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Studies on the metabolism of the polysaccharide hyaluronan has previously been hampered by the lack of radioactive hyaluronan of high molecular weight (MW) and high specific activity. In the present study 125I-tyrosine (T)-labelled hyaluronan was produced after CNBr-activation of the polysaccharide. A specific activity of approximately 0.1 MBq microgram-1 was achieved using 100 micrograms of 0.5 x 10(6) Da hyaluronan labelled for 2 h with 18 MBq 125I. The 125I-T-hyaluronan kept a high MW-profile upon gel filtration chromatography and was found to be cleared from the circulation with the kinetics and organ distribution reported for biosynthetically labelled hyaluronan of high MW. The 125I-labelled polysaccharide is also taken up by liver endothelial cells both in vivo and in vitro, indicating that the labelling does not interfere with the binding to specific cell-surface receptors found on these cells. The intracellular degradation is slower than that earlier reported for biosynthetically labelled hyaluronan and seems to be halted at the level of low MW oligo- or mono-saccharides that eventually leave the organism via the urine. Scintigraphic images of rats after intravenous injection of 125I-T-hyaluronan showed rapid uptake in the liver and a redistribution of radioactivity from liver to urine with time. Our results indicate that the 125I-T-hyaluronan is suitable for studies of hyaluronan-metabolism in a number of ways. The gamma emitters 125I and 131I are easy to monitor and can be used also for in vivo 3D-imaging using single photon emission computer tomography.

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Because macrophages secrete lipoprotein lipase (LPL), we sought to determine if LPL activity would influence the metabolism of chylomicrons by macrophages. In initial studies, we showed that normal chylomicrons were a substrate for the macrophage's LPL activity. Uptake of normal chylomicrons occurred in a saturable fashion and was effectively competed for by human chylomicrons, very low density lipoproteins (VLDL), and rabbit beta-VLDL, but not by acetyl-low density lipoprotein (LDL), and only modestly by native LDL.When apoprotein C-ll-deficient chylomicrons were incubated with macrophages, no hydrolysis of triglyceride occurred, yet saturable uptake of chylomicron protein and lipid occurred, demonstrating that LPL activity is not a prerequisite for saturable uptake. However, addition of apo C-ll led to marked hydrolysis and enhanced uptake of protein and lipid moieties. When albumin was present in the medium, there was approximately equal enhancement of cellular content of triglyceride and cholesteryl ester, despite the fact that chylomicrons are triglyceride-rich. This was due to uptake of a triglyceride-depleted particle produced by LPL, as well as a preferential re-release of triglyceride. These studies suggest potential pathways by which triglyceride-rich lipoproteins could contribute to accumulation of cholesteryl esters in macrophages, even while only small amounts of triglyceride accumulate. (Arteriosclerosis 3:433-440, September/October 1983) C onsiderable attention has been focused on the potential pathogenetic role of the macrophage in the atherogenic process.12 Macrophages in cell culture have been shown to bind, internalize, and degrade biologically and chemically modified cholesterol-rich lipoproteins, or beta very low density