Apolipoprotein (apo) B plays an important role in plasma lipid transport and in the maintenance of cholesterol homeostasis. Attempts to determine the structure of apo B have been hampered by technical obstacles resulting from its chemical and physical properties. Recently monoclonal antibodies (Mabs) against human apo B have been used as probes to study apo B structure and heterogeneity. Certain Mabs are capable of blocking binding of low density lipoprotein (LDL) apo B to the cell surface LDL receptor, which presumably reflects the proximity of their antigenic determinants to the receptor recognition domain. The distribution of antigenic determinants recognized by Mabs has been studied on the hepatic (apo B-100) and intestinal (apo B-48) forms of apo B and on fragments generated by limited proteolysis of apo B. Some Mabs are specific for apo B-100, whereas others cross-react with apo B-48. Apo B-100 specific Mabs coupled to Sepharose have been used to isolate separately apo-B-containing lipoproteins of intestinal and hepatic origin and their respective lipid and apolipoprotein compositions have been determined. Using the separated fractions it has been shown that apo B-100, but not apo B-48, can react with the LDL receptor. Most Mabs failed to react with apo B which had been delipidated and resolubilized, but in some cases immunoreactivity could be recovered if the solubilized apo B were reincorporated into lipid vesicles. These experiments showed that different determinants had different lipid requirements for their expression. Within an individual there is immunochemical heterogeneity in apo-B-containing lipoproteins in the expression of apo B antigenic determinants which can be detected by Mabs. Intersubject differences in reactivity of lipoprotein subfractions with Mabs have also been observed and in some cases appear to represent genetic polymorphism of apo B.
Distribution of Lipoprotein (a) in Plasma Lipoprotein Subfractions
