Improved Method for Enzymic Determination of Cholesterol in Lipoproteins Separated by Electrophoresis on Thin Layer Agarose Gels

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Abstract
The cholesterol of lipoproteins, separated electrophoretically on thin layer agarose films, is visualised and quantitated by incubating the gels in an enzymic reagent containing cholesterol esterase and cholesterol dehydrogenase. The individual fractions are quantitated by scanning densitometry. No sample pretreatment is necessary. All major fractions are detected readily. The accuracy of the determination is similar to that of ultracentrifugation. On average, imprecision is 3.1% for .beta.-, 7.0% for pre.beta.-, and 4.8% for .alpha.-lipoprotein cholesterol. Concentration and colour development are linear up to 8 mmol/l cholesterol in a given lipoprotein fraction. The results from the direct enzymic procedure for .beta.-, pre.beta.- and .alpha.-lipoprotein cholesterol are compared with those from quantitative lipoprotein electrophoresis after precipitation with phosphotungstic acid and bivalent cations and with those from different precipitation methods using dextran sulphate and polyethylene glycol. The new method has several advantages: high specificity; lack of dependence on the actual composition of the lipoproteins; lack of interference from coprecipitated proteins in the gel, e.g. fibrinogen or paraproteins; and insensitivity to lipolysis and high free fatty acid concentrations caused by heparin application or ageing of the specimen (at least for .alpha.-lipoprotein cholesterol quantitation). In its convenience and simplicity of operation, and the simple calculation of results, the method is similar to standard protein electrophoresis. The proposed method is therefore suggested as a standard method for elucidating lipoprotein disorders.