Incorporation of ⁷⁵Se-Selenomethionine into Human Apoproteins: II. Characterization of Metabolism of Very-low-density and Low-density Lipoproteins in Vivo and in Vitro

This investigation is designed to explore the potential role of apoVLDL as a precursor of a polypeptide component of human LDL. Attention was directed to the chromatography-defined Sf-I polypeptide fraction of apoVLDL, which has been previously shown to be immunologically and chemically indistinguishable from the major component of apoLDL^1–3. In VLDL isolated from blood drawn within two hours following ⁷⁵Se-SM injection, the Sf-I polypeptide fraction of apoVLDL was highly enriched with isotope, providing an appropriate preparation for in-vitro tracer studies. Conversion of ⁷⁵Se-VLDL to ⁷⁵Se-LDL occurred in vitro in the presence of normal plasma at 37° C., and this conversion was augmented by post-heparin plasma. No conversion to HDL lipoproteins could be detected. Injection of heparin in vivo resulted in acute reciprocal changes in the radioactivity contained within serum apoVLDL and apoLDL. These findings suggest that a component of the Sf-I polypeptide fraction of apoVLDL can be metabolized into the apoprotein of LDL in man. Thus, the biochemical and immunologic similarities between the Sf-I fractions of apoVLDL and apoLDL may result from a physiologic “precursor-product” relationship between the apoprotein moieties of these two lipoprotein species. A method for further investigation of the metabolism of human apoprotein is suggested.