The lipocalin α1-microglobulin binds heme in different species

The lipocalin α1-microglobulin (α1m), found in plasma and tissues of various vertebrates, is brown, forms complexes with other proteins and has immunomodulatory effects in vitro, but the physiological function is not yet established. Human α1m was recently shown to bind heme and, after cleavage of a C-terminal tetrapeptide, initiate heme degradation, thus suggesting a heme-scavenger function. In this work the heme-binding of α1m was characterized using heme immobilized on agarose beads, spectrophotometry, and electrophoresis. α1m, both in plasma and in purified form, displayed a concentration-dependent binding to heme–agarose. The apparent dissociation-constant was estimated to be around 2 × 10−6 M for both free α1m and the IgA–α1m complex. Incubation with free heme resulted in two forms of α1m with different electrophoretic mobility. α1m, identified on Western blotting, was found in eluates from heme–agarose after incubation with human biological fluids as well as sera from non-human species, indicating evolutionary conservation of the heme-binding property. Heme-binding could be instrumental for isolating new α1m-homologues.