ABSTRACT

Newly synthesised chylomicrons and very low density lipoproteins (VLDL) have lipid cores composed of triglyceride with some cholesteryl ester which is surrounded by a monolayer of phospholipid, free cholesterol, and protein (reviewed by Havel and Kane, 1995). The primary protein component of hepatic VLDL is apolipoprotein B-100. This is a single polypeptide with a molecular weight of 512,000. Computer modeling has been used by Segrest et al. (1994) to help further elucidate the nature of apoB. The amino terminal 20% of apoB contains an alpha helix rich domain typical of globular proteins which is stabilized by seven disulfide bonds (Yang et al., 1990). The remainder of the protein contains two amphipathic beta sheet domains and two amphipathic alpha helix domains which impart the lipid binding properties to the protein. Newly made VLDL also contain small amounts of apoE and apoC proteins. The primary structural protein of intestinal chylomicrons is the 241,000 molecular weight protein, apoB48. ApoB-48 is a product of the same gene which encodes apoB-100 (For a recent review see Chan et al., 1997). In the intestine, the gene product is deaminated posttranscriptionally at cytidine 6666 of the mRNA to produce a uridine. This converts codon 2153 of the message to a stop codon. Some species, including mice and rats, are also capable of editing apoB mRNA in the liver. These species produce apoB-48 particles in the liver as well as the intestine. Newly synthesized chylomicrons also contain apoAI and apoAIV.