Qualitative Studies on the Interconversion of High Density Lipoproteins Facilitated by the Plasma Lipid Transfer Complex
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Human lipoprotein-free plasma contains proteins which facilitate the movement of cholesteryl ester, phospholipid, and triglyceride between the various lipoprotein particles. During lipolysis of very low density lipoproteins (VLDL), lipid and protein constituents transfer to HDL. The purpose of the present investigation was to assess the role, if any, of the lipid transfer complex (LTC) in this transfer process. The LTC was isolated from lipoprotein-free plasma by column chromatography on phenyl-Sepharose and carboxymethyl cellulose. VLDL-remnants were prepared by the action of . bovine milk lipoprotein lipase (LPL). HDL subfractions were isolated by zonalultracentrifugation. The interconversions of the HDL subfractions were monitored by gradient gel electrophoresis. Incubation of HDL4 with VLDL-remnants in the presence of LTC caused a breakdown of HDL4 into smaller and larger HDL3-like particles. This breakdown was dependent on the presence of the LTC and did not occur with VLDL substituted for the VLDL remnants. Plasma HDL2 and HDL3 fractions were not altered by incubation with LTC and VLDL remnants. Preincubation of LTC with 2mM p-chloromercuriphenyl sulfonate (PCMPS) , an inhibitor of the triglyceride transfer protein, prevented the breakdown of HDL4. These in vitro studies suggest that the lipid transfer complex, and possibly the triglyceride transfer protein, play a role in the reorganization of HDL during the lipolysis of VLDL.