Measurement of serum apolipoprotein B by radioimmunoassay

A sensitive and specific double antibody radioimmunoassay for the major apolipoprotein (apo B) of human serum very low density lipoprotein (VLDL) and low density lipoprotein (LDL) is described. Using anti‐LDL and anti‐apo B antibodies the immunoreactivity of LDL and apo B were compared. Human LDL and its isolated apo B were not immunologically identical when each antiserum was used with its homologous label; a population of antibodies was selected which reacted with antigenic sites unique to the antigen itself as well as to those which were common to the closely related protein. When the heterologous label was used with either antiserum, a population of antibodies directed against antigenic sites shared by the LDL and apo B molecules was selected. Apo B in sera samples can be measured using either anti‐LDL or anti‐apo B antibodies provided that intact LDL was used for preparation of the iodinated tracer and standard. Serum apo B levels in healthy normolipi‐daemic males and females were 0.93 ±0.25 g/l (range 0.58–1.39) and 0.90 ± 0.15 g/l (range 0.58–1.12), respectively. The total cholesterol and apo B, and phos‐pholipid and apo B concentrations for both males and females were significantly correlated (P<0.05). In another normolipidaemic population ( n = 52), total serum apo B values correlated positively with LDL cholesterol ( r= 0.92, P< 0.001). Apo B was measured in sera from patients with abetalipoproteinaemia, familial hypercholesterolaemia and Tangiers disease. Apo B was not detected in the serum of subjects with abetalipoproteinaemia, while the apo B level in the familial hypercholesterolaemic subjects was significantly elevated (range 3.26–4.94 g/l) compared to normals (P<0.001). Serum apo B (0.80 g/l) of the subject with Tangier disease was within the normal range.