Uptake and degradation of iodine-labelled chylomicron remnant particles by monolayers of rat hepatocytes

Abstract

Rat chylomicrons were labeled with 125I with 69-72% of the iodine in the protein moiety. Less than 1 nmol of iodine nmol of protein was incorporated. Of the peptide radioactivity 44-56% was in apolipoprotein A-1, 30-40% in the C peptides and 11-15% in apolipoprotein B. The arginine-rich peptide, which accounted for about 14% of the chylomicron protein mass as determined by scanning of sodium dodecyl sulfate-polyacrylamide gels, contained very little radioactivity. Chylomicron remnants generated with postheparin plasma from iodine-labeled chylomicrons showed a relative increase in the percentage of the arginine-rich peptide (76-90% of the apolipoprotein mass according to gel scanning). The major portion of the peptide iodine label was present in apolipoprotein A-1 (43-57%), B (22-32%) and C peptides (17-35%). When iodine-labeled chylomicron remnants were added to rat hepatocytes in primary culture, labeled peptides were taken up and degraded by the hepatocytes by a saturable process. The Vmax for the uptake was calculated to be 300 ng of protein/h per mg of cell protein and the apparent Km as 7.7 .mu.g of protein/mg of cell protein. A larger proportion of the 125I-labeled lipids of the remnants (mainly polar lipids) was taken up. This suggests that these may also enter the cells by a mechanism that does not involve particulate uptake, such as phospholipid exchange. The degradation of labeled peptides was inhibited by colchicine, concanavalin A, chloroquine and NH4Cl, which also inhibit degradation of the cholesteryl ester portion. All these drugs exerted their inhibition mainly after the uptake of labeled peptide. No degradation occurred at 4.degree. C, and also the uptake was markedly decreased. The uptake of labeled chylomicron remnant peptide was 77 times as effective as that of labeled sucrose, which is likely to be taken up randomly by pinocytosis.