ABSTRACT
Since that time, evidence has progressively accumulated for a close association between serum apo B and premature coronary atheroma (see Chapters 5 and 12). Until recently, the structure and properties of the apo B molecule remained a mystery because its enormous size, insolubility even when only partially dilipidated and tendency to aggregate make it resistant to many biochemical techniques. Now, as a result of advances in immunochemistry, the study of specific proteolytic fragments and molecular genetic techniques, fascinating details of its biochemistry have emerged. The apo B gene is situated on chromosome 2. Its messenger ribonucleic acid (mRNA) contains 14 121 nucleotides and is thus the largest mRNA known. It encodes a 4563 amino acid protein, the N-terminal 27 amino acids of which are cleaved resulting in a 4536 amino acid native apo B100. The 27 residue terminal portion is hydrophobic and large enough to span a biological membrane. It may thus be important in the membrane transport and anchoring of apo B during the synthesis and secretion of the apo B-containing lipoproteins. Apolipoprotein B is synthesized in the rough endoplasmic reticulum, and triglycerides and phospholipids are synthesized in the smooth endoplasmic reticulum, becoming bound to the apo B before its appearance in the Golgi complex. There,
carbohydrate is acquired before secretion of the nascent VLDL. N-linked oligosaccharides comprise 8-10% of the mass of apo B.
