Molecular cloning and expression analysis of feline α1-microglobulin

Structure, Functions, and Physiological Roles of the Lipocalin α1-Microglobulin (A1M)

α₁-microglobulin (A1M) is found in all vertebrates including humans. A1M was, together with retinol-binding protein and β-lactoglobulin, one of the three original lipocalins when the family first was proposed in 1985. A1M is described as an antioxidant and tissue cleaning protein with reductase, heme- and radical-binding activities. These biochemical properties are driven by a strongly electronegative surface-exposed thiol group, C34, on loop 1 of the open end of the lipocalin barrel. A1M has been shown to have protective effects in vitro and in vivo in cell-, organ-, and animal models of oxidative stress-related medical conditions. The gene coding for A1M is unique among lipocalins since it is flanked downstream by four exons coding for another non-lipocalin protein, bikunin, and is consequently named α₁-microglobulin-bikunin precursor gene (AMBP). The precursor is cleaved in the Golgi, and A1M and bikunin are secreted from the cell separately. Recent publications have suggested novel physiological roles of A1M in regulation of endoplasmic reticulum activities and erythrocyte homeostasis. This review summarizes the present knowledge of the structure and functions of the lipocalin A1M and presents a current model of its biological role(s).

Ci-AMBP: a highly conserved member of the microglobulin superfamily of proteinase inhibitors in grass carp, Ctenopharyngodon idellus

A full-length cDNA clone encoding grass carp (Ctenopharyngodon idellus) α1-microglobulin/bikunin precursor (Ci-AMBP) was isolated by subtracted differential hybridization screening from a liver cDNA library. The deduced amino acid sequence shared approximately 50% sequence identity with its mammalian counterparts, but more than 90% identity with another fish species. AMBPs are the precursors of the plasma glycoproteins α1-microglobulin (α1m) and bikunin. Both peptide structures and their chromosomal organization were well conserved in Ci-AMBP. The α1m and bikunin polypeptides are separated by the typical tetrapeptide R-A-R-R that provides an endoproteolytic cleavage site for maturation. The genetic organization of domains and functional motifs indicated that Ci-AMBP is a typical member of the lipocalin and Kunitz-type protease inhibitor superfamilies. Expression of the Ci-AMBP gene in different tissues/organs was evaluated using semi-quantitative RT-PCR and, in contrast to the restricted expression in other species, transcripts were detected in a wide range of tissues. The most abundant expression occurred in the secretory organs, which supports the roles of α1m and bikunin in the immune response to diseases and in the stress response.

Pathological conditions involving extracellular hemoglobin: molecular mechanisms, clinical significance, and novel therapeutic opportunities for α(1)-microglobulin

Hemoglobin (Hb) is the major oxygen (O(2))-carrying system of the blood but has many potentially dangerous side effects due to oxidation and reduction reactions of the heme-bound iron and O(2). Extracellular Hb, resulting from hemolysis or exogenous infusion, is shown to be an important pathogenic factor in a growing number of diseases. This review briefly outlines the oxidative/reductive toxic reactions of Hb and its metabolites. It also describes physiological protection mechanisms that have evolved against extracellular Hb, with a focus on the most recently discovered: the heme- and radical-binding protein α(1)-microglobulin (A1M). This protein is found in all vertebrates, including man, and operates by rapidly clearing cytosols and extravascular fluids of heme groups and free radicals released from Hb. Five groups of pathological conditions with high concentrations of extracellular Hb are described: hemolytic anemias and transfusion reactions, the pregnancy complication pre-eclampsia, cerebral intraventricular hemorrhage of premature infants, chronic inflammatory leg ulcers, and infusion of Hb-based O(2) carriers as blood substitutes. Finally, possible treatments of these conditions are discussed, giving a special attention to the described protective effects of A1M.