Bovine alpha 1-microglobulin/bikunin. Isolation and characterization of liver cDNA and urinary alpha 1-microglobulin

cDNA coding for alpha 1-microglobulin, an immunoregulatory plasmaprotein, was isolated from bovine liver. The sequence of a total of 1258 nucleotides revealed an open reading frame of 352 amino acids. This included alpha 1-microglobulin, 182 amino acids, and bikunin, the light chain of the plasmaprotein inter-alpha-inhibitor, 147 amino acids. The two proteins were connected by a basic tetrapeptide, R-A-R-R, which conforms to the consensus sequence recognized by endoproteolytic cleavage enzymes. The deduced amino acid sequence showed a high degree of identity with alpha 1-microglobulin and bikunin sequences from other species, and the alpha 1-microglobulin part displayed sequence motifs typical for members of the lipocalin protein superfamily. A single alpha 1-microglobulin/bikunin mRNA with a size of around 1300 nt was found in bovine liver. The mature alpha 1-microglobulin protein was isolated from bovine urine, and partly characterized. It was found to be a globular molecule with an apparent molecular weight of 23,300, containing one N-linked and at least on O-linked oligosaccharide, one intra-chain disulfide bridge and an electrophoretic heterogeniety with a pI-value of 4.1-5.2.