Cell surface proteins of normal and neoplastic lymphocytes were labeled with iodide-125I by lactoperoxidase-catalyzed iodination. Incubation of 125I-labeled iodide cells in vitro resulted in the release of iodinated surface proteins at a rapid rate which was dependent on cellular respiration and protein synthesis.

Comparisons by disc electrophoresis showed a marked similarity between urea-soluble surface proteins extracted from iodinated cells and iodinated material released by the cells during in vitro incubation.

The rate of release of cell surface proteins from thymus cells was three times faster than that of spleen cells or bone marrow-derived thoracic duct lymphocytes. In addition, different proteins were released at different rates as evidenced by the rate of release of 125I of rabbit anti-mouse immunoglobulin specifically bound to mouse spleen cells and comparisons by disc electrophoresis of urea-soluble iodinated surface proteins extracted from cells before and after incubation.

The results suggest that a dynamic state exists at the cell surface. The possible role of the release of cell surface proteins in cell regulation and communication is discussed.

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