples is difficult due to the very low concentration in plasma, structural similarity to the N-terminal 48% of apo B-100 and lack of an appropriate standard for apo B-48. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), followed by coomassie blue staining, has been used for many years to measure apo B-48 levels in triacylglycerol (TAG)-rich lipoprotein samples. The raising of antiserum to apo B-48 has led to development of more sensitive and specific methods including immunoblotting and enzyme-linked immunosorbant assays (ELISAs). This has enabled direct measurement of apo B-48 in plasma without the need for separation into TAG-rich lipoproteins. A high degree of variability was observed in the apo B-48 concentrations reported in the literature both within and between the SDS-PAGE, immunoblotting and ELISA methods.

Lipoprotein particle abnormalities and the impaired lipolysis in renal insufficiency

BACKGROUND

Increased concentrations of very low- (VLDL) and intermediate-density (IDL) lipoproteins in chronic renal failure (CRF) are thought to result from a defect(s) in degradation of plasma triglyceride (TG)-rich lipoproteins. The purpose of this study was to identify lipoprotein abnormalities associated with the reduced lipolytic rate constant, k1, of combined VLDL and IDL substrate from renal patients and asymptomatic controls.

METHODS

The VLDL + IDL were isolated from 18 predialytic patients (CRF-I), 8 patients on hemodialysis (CRF-II) and 10 asymptomatic controls. The lipolytic rate constant (k1) of VLDL + IDL was measured by an assay using bovine milk lipoprotein lipase and determination of TG before and after incubation by gas chromatography (GC). Neutral lipids were measured by GC and apolipoproteins by electroimmunoassays; the apolipoprotein-defined TG-rich lipoproteins including Lp-B:C, Lp-B:C:E and Lp-A-II:B:C:D:E were determined by immunoaffinity chromatography.

RESULTS

The k1 values of VLDL + IDL were significantly (P < 0.001) lower in CRF-I and CRF-II patients (0.0341 and 0.0352 min-1, respectively) than controls (0.0515 min-1). The levels of apolipoproteins B, C-III and E, and TG-rich Lp-B:C, Lp-B:C:E and Lp-A-II:B:C:D:E particles normalized to 100 mg TG per VLDL + IDL were significantly higher in both groups of CRF patients than in controls. All three TG-rich lipoproteins were characterized by significantly increased percent contents of free (FC) and esterified (CE) cholesterol and a decreased percentage of TG. The k1 values of the combined CRF-I and CRF-II patient groups showed significant negative correlations (P < 0.001) with FC (r=-0.81) and CE (r=-0.63) and a positive correlation with TG (r=0.72). Among lipoprotein particles, only Lp-A-II:B:C:D:E levels showed a significant negative correlation with k1 values (r=-0.47, P < 0.03).

CONCLUSIONS

This study shows that the abnormal neutral lipid composition of all three TG-rich lipoprotein particles and increased concentrations of Lp-A-II:B:C:D:E particles represent the main factors affecting the in vitro lipolytic rates of VLDL + IDL substrate in both the CRF patients before dialysis and patients on hemodialysis.

A simple and sensitive method in using the ratios of cholesteryl ester molecular species as indexes of oxidative stress in plasma and lipoprotein fractions

To develop a simpler method for lipid peroxidation which may replace the classic gold standard of measuring the loss of polyunsaturated fatty acids, we investigated the use of ratios of molecular species of neutral lipids (MSNL) separated by gas chromatography in a single step. We sub-fractionated lipoproteins and subjected each to oxidation. Among different combinations of ratios of MSNL, we found that the ratios of cholesteryl esters (CE) containing C20/C16 (R1) and C18/C16 (R2) decreased most rapidly with increasing Cu2+ concentration and with increasing oxidation time, for all lipoproteins. We suggest that these two CE ratios can be used as oxidative indexes for all lipoproteins and thus for intact plasma. Experimental evidence showed that the oxidative index of whole plasma is the weighted average index of its lipoproteins. This method was validated with thiobarbituric acid method. Normal healthy asymptomatic males had R1 value 0.496 +/- 0.130, and R2 4.674 +/- 0.848, and females had R1 0.535 +/- 0.117 and R2 4.569 +/- 0.733 with no significant gender differences. Both ratios showed low variabilities within each individual. The method was tested to be feasible in monitoring vitamin E supplementation study. This CE-ratio method is concentration independent. It is simple, rapid, and highly reproducible, and, suitable for screening plasma on large scale.