Measurement of apolipoprotein B radioactivity in whole blood plasma by precipitation with isopropanol.

A method to measure apolipoprotein B radioactivity in whole blood plasma is described that is suitable for routine use in kinetic experiments in vivo. Radiolabeled apolipoprotein B is precipitated from plasma diluted 15- to 30-fold in the presence of carrier low density lipoproteins by 50% isopropanol. The amount of radioiodine in apoB is estimated from the difference between total radioiodine concentration in whole plasma and the fraction soluble in 50% isopropanol. Addition of up to 100 microliters of plasma to radioiodinated lipoproteins did not alter the percent of radioiodine precipitated in 1500 microliters of 50% isopropanol. The percent of radioiodine precipitated by isopropanol 3 min after intravenous injection of homologous radioiodinated very low density lipoproteins, intermediate density lipoproteins, and low density lipoproteins into rabbits was almost identical to that in the injected lipoproteins (y = 1.009 X +/- 0.462; r = 0.997).