A new approach for assessing cumulative lysosomal degradation of proteins or other macromolecules.

A general method is proposed for the direct estimation of the degradation in various tissues of macromolecules that are metabolized by a lysosomal mechanism. This involves coupling to the macromolecule a small molecule that is cleaved from it only after entry into the lysosome, that is not metabolized but is “trapped” in the lysosome, and that therefore accumulates as a direct function of the amount of macromolecule degraded. The feasibility of the method was shown using low density lipoprotein and serum albumin doubly labeled with covalently bound [¹⁴C]sucrose and ¹²⁵I. Uptake by normal fibroblasts, measured in terms of ¹⁴C accumulated in the cells, correlated very closely with uptake measured in terms of ¹²⁵I-labeled metabolites in the medium plus ¹²⁵I in the cells.