A series of small peptides, such as might arise in the course of intralysosomal protein digestion, were screened for the ability to escape, intact, from mouse peritoneal macrophage lysosomes. Inability to penetrate lysosomal membranes was inferred from a peptide's induction of lysosomal swelling, or vacuolization, in cultured macrophages.
Two of the peptides tested, (D-Glu)2 and (D-Ala)3, induced vacuolization. Neither peptide was susceptible to hydrolysis by enzymes in macrophages or in the serum-containing culture medium. Their morphological effect was inhibited by parafluorophenylalanine, an inhibitor of pinocytosis. Once formed by either peptide, the vacuoles persisted for several hours in peptide-free medium. Quantitative studies of radioactively labeled (D-Glu)2 confirmed the morphological evidence that (D-Glu)2 is taken up by pinocytosis and stored, intact, in macrophage lysosomes.
The majority of the peptides which failed to induce vacuolization—(L-Ala)2, L-Ser·L-Ala, L-Val·L-Ala, L-Ala·L-Thr, Gly·D, L-Phe, L-Ala·D-His, (L-Ala)3, (L-Glu)2, and D-Leu·L-Tyr—were found to be susceptible to hydrolysis by cellular or serum peptidases. Their failure to induce vacuolization was attributed to their hydrolysis to subunits capable of penetrating lysosomal membranes.
Some of the peptides which had failed to induce vacuolization—(D-Ala)2, D-Ser·D-Ala, D-Val·D-Ala, Gly-D-Asn, D-Ala·D-Thr, and D-Arg·D-Val—were found to be indigestible. Except for the cytotoxic peptide D-Arg·D-Val, peptides in this category all had lower molecular weights and volumes than (Glu)2 or (Ala)3. It is inferred that these peptides are small enough to escape from macrophage lysosomes, while (Glu)2 and (Ala)3 are too large to escape intact. The implications of this inference for the mechanism of intracellular digestion of pinocytosed proteins are discussed.