Tissue sites of degradation of low density lipoprotein: Application of a method for determining the fate of plasma proteins

Abstract

A method for determining tissue sites of plasma protein degradation is described as applied to studies of low-density lipoprotein (LDL) catabolism in swine. The method is based on the fact that sucrose is not degraded by lysosomal enzymes and thus accumulates in lysosomes. [14C]sucrose was activated with cyanuric chloride and covalently coupled to the LDL protein. Studies in cultured fibroblasts established that the sucrose 14C accumulates intracellularly in degradation products at a rate equal to the rate of degradation of 125I-labeled LDL simultaneously measured. In vivo the fractional catabolic rate of [14C]sucrose-LDL was the same as that of 125I-labeled LDL. 14C-labeled degradation products in all major tissues were determined 24 h after injection of [14C]sucrose-LDL. About 75% of the LDL degraded (calculated from analysis of the plasma decay curve) was accounted for in the 14C-labeled degradation products accumulated in the tissues examined; only 4% appeared in the urine. In 3 studies, 37.9, 39.6 and 37.8% of the LDL degraded was recovered in the liver. Results were similar at 48 h (38.7 and 39.9% hepatic degradation), but urinary losses were then about 10%, and about 4% was lost in bile. All extrahepatic tissues examined contained 14C-labeled degradation products. The concentration was highest in the adrenal glands, 2-5 times that in liver and 10 times that in the next most active tissues. In principle, this approach should be applicable to studies of the tissue sites of degradation of any of the plasma proteins.