Effect of pH and Lysosomotropic Agents on Membrane-Associated and Internalized125I-Iodinated Human Growth Hormone in Cultured Human Lymphocytes: A Quantitative Biochemical and Electron Microscopic Study*

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Abstract
When 125I-iodinated human GH ([125I]iodo-hGH) interacts with cultured human lymphocytes at 15 C, the reaction is reversible, but at 37 C the reaction becomes less dissociable as a function of incubation time. Acidification of the incubation medium results in rapid ligand dissociation at 15 C, but at 37 C the acid-dissociable component decreases as a function of incubation time. Under conditions where approximately 50% of the ligand is internalized by the cell, 90% is nondissociable. When the 37 C incubation is carried out in the presence of 25 mM NH4Cl, cell-associated radioactivity is increased. Under these conditions approximately 90% of cell-associated radioactivity also is nondissociable. Using quantitative electron microscopic autoradiography, the proportion of [125I]iodo-hGH associated with the plasma membrane and internalized by the cell is indistinguishable in the presence or absence of NH4Cl. Irreversible [125I]iodo-hGH association with cultured human lymphocytes is due to time- and temperature-dependent effects in the plasma membrane of the cell. These effects cannot be distinguished from internalization by acidification. Furthermore, lysosomotropic agents increase cell-associated radioactivity, but the proportion internalized is not increased.

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