QUANTITATION OF THE TRANSFER OF SURFACE PHOSPHOLIPID OF CHYLOMICRONS TO THE HIGH-DENSITY LIPOPROTEIN FRACTION DURING THE CATABOLISM OF CHYLOMICRONS IN THE RAT

Small chylomicrons (CM) labeled with cholesterol, cholesterol ester, phopholipid, and, in some cases, protein, were used to study the fate of these constituents as the CM are catabolized in the circulations of the hepatectomized and intact rat. In the hepatectomized animal after 1/2 h, CM are greatly reduced in volume, surface area, and diameter. During this period, the CM lost >92% of the mass of their triacylglycerol, >77% of the mass of their phospholipid, and >39% of their protein. Compared to the injected CM, the chemically altered particles, called CM 'remnants', have a reduction in volume of 96% and in surface area of 88%. The labeled cholesterol esters remain with the Cm remnants but, strikingly, a major fraction of the labeled phospholipids and labeled soluble apoproteins leave the CM and are found in the high density lipoprotein (HDL) fraction. The chemical composition of this HDL fraction contains relatively more phospholipid and less cholesterol ester than normal rat HDL. Because of the difference in composition of HDL between normal rats and those given CM, we estimate that the HDL phospholipid pool increased by ≡25% by the infusion of ≡4-5 mg of CM phopholipid. Approximately 5 mg of phospholipid is secreted on CM by a fed rat in 1 hr. These findings in the hepatomized rats indicate that a major fraction of the phospholipid and a minor fraction of the protein (soluble non-B apoproteins) of newly secreted CM are transferred from the CM to the HDL fraction during remnant formation. The same process probably occurs in intact rats except that the remnant particles are rapidly removed from the plasma by the liver and a smaller fractions of the surface of the CM enters the HDL fraction.