Catabolism of Very Low Density Lipoprotein B Apoprotein in Man

Michael F. Reardon; Noel H. Fidge; Paul J. Nestel

doi:10.1172/JCI108999

Catabolism of Very Low Density Lipoprotein B Apoprotein in Man

Michael F. Reardon, Noel H. Fidge, and Paul J. Nestel

Published March 1, 1978 - More info

View PDF

Abstract

The turnover and the catabolic fate of the B apoprotein of very low density lipoprotein (VLDL-B) was studied in 15 normal and hyperlipidemic subjects using reinjected autologous VLDL labeled with radioiodine. The specific radioactivity-time curve of the B apoprotein in total VLDL (S_f20-400) was multiexponential but conformed to a two-pool model during the first 48 h of catabolism. The flux was highest in several hypertriglyceridemic subjects. The mass of pool A exceeded the intravascular content of VLDL-B by 30% on average, indicating extravascular metabolism of VLDL. The two-pool model might reflect the input of several populations of particles or heterogeneity of catabolic processes or pools. The flux of B apoprotein was also measured in several subclasses of VLDL, in smaller intermediate density lipoproteins, and in low density lipoproteins (LDL). In three subjects the flux was similar in S_f 60-400 and in S_f 12-60 lipoproteins, suggesting that VLDL was catabolized at least to a particle in the density range S_f 12-60. Subsequent catabolism appeared to proceed by two pathways: in normotriglyceridemic subjects, B apoprotein flux in the S_f 20-400 and in S_f 12-20 lipoproteins was similar, whereas in hypertriglyceridemic subjects flux through S_f 12-20 accounted for only part of the VLDL-B flux.

The flux of low density lipoprotein B apoprotein (LDL-B), which is believed to be derived from VLDL catabolism, was calculated from the area between the specific activity time curves of VLDL-B and LDL-B. In subjects with normal plasma triglyceride concentration, LDL-B flux was from 91% to 113% of that of VLDL-B; but in three hypertriglyceridemic subjects showing high rates of VLDL-B transport, LDL-B flux was only one-third that of VLDL-B. This suggests that when VLDL-B flux is high, VLDL is substantially catabolized by a route other than through LDL and possibly leaves the circulation as a particle in the S_f 20-60 density range.