Evidence that two synthetic pathways contribute to the apolipoprotein B pool of the low density lipoprotein fraction of rabbit plasma.

The plasma specific activity of apolipoprotein B was determined in normal rabbit injected with homologous radioiodinated very low and intermediate density lipoproteins (VLDL and IDL). The specific activity was followed in the same density range of the injected lipoproteins and in two low density lipoprotein subfractions (LDL1 and LDL2). VLDL B (d less than 1.006 g/ml) is cleared from plasma mainly in IDL (d = 1.006-1.019 g/ml), with minor radioactivity recovery in LDL1 (d = 1.019-1.040 g/ml) and LDL2 (d = 1.040-1.063 g/ml) fractions. The main catabolic product of IDL B is, on the other hand, LDL1 B. LDL2 B, which represent more than 20% of the whole LDL B plasma pool in this animal species, is not derived to a significant extent from either VLDL or IDL and, conceivably, is synthesized independently.