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Electrophoresis of rat apolipoprotein B (apoB) on 5% polyacrylamide gels in the presence of NaDodSO4 separates three major components: PI, which comigrates with human low density lipoprotein (LDL) apoB; PII, a slightly faster-moving satellite band; and PIII, which migrates somewhat more slowly than myosin heavy chain. The proportion of PIII decreases with increasing density of the parent rat lipoprotein, from 90% an 70%, respectively, in chylomicrons and very low density lipoproteins (VLDL), to 7% in the major LDL2 (density 1.038-1.063 g/ml) fraction. A major component that comigrates with rat PIII is a marker for human chylomicron apoB, being absent from human VLDL, intermediate density lipoprotein (IDL), and LDL. Preliminary immunological and peptide mapping data show that rat apoB PI and PIII are closely related structurally, with the latter possibly being a large fragment of the former. Both peptides are synthesized in rat liver and found in Golgi secretory vesicles. Kinetic tracer experiments show that rat PI and PIII are present on separate VLDL particles, both of which are extensively removed from the circulation at the remnant stage, and that the declining PIII-to-PI/II ratios in IDL and LDL may be attributed to the more rapid turnover of PIII-containing lipoproteins at all levels, particularly within the LDL density range.

Low density lipoproteins and the triglyceride-rich lipoproteins of human serum each contain proteins of high molecular weight termed apolipoprotein B, which have previously been thought to be identical. We have isolated four species of apolipoprotein B with unique molecular weights and amino acid compositions. We have assigned numerical designations to these species in a centile system based upon their relative apparent Mr in NaDodSO4. One which we term B-100, with an apparent Mr of 549,000 +/- 7650 (SD) determined by NaDodSO4 gel electrophoresis, predominates in low density and very low density lipoproteins and is also present in chylomicrons from thoracic duct lymph or from plasma. Substantial amounts of two large proteins designated B-74 (apparent Mr 407,000 +/- 5790) and B-26 (apparent Mr 144,500 +/- 8970), which appear to be complementary fragments or constituents of the B-100 protein, are found in the low density lipoproteins of many individuals. A distinct protein, B-48, with an apparent Mr of 264,000 +/- 8150 is a major and constant constituent of chylomicrons from thoracic duct lymph or from plasma.

A protein that binds to a lecithin-stabilized triglyceride emulsion has been separated from plasma after removal of major lipoprotein classes by ultracentrifugation at density 1.21 g/ml. This protein, rich in proline, has been purified to electrophoretic and immunochemical homogeneity by subsequent gel and ion-exchange chromatography. In native plasma and after purification, it exists as a large particle exceeding 10(6) daltons, but a single component with a molecular weight of about 74,000 is found upon polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Although the purified protein contains very little bound lipid and is not present in the major lipoprotein classes from post-absorptive individuals, it is present in chylomicrons. Its concentration in plasma varies from 12 to 41 mg/dl and is significantly correlated with that of cholesterol in lipoproteins of very low and low density but not in those of high denisty.