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Journal of Lipid Research, Vol. 14, 357-363, May 1973
Copyright © 1973 by Lipid Research, Inc.

Comparison of the metabolic behavior in vitro of the apoproteins of rat serum high density lipoprotein₂ and high density lipoprotein₃

Bernard Rubenstein and David Rubinstein

Department of Biochemistry, McGill University, Montreal, Canada

Rat serum high density lipoproteins were divided into two fractions, HDL₂ (d 1.063-1.12) and HDL₃ (d 1.12-1.21). These fractions were compared on the basis of (a) the pattern of the apolipoprotein peptides obtained on polyacrylamide gel electrophoresis in 7 m urea, (b) the exchange of some of the peptides with those in very low density lipoproteins (VLDL), and (c) the incorporation by perfused rat liver of [³H]leucine into the peptides of the HDL₂ and HDL₃ secreted into the perfusate. Among the peptide bands of HDL₃, one is absent and another present only in trace amounts in HDL₂. After electrophoresis on polyacrylamide gel for 24 hr, a major peptide band of HDL₂ is split into three distinct areas, whereas it remains as a single area in HDL₃. Both HDL₂ and HDL₃ exchange prelabeled protein with VLDL. However, the exchange is much more limited in HDL₃, even though it contains most of the protein found in circulating rat HDL. Analysis of the individual peptides, separated by polyacrylamide gel electrophoresis after incubation with VLDL, reveals that in HDL₃ the exchange is limited to two peptides, whereas a third, although present in both subfractions of rat HDL, exchanges only when found in HDL₂. This peptide represents most of the exchange with VLDL. Perfused rat liver incorporates [³H]leucine into HDL of the perfusate, primarily into HDL₂. Most of the radioactivity is found in those peptides that do not take part in the exchange with VLDL. These data lead to the conclusions that there are functional and structural differences between HDL₂ and HDL₃ and that some of the peptides of HDL may be derived from exchange with, and breakdown of, VLDL. Others are secreted, at least in part, directly into the circulation by the liver.

Supplementary key words very low density lipoproteins • apolipoproteins • perfused liver

Submitted on September 14, 1972
Accepted on January 22, 1973

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