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 apo VLDL as a precursor of a polypeptide component of human LDL. Attention was directed to the chromatography-defined Sf-I polypeptide fraction of apo VLDL, which has been previously shown to be immunologically and chemically indistinguishable from the major component of apoLDL.1-3 In VLDL isolated from bloow drawn within two hours following 75Se-SM injection, the Sf-I polypeptide fraction of apo VLDL 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 degrees 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 apo VLDL and apoLDL. These findings suggest that a component of the Sf-I polypeptide fraction of apo VLDL 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.