HOMOGENEOUS ASSAY FOR MEASURING LOW-DENSITY-LIPOPROTEIN CHOLESTEROL IN SERUM WITH TRIBLOCK COPOLYMER AND ALPHA-CYCLODEXTRIN SULFATE

We have developed a fully automated method for measuring LDL-cholester ol (LDL-C) in human serum without the need for prior separation, using a nonionic surfactant, polyoxyethylene-polyoxypropylene block copolye ther (POE-POP), and a sodium salt of sulfated cyclic maltohexaose, alp ha-cyclodextrin sulfate. Of the surfactants tested, POE-POP with a hig her molecular mass of the POP block and a greater hydrophobicity reduc ed the reactivity of cholesterol in lipoprotein fractions; the reactiv ity in descending order was LDL much greater than VLDL > chylomicron a pproximate to HDL. Gel filtration chromatographic studies revealed tha t POE-POP removed lipids selectively from the LDL fraction and allowed them to participate in the cholesterol esterase-cholesterol oxidase c oupling reaction system. By contrast, alpha-cyclodextrin sulfate reduc ed the reactivity of cholesterol, especially in chylomicrons and VLDL. A combination of POE-POP with alpha-cyclodextrin sulfate provided the required selectivity for the determination of LDL-C in serum in the p resence of magnesium ions and a small amount of dextran sulfate withou t precipitating lipoprotein aggregates. There was a good correlation b etween the results of LDL-C assayed by the proposed method and the bet a-quantification reference method involving 161 sera with triglyceride concentrations ranging from 0.3 to 22.6 mmol/L.