Apolipoprotein B: structural and metabolic heterogeneity.

Abstract

The large number of protein species present in the lipoprotein complexes in blood suggests a number of biochemical roles of proteins in lipoprotein metabolism and perhaps also in nontransport functions. One group of apolipoproteins, the B proteins, clearly participates in organizing the struc­ ture of lipoprotein complexes. At least one of these proteins (B-l00) also interacts with high-affini ty receptors that function in the endocytosis of lipoproteins (6, 16). Unlike many of the smaller apolipoproteins, there is no measureable concentration of soluble B protein in the aqueous medium in equilibrium with B protein bound to lipoproteins. Also, the B proteins do not appear to transfer between particles but maintain organized lipo­ protein complexes throughout successive stages of metabolism until the end products are removed from plasma. In all, their behavior resembles that of the intrinsic proteins of cell membranes. Because of transfer and exchange of lipids and non-B apolipoproteins, the B proteins may be the only constit­ uents that remain fixed throughout the metabolism of a given particle. All the B proteins share the property of insolubility in water after delipidation by organic solvents and are rapidly precipitated by 4. 2 M tetramethylurea, a solvent that solubilizes the smaller apolipoproteins (23). In humans there appear to be two original species of apo B and two principal pathways of metabolism. The species with the larger molecular weight is secreted by liver into the space of Disse in nascent particles of very low density lipoproteins (VLDL). Metabolism of these particles mediated by the lipoprotein lipase system yields a succession of smaller particles poorer in triglycerides. The end products of this process, VLDL remnants,