Direct measurement of apoprotein B specific activity in ¹²⁵I-labeled lipoproteins

A method for determining apoprotein B specific activity in radioiodinated lipoproteins is described and validated. It utilizes organic solvents and tetramethylurea in the isolation of apoprotein B from other radiolabeled contaminants, both lipid and protein, in exogenously labeled VLDL. The contaminants are also removed from those lipoprotein classes subsequently derived from VLDL, namely IDL and LDL. The procedure requires approximately 50 μg of apoB per analysis, allowing specific activity determinations in triplicate on 3-ml plasma samples with a standard error of <6%. Finally, data from a study of apoprotein B turnover in VLDL, IDL, and LDL in a human subject is presented to demonstrate the potential of this method in further elucidating the kinetic interrelationships between these lipoprotein classes.