Synthesis of apoferritin in mouse peritoneal macrophages. Characterization of 20 S particles.

Apoferritin particles were found in mouse peritoneal macrophages cultured in vitro. They were found as 20S particles in the "ribosomal fraction" of macrophages labeled with L-[14C]glutamic acid. Possibilities that they were breakdown products of ribosomes or of other well-known contaminants of the ribosomal fraction were excluded because they did not incorporate [5-3H]uridine. They were resistant to RNase and were relatively resistant to detergent. The antibody against horse spleen apoferritin precipitated about 70% of the particles in the 20S region, judging by measurement of radioactivity. On in vitro incubation with Fe2+ and suitable oxidizing agents the sedimentation coefficient of 80% of the 20S particles changed to about 60S, which corresponds to that of ferritin. SDS-polyacrylamide gel electrophoresis revealed the presence of subunit structures with the same molecular size as that of mouse liver apoferritin. Under the electron microscope, the particles appeared spherical with a relatively uniform diameter of about 130 A.