Abstract
Rat and human very low density lipoproteins (VLDL) were fractionated by zonal ultracentrifugation, yielding sharply defined fractions with narrow sedimentation limits. Sedimentation coefficients for the individual fractions were determined at two densities with the analytical ultracentrifuge, and the results were analyzed to yield buoyant densities and molecular weights for the particles in each fraction. For the rat lipoproteins, the weight concentrations of triglycerides, cholesterol, phospholipid, and protein were determined for each fraction, and their molar concentrations of apolipoprotein B were measured with a radioimmunoassay. For the human lipoproteins the corresponding values were taken from Patsch et al. (Patsch, W., J. R. Patsch, G. M. Kostner, S. Sailer, and H. Braunsteiner. 1978. Isolation of subfractions of human very low density lipoproteins by zonal ultracentrifugation. J. Biol. Chem. 253:4911-4915). From these data, a ratio of the number of apoB peptides to the number of lipoprotein particles was calculated for each fraction. This ratio was close to 1 for all VLDL fractions, ranging in particle diameter from about 40 to 80 mm and 30 to 50 mm, respectively, for rat and human VLDL. The majority rat VLDL contain B-48 rather than B-100 as their (single) apoB peptide. Based on these data, we proposed that only a single copy of B-48 is required for VLDL assembly in rat liver, unless nascent hepatic VLDL contain additional apoB peptides which are uniformly lost from the plasma VLDL particles when they are analyzed.
Highlights
Rat and human very low density lipoproteins (VLDL) were fractionated by zonal ultracentrifugation, yielding sharply defined fractions with narrow sedimentation limits
B-48 contains the first 2152, or roughly one half, of the 4536 amino acids in B-100. This raises the additional question of whether assembly of a TG-rich lipoprotein requires apoB on a mass or molar basis: that is, do B-48-conraining particles carry one or two B-48 peptides? We have examined these questions in regard to rat and human VLDL
The results presented here show that both rat and human plasma VLDL contain an average of one apoB peptide per particle
Summary
Rat and human very low density lipoproteins (VLDL) were fractionated by zonal ultracentrifugation, yielding sharply defined fractions with narrow sedimentation limits. A ratio of the number of apoB peptides to the number of lipoprotein particles was calculated for each fraction This ratio was close to 1 for all VLDL fractions, ranging in particle diameter from about 40 to 80 mm and 30 to 50 mm, respectively, for rat and human VLDL. The majority rat VLDL contain B-48 rather than B-100 as their (siagle) apoB peptide.819. A. Supplementary key words zonal ultracentrifugation buoyant densities radioimmunoassay apoB-48 assembly of VLDL particles secretion are unknown, it has long been known that human plasma LDL contain about 500 kDa apoB per particle, originallyassumed to represent two copies of a 250 kDa peptide [3]. The peptide molecular mass was estimated to be close to 400 kDa [4], prompting the proposal that the LDL particle contains one copy, rather than two, of apoB. ‘To whom correspondence and reprint requests should be addressed at: VA Wadsworth Medical Center, Lipid Research Laboratory, Bldg. 113, Rm. 312, Wilshire and Sawtelle Blvds., Los Angeles, CA 90073
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