| Summary: | The fate of 125I-labeled apolipoproteins was studied in vivo in rats that had received intravenous injections of 125I-labeled rat HDL and 125I-labeled human HDL, LDL, and VLDL.Plasma decay curves of rat and human HDL were exponential with similar half-lives in the circulation (11–12 hr). After injection, low molecular weight apolipoproteins (apoLP-alanine of human HDL and fraction HS-3 of rat HDL) were found to redistribute to other lipoproteins, predominantly VLDL. Decay curves of individual HDL proteins were constructed after lipoprotein fractionation, delipidation, and polyacrylamide gel electrophoresis. It was found that the half-lives of the different HDL apoproteins were not identical. A major rat HDL protein (52% of total counts) had a circulating half-life (t½) of 12.5 hr. Two others had a t½ of 8–9 hr while the t½ of several others was 11–12 hr. The t½ of three well-characterized human HDL apoproteins, apoLP-glutamine I, apoLP-glutamine II, and apoLP-alanine, were 13.5, 9.0, and 15.0 hr, respectively.The fate of 125I-labeled human VLDL and LDL apoproteins in rats was similar to that described previously in humans. After injection of 125I-labeled human VLDL into rats, apoLP-glutamic acid and apoLP-alanine rapidly transferred to rat HDL and were lost thereafter from the circulation from both VLDL and HDL. The apoLDL moiety of human VLDL moved metabolically to the LDL density range (d = 1.019–1.063) through a lipoprotein of intermediate density (d = 1.006–1.019).Analysis of tissue radioactivity at time intervals after the injection of labeled lipoproteins suggests that the liver may play an important role in the clearance of all lipoprotein preparations from plasma.
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