Rat alpha 1-microglobulin: co-expression in liver with the light chain of inter-alpha-trypsin inhibitor.

A 1162 bp rat liver cDNA clone encoding the immunoregulatory plasma protein alpha 1-microglobulin was isolated and sequenced. The open reading frame encoded a 349 amino acid polyprotein, including alpha 1-microglobulin, 182 amino acids, and bikunin, the light chain of the plasma protein inter-alpha-trypsin inhibitor, 145 amino acids. The alpha 1-microglobulin/bikunin mRNA was found only in the liver when different tissues were examined. Free alpha 1-microglobulin and a polyprotein, containing both alpha 1-microglobulin and inter-alpha-trypsin inhibitor epitopes, were found in the microsomal fraction from rat liver homogenates.