Effects of protease inhibitors on ingestion and digestion of soluble antigen-antibody complexes by macrophages

The inhibitory effects of diisopropyl fluorophosphate (DFP) and p-tosyl-L-lysine chloromethyl ketone (TLCK) on the ingestion and digestion of 125I-alpha-amylase [B alpha A, EC 3.2.1.1] complexed with antibody by guinea pig peritoneal macrophages were studied by sucrose density gradient fractionation. When macrophages were exposed to the complex at 37 degrees C and homogenized after washing, most of the trichloroacetic acid (TCA)-insoluble radioactivity was recovered in two fractions which behaved like the plasma membrane and lysosomes in the density gradient, and the TCA-soluble radioactivity was found in the fraction not moving into the gradient. The complex bound to Fc receptors on the cell surface, therefore, was shown to be internalized, transported to the lysosomes, and finally digested to amino acid catabolites. The inhibition with DFP caused an accumulation of the complex in a region between the plasma membrane and lysosomes. This distribution of radioactivity resembled that of cytochalasin B-inhibited macrophages. TLCK also inhibited the digestion of the complex, but the distribution of radioactivity revealed a marked accumulation of the complex in the region of lysosomes. These results provide strong support to the hypothesis that DFP blocks the ingestion of the complex, as in the case of cytochalasin B, whereas TLCK inhibits the intralysosomal proteolysis of the complex.