[Intestinal apolipoprotein metabolism]

Intestinal apolipoproteins play a major role in lipoprotein metabolism. Apolipoproteins (Apo) AI, AIV and B48 are synthesized with chylomicrons in the mucosa and secreted into intestinal lymph. Apo B48 and B100 are coded for by the same gene on chromosome 2 and are identical in their aminoterminus. While apo B100 mRNA can be translated to the 4536 amino acid protein, the apo B48 mRNA is edited to contain a stopcodon at amino acid 2152. Apo B48 seems to be important for the secretion of chylomicrons and stays with chylomicrons during their lipolysis in plasma. Hydrolysis only takes place after the particle has acquired apo CII, the obligate cofactor for the enzyme lipoprotein lipase. The ligand for final irreversible uptake into the liver, mediated by a putative chylomicron-remnant receptor, seems to be apo E which only associates during lipolysis in plasma. Although apo E is not synthesized to a significant extent in the gut it was reported to influence intestinal cholesterol absorption: carriers of the genetic apo E2 isoform seem to absorb cholesterol to a lesser extent than those with apo E4 as compared to apo E3. Despite the almost exclusive synthesis of apo AIV and the significant extent of apo AI synthesis in the human gut these two apoproteins do not seem to have a chylomicron specific function. They seem to be more involved in the "reversed cholesterol transport", believed to be antiatherogenic. After entering the plasma compartment apo AI and AIV leave the chylomicrons during lipolysis as "surface remnants" to become high density- (HDL)-and apo AIV containing lipoproteins.(ABSTRACT TRUNCATED AT 250 WORDS)