Measurement of plasma small-dense LDL concentration by a simplified ultracentrifugation procedure and immunoassay of apolipoprotein B.
BACKGROUND
Existing methods for detecting small-dense low-density lipoprotein (SD-LDL) are either semiquantitative (e.g., gradient gel electrophoresis) or require specialised laboratory methods (e.g., density-gradient ultracentrifugation, DGU).


METHODS
We report a method in which plasma was adjusted to a density (D) of 1.044 and 1.060 g/ml, respectively, in two tubes, both of which underwent ultracentrifugation (UC). A measure of SD-LDL apolipoprotein B (apo B) was obtained by subtraction of the apo B concentration in D>1.060 g/ml lipoproteins from that in D>1.044 g/ml lipoproteins to correct for apo B associated with lipoprotein (a) [Lp(a)]. This procedure was evaluated in paired plasma samples in healthy men (n=62) and in age-matched healthy women (n=74) and in age-matched primary dyslipidaemic men (n=72) and women (n=29) and compared with an established density-gradient ultracentrifugation (DGU) method.


RESULTS
The dyslipidaemic patients had either decreased high-density lipoprotein cholesterol (HDL-C) and/or increased triglycerides. In dyslipidaemic men, SD-LDL apo B level (23 [5-77] mg/dl) was significantly higher than in healthy men (P<0.001). In dyslipidaemic women, the SD-LDL apo B levels (11 [4-71] mg/dl) were significantly higher than in healthy women (7 [1-45] mg/dl; P<0.005). The concentration of SD-LDL apo B correlated inversely with HDL-C in both women (r=-0.280: P<0.005) and men (r=-0.464; P<0.0001) and positively with triglyceride concentration in both women (r=0.213; P<0.05) and men (r=0.592: P<0.0001). Correction for apo B in Lp(a) increased the analytical variation, which was 12% for apo B at D=1.044-1.060 g/ml and 9% for apo B measured at D>1.044 g/ml. Although the correlation between the new method and DGU results was high (r=0.830; P<0.0001, n=43), the concentration of apo B at D>1.044 g/ml correlated strongly with both corrected results (r=0.978; P<0.0001; n=237) and also with SD-LDL isolated using the DGU method (r=0.832; P<0.0001). Results at D>1.044 g/ml showed the expected correlations both with HDL-C (r=-0.465: P<0.0001) and triglycerides (r=0.526; P<0.0001).


CONCLUSIONS
The new method gave results consistent with earlier published findings using other techniques. Further simplification of the method using a single-density spin at D>1.044 g/ml appears feasible and may provide an easier quantitative method for clinical use.
Menys
Valentine C
VC
Department of Medicine, Manchester Royal Infirmary, Oxford Road, M13 9WL, Manchester, UK. Valentine.Menys@CMMC.nhs.uk
Liu
Yifen
Y
Mackness
Michael I
MI
Caslake
Muriel J
MJ
Kwok
See
S
Durrington
Paul N
PN
D000368:Aged; D001055:Apolipoproteins B / Q000097:blood*; D002499:Centrifugation, Density Gradient; D008076:Cholesterol, HDL / Q000097:blood; D008078:Cholesterol, LDL / Q000097:blood*; D002847:Chromatography, Agarose; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D006801:Humans; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors / Q000627:therapeutic use; D006949:Hyperlipidemias / Q000097:blood / Q000188:drug therapy; D007118:Immunoassay; D017270:Lipoprotein(a) / Q000097:blood; D008297:Male; D008875:Middle Aged; D012727:Sex Characteristics; D014280:Triglycerides / Q000097:blood
D001055:Apolipoproteins B; D008076:Cholesterol, HDL; D008078:Cholesterol, LDL; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors; D017270:Lipoprotein(a); D014280:Triglycerides