Determination of apolipoprotein E in high density lipoprotein fraction by immunofixation method and turbidimetric immunoassay after precipitation.

Apolipoprotein E (apoE) in high density lipoprotein (HDL) fraction (HDL-fr) was determined by the immunofixation method and turbidimetric immunoassay (TIA) after precipitation with phosphotungstic acid/MgCl2 in normolipidemic control subjects and patients with type IV hyperlipemia and hyper HDL-cholesterolemia. Immunofixation assay revealed two major bands of apoE in whole serum: one in the alpha-area and the other in the pre beta-area. ApoE in alpha-area (alpha-apoE) was identical to alpha-apoE in HDL-fr separated by ultracentrifugation but not to alpha-apoE in HDL-fr separated by precipitation (pHDL-fr). alpha-ApoE in pHDL-fr lacked the slower area of the band. Agarose column chromatography and gradient gel electrophoresis indicated that alpha-apoE belongs to early fractionated HDL, and that the precipitatable alpha-ApoE belongs to the higher molecular size HDL alpha-ApoE (%) estimated by immunofixation showed a strong positive correlation with apoE (%) in pHDL-fr. ApoE (%) in pHDL-fr was higher in case of hyper HDL-cholesterolemia and lower in type IV hyperlipemia than in controls, and was inversely correlated with serum triglycerides (TG) and positively with HDL-cholesterol (especially HDL2-cholesterol) in these subjects. It is suggested that the variation of apoE in pHDL-fr depends on the level of HDL2. Also, it may be suggested that apoE in pHDL-fr and precipitatable alpha-apoE belong to low and high molecular HDL2, respectively, and that apoE content in these particles is correlated with HDL2.