Formation of the alpha 1-microglobulin chromophore in mammalian and insect cells: a novel post-translational mechanism?

alpha 1-Microglobulin is an immunosuppressive plasma protein synthesized by the liver. The isolated protein is yellow-brown, but the hypothetical chromophore has not yet been identified. In this work, it is shown that a human liver cell line, HepG2, grown in a completely synthetic and serum-free medium, secretes alpha 1-microglobulin which is also yellow-brown, suggesting a de novo synthesis of the chromophore by the cells. alpha 1-Microglobulin isolated from the culture medium of insect cells transfected with the gene for rat alpha 1-microglobulin is also yellow-brown, suggesting that the gene carries information about the chromophore. Reduction and alkylation or removal of N- or O-linked carbohydrates by glycosidase treatment did not reduce the colour intensity of the protein. An internal dodecapeptide (amino acid positions 70-81 in human alpha 1-microglobulin) was also yellow-brown. The latter results indicate that the chromophore is linked to the polypeptide. In conclusion, the results suggest that the alpha 1-microglobulin gene carries information activating a post-translational protein modification mechanism which is present in mammalian and insect cells.