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 125I-HSA showed that intracellular isotope is localized in perinuclear granules, or secondary lysosomes.

Following a pulse of 125I-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 125I-HSA is reversibly inhibited by reduced temperature.

The TCA-soluble radioactive material excreted by pulsed macrophages was identified as monoiodotyrosine.

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