Metabolism of apolipoprotein A-I of chylomicrons in rats and humans

After intravenous injection of a large amount of small chylomicrons into intact rats, the concentration of apolipoprotein (apo) A-I was increased by about 40% and remained at this elevated level as most of the chylomicron triglycerides were removed from plasma during the ensuing hour. This apo A-I rapidly left the chylomicrons and was transferred to lipoproteins of higher density. Such transfer of apo A-I did not occur when chylomicrons were incubated at comparably high concentrations with rat serum. In normolipidemic humans, the concentration of apo A-I and apo A-II, as well as phospholipids, increased in the light subfraction of high density lipoproteins (HDL2) 4 to 7 h after ingestion of a meal containing 1.5 g cream fat per kilogram body weight. The concentration of these components increased in the heavy subfraction of high density lipoproteins (HDL3) after 12 to 24 h. The concentration of apo E in plasma was unaffected by fat ingestion, but the concentration of this protein increased in lipoproteins of density less than 1.006 g∙mL−1 and fell in lipoproteins of higher density. It is concluded that apo A-I in rat chylomicrons is transferred quantitatively to HDL as chylomicron remnants are formed. Chylomicron apo A-I and apo A-II appear to be transferred similarly to HDL in humans, whereas apo E is transferred from HDL to chylomicrons after chylomicrons enter the blood.