Rapid protein uptake and digestion in proximal tubule lysosomes

The aim of the present study was to determine the initial time course for the handling of protein by the proximal tubule cells in rat kidney, using 125I-labeled cytochrome C as a tracer. The uptake of the protein was followed by electron microscope autoradiography in kidneys from rats fixed by perfusion after i.v. injection of cytochrome C. Protein degradation was studied by incubating cortical slices, taken from rats, injected with the same label, and measuring the release of trichloroacetic acid (TCA)-soluble radioactivity from the slices. It was shown by autoradiography that lysosomes in proximal tubule cells start to accumulate labeled cytochrome C within 3 min after injection of the protein, and that the concentration of label in lysosomes increases during the first 30 min after injection, whereas it decreases in endocytic vacuoles. The catabolism of protein as measured in cortical slices began within 13 min after the i.v. injection. Other experiments showed that the accumulation of cytochrome C in the kidneys is very fast. The maximum accumulation, 37% of the injected dose, was reached seven minutes after injection. The results show that protein uptake in proximal tubule cells, transport into lysosomes and the digestion of protein is a more rapid process than previously reported in ultrastructural studies.