The uptake and digestion of iodinated human serum albumin by macrophages in vitro.

Mouse peritoneal macrophages take up I*-HSA from their medium during in vitro cultivation. Conditions which promote I*-HSA uptake are the same as those which stimulate formation of pinocytic vesicles. Autoradiography of cells pulsed with (125)I-HSA showed that intracellular isotope is localized in perinuclear granules, or secondary lysosomes. Following a pulse of (125)I-HSA, intracellular radioactivity decreases and the amount of TCA-soluble isotope in the medium increases correspondingly. About 50% of the intracellular isotope is lost in 5 hr. The release of isotope from pulsed cells is not inhibited by parafluorophenylalanine, 2,4-dinitrophenol or by a reduction of the serum concentration of the medium. However, the processing of ingested (125)I-HSA is reversibly inhibited by reduced temperature. The TCA-soluble radioactive material excreted by pulsed macrophages was identified as monoiodotyrosine.