Radiolabeled sucrose covalently linked to protein. A device for quantifying degradation of plasma proteins catabolized by lysosomal mechanisms.

A general method is described for assessing the degradation of proteins metabolized by lysosomal mechanisms. The method depends on the lysosomal trapping of sucrose which is covalently bound to the protein of interest and thus caried into the lysosome with it. The validity of the method was demonstrated in vitro in studies of the catabolism of low density lipoprotein (LDL) by cultured fibroblasts. Sucrose-derivatized LDL was not distinguished from 125I-LDL by fibroblasts, either in terms of surface binding or rate of uptake. 14C from [14C]sucrose-LDL accumulated in the cells as predicted; very little appeared in the trichloroacetic acid-soluble fraction of the medium (2% of total uptake). 14C-labeled metabolites in the cells (modal apparent Mr = 1000-2000) were separated from undegraded LDL by gel filtration. LDL degradation calculated from the 14C metabolites accumulating intracellularly was in excellent agreement with that calculated from paired studies using 125I-LDL. Finally, the validity of the method was demonstrated in vivo using asialofetuin, a protein previously shown to be selectively taken up and degraded by the liver. In principle, the method described should be applicable to the study of the sites of degradation of any of the plasma proteins.