Metabolism of Very Low Density Lipoprotein of Sf 100-400 in Type V Hyperlipoproteinaemia

The metabolism of the apolipoproteins of very low density lipoprotein (VLDL) was studied in three subjects with primary type V hyperlipoproteinaemia (HLP) after injection of autologous 125I-labelled VLDL of Sf 100–400. The fractional disappearance rate of total radioactivity was stable during the two-week period of study. Transfer of apolipoprotein C (apo-C) from VLDL to high density lipoprotein (HDL) occurred both in vivo and in vitro upon mixing the labelled VLDL preparation with serum and significant amounts of 125I-labelled apoli-poprotein B (apo-B) and apo-C were recovered in the chylomicron fraction (Sf >400) in both situations. The time-courses of the disappearance of specific activity from VLDL (Sf 20–400) and its appearance in low density lipoprotein (LDL) are consistent with apo-B in VLDL (VLDL-B) being the precursor of that in LDL. In comparison to studies in which the whole VLDL class (Sf 20–400) was labelled, the rate of synthesis of VLDL-B was slightly decreased and the fractional rates of catabolism (FCRs) of apo-B in VLDL and in the whole spectrum of VLDL+chylomicrons were markedly depressed. In turn, the rate of formation of LDL was low, and its FCR was probably slightly decreased. The findings are consistent with the belief that in these three subjects the type V HLP is to a great extent due to impaired rates of formation and breakdown of apo-B in VLDL. They do not exclude the possibility that the disorder is also caused by an increased rate of formation of chylomicrons.