A new model of human VLDL metabolism based on simultaneous studies of its APOB and triglyceride

Previous studies which separately examined the behavior of the B apolipoprotein and of the triglyceride of VLDL had raised the possibility that each was metabolized in a different manner. The present studies have explored this possibility by simultaneously studying each of these VLDL constituents in thirteen humans. 125I-labeled Sf 60-400 lipoproteins were injected. The fate of their apo B (a reflection of the metabolism of the lipoprotein particle) was followed in this, and in the smaller Sf 12-60 lipoproteins. In everyone, the larger particle was catabolized to the smaller one. Furthermore, the smaller particle was derived exclusively from the larger one. The behavior of the lipoproteins' triglyceride was examined by following the specific activity of 3H-triglyceride which had been endogenously labelled by injecting 2-3H-glycerol. Some of the larger lipoproteins' triglyceride appeared in the smaller particles. However, in contrast to the apo B, the triglyceride in the smaller particle was not derived exclusively from that in the larger particle. In eleven subjects, the triglyceride specific activity curves above demonstrated that some of the triglyceride entered the smaller circulating particle from a source other than the large particle.