Localization of serum-derived alpha 2 macroglobulin in cultured cells and decrease after Moloney sarcoma virus transformation.

NRK cells and many other cultured fibroblasts were found to contain the protease inhibitor, alpha 2 macroglobulin (alpha 2M). This alpha 2M is present as a result of uptake of alpha 2M from the calf serum in the culture medium. Some of this alpha 2M is released back into the medium. In radiolabeling experiments with 14C-amino acids, no radioactivity was detected in intracellular or extracellular alpha 2M. Fluorescence microscopy of fixed cells using rhodamine-labeled antibodies indicated that alpha 2M is present in vesicular organelles different from primary lysosomes. Fluorescence microscopy of living cells shows that rhodamine-labeled alpha 2M (rhodamine-alpha 2M) is taken up into similar structures. Of the many cell lines examined, Moloney sarcoma virus-transformed cells had the lowest amounts of alpha 2M. Some of the effects of serum on the behavior of cultured cells could be a consequence of inhibition of cellular proteases by alpha 2M.