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A B S T R A C T We have developed a double antibody radioimmunoassay (RIA) for human apolipoprotein B (ApoB). The assay measures not only the ApoB content of 8-lipoproteins (low density lipoproteins [LDL]) but also that contained in the other lipoproteins in plasma.Purified lymph and plasma chylomicrons and plasma very low density lipoproteins (VLDL) produced displacement curves in the assay system which paralleled those produced by pure LDL. Thus, the ApoB found in chylomicrons, VLDL, and LDL were immunologically identical. ApoB accounted for about 25 and 35%, respectively, of the total protein of chylomicrons and VLDL by RIA. VLDL and LDL preparations from normal and hyperlipoproteinemic subjects also produced parallel displacement curves, suggesting that the ApoB of normal and hyperlipoproteinemic subjects were immunologically identical. High density lipoproteins and abetalipoproteinemic plasma displaced no counts, nor did the sera of several animal species produce any useful displacement curves in this system.The fasting total plasma ApoB concentration of normal subjects was 83±16 mg/dl (mean+SD). ApoB levels were high in Type II (162±16), and less so in Type IV (112±24) and Type V (105±17). When plasma ApoB concentration in Type IV patients was graphed against plasma glycerides, two subpopulations, which may represent different genetic or biochemical subgroups, were apparent. ApoB concentration in individuals on constant diet and drug regimen was stable over weeks to months. Greater than 90% of ApoB of normal and Type II subjects was in the d> 1.006 plasma fraction. By contrast, only 50-80% of ApoB was in the d > 1.006 fraction in Types IV and V. Thus, hypertriglyceridemia was associated primarily with a redistribution of ApoB to the lighter density fractions; by contrast, in hypercholesterolemia absolute ApoB concentration was markedly increased.

The major apoprotein of high density lipoprotein is apolipoprotein A-I (ApoA-I). In addition to being a structural component of this class of lipoproteins, ApoA-I also has a physiologic role as an activator of lecithin-cholesterol acyl transferase, an enzyme important in the metabolism of all lipoproteins.To measure ApoA-I content in human plasma, to assess its immunologic activity in hyperlipoproteinemia, and to carry out certain structural studies of high density lipoproteins, we have developed a double antibody radioimmunoassay. ApoA-I, isolated by gel filtration, was used to produce monospecific antisera.

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A B S T R A C T Smaller very low density lipoprotein (VLDL) remnants interact more readily with tissues than do larger "intact" VLDL. This may be related to changes in the availability of VLDL apoproteins on the surface of the lipoproteins. To test this hypothesis VLDL were incubated at 37°C with bovine milk lipase (LPL), and the abilities of LPL-treated VLDL preparations to compete with 1251-low density lipoproteins (LDL) for interaction with cultured normal human fibroblasts were measured. At the same time, the immunologic activities of these preparations were also tested by double antibody radioimmunoassay. Triglyceride (TG) contents of VLDL fell by 30-90% during incubation with LPL and, on zonal ultracentrifugation, VLDL of faster Svedberg unit of flotation (Sfl.063) rates (>150) were gradually converted to smaller VLDL with lower Sf rates (21-60).LPL-treated VLDL competed two to five times more effectively with 1251-LDL for binding to cellular receptors than did control VLDL. Control VLDL incubated with heat-inactivated LPL at 37°C, or with active LPL at 4°C had unaltered cell reactivities and TG contents compared with VLDL incubated without any enzyme. The direct uptake and degradation of LPLtreated VLDL was also assessed by using VLDL 125I1 labeled in apoprotein (Apo)B. LPL-treated VLDL-1251-ApoB were taken up and degraded by fibroblast at greater rates than were control VLDL-'25I-ApoB. Thus, hydrolysis of VLDL lipids was accompanied by an increased ability of VLDL to interact with fibroblasts. The immunoreactivity of ApoB in the same VLDL preparations, expressed as the "apparent ApoB contents" of LPL-treated VLDL, increased by 10-50% (P <0.02) in those assays that contained anti-LDL antisera, but the ApoB of control VLDL remained constant. However, assays that contained antisera directed against ApoB isolated from VLDL did not distinguish Received for publication 7 August 1978 and in revised form 9 July 1979. 1288 between LPL-treated and control VLDL. Thus, VLDL lipid hydrolysis was accompanied by changes in the immunoreactivity of VLDL-ApoB, which probably reflect changes in the disposition of ApoB on the surface of VLDL. The altered disposition of ApoB on VLDL "remnants" may be related to their enhanced interaction with cells.