Selective release of peptides from lysosomes.

We demonstrate a selective release of peptides by lysosomes in vitro. A lysosomal fraction from human fibroblasts that had previously endocytosed [3H]ribonuclease A was incubated for 2 h, and radioactivity released into the medium and radioactivity retained within lysosomes were analyzed. A variety of radiolabeled molecules including peptides of an appropriate size to serve as antigens for T cell-mediated immunity were released. One small peptide was predominantly released, while others, as well as intact ribonuclease A, were predominantly retained. A 4-5-fold range was also evident in the relative release of three 3H-labeled tripeptide probes of similar charge derived from the sequence of ribonuclease A. This selectivity and the fact that similar peptide degradation fragments were also released and retained by intact cells after endocytosis of [3H]ribonuclease A argues strongly that the release observed in vitro is physiological and not due to damaged lysosomal membranes.