Fate of milk 125I-labelled lipoprotein lipase in cells in culture. Comparison of lipoprotein lipase- and non-lipoprotein lipase-synthesizing cells.
Radioiodinated lipoprotein lipase, isolated from bovine milk (125I-labeled milk lipoprotein lipase) was shown to retain full hydrolytic activity towards its native substrate, i.e., chylomicron triacylglycerol. The 125I-labeled enzyme interacted with various cells in culture by being bound to the cellular surface, internalized and degraded. Cellular binding of the labeled enzyme occurred in the presence or absence of substrate and was related to enzyme concentration. Heparin reduced cellular binding by 50% but inhibited uptake and degradation more extensively. Cellular uptake was not affected by chloroquine or NH4Cl, but degradation of the labeled enzyme was blocked. Uptake and degradation were not inhibited by mannose 6-phosphate. The interaction between the exogenous enzyme and cells which do not synthesize lipoprotein lipase, i.e., fibroblasts and endothelial cells, resulted in a high ratio of surface binding to degradation. In heart cell cultures and preadipocyte cultures, which produce lipoprotein lipase, the ratio of enzyme catabolized to that bound was high at all time points examined. Since in the intact organism lipoprotein lipase acts at the luminal surface of vascular endothelium, it seems expedient that these cells are able to bind the enzyme, but will catabolize it only slowly. The rapid and extensive degradation of the 125I-labeled lipoprotein lipase in heart cells and preadipocytes may be related to the metabolism of the endogenously produced lipoprotein lipase.
D000273:Adipose Tissue / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D002478:Cells, Cultured; D002914:Chylomicrons / Q000378:metabolism; D004727:Endothelium / Q000378:metabolism*; D005260:Female; D005347:Fibroblasts / Q000378:metabolism; D006493:Heparin / Q000494:pharmacology; D006801:Humans; D008071:Lipoprotein Lipase / Q000378:metabolism*; D008297:Male; D008892:Milk / Q000201:enzymology*; D009206:Myocardium / Q000378:metabolism*; D051381:Rats
D002914:Chylomicrons; D006493:Heparin; D008071:Lipoprotein Lipase