Interrelationship between rat serum very low density and high density lipoproteins.

An exchange of phospholipids and certain peptides among various classes of rat serum lipoproteins has been demonstrated and its nature has been investigated. [(32)P]-Phospholipid from isolated VLDL, prepared in vivo, was transferred to HDL, and to a much lesser extent to LDL, in vivo and in vitro. This difference between HDL and LDL can be abolished by ultracentrifugation of the serum at d 1.21. Unlabeled VLDL acquired [(32)P]phospholipid from HDL of serum. Phospholipid associated with the alpha-lipoprotein component of VLDL exchanged more readily than that associated with the beta-lipoprotein component of VLDL. Generally, the phospholipid species exchange in proportion to their distribution in the lipoproteins. Radioactivity from (3)H-labeled protein of VLDL was transferred to HDL while HDL (3)H-labeled protein in serum was transferred to VLDL during a 20-min incubation. LDL was not involved in the transfer of protein. Protein associated with the alpha-lipoprotein component of VLDL exchanged more readily than that associated with beta-lipoprotein component. Analysis of tritiated apoproteins of VLDL and HDL by polyacrylamide gel electrophoresis revealed that three of the six peptide bands of apo-VLDL exchanged between VLDL and HDL. The data raise the possibility that intact subunits of the VLDL and HDL are being exchanged.