Redox properties of the lipocalin α1-microglobulin: Reduction of cytochrome c, hemoglobin, and free iron

α1-Microglobulin (α1m) is a 26-kDa plasma and tissue glycoprotein. The protein has a heterogeneous yellow-brown chromophore consisting of small unidentified prosthetic groups localized to a free thiol group (C34) and three lysyl residues (K92, K118, and K130) around the entrance to a hydrophobic pocket. It was recently reported that α1m can bind heme and that a C-terminally processed form of α1m degrades heme. It is shown here that α1m has catalytic reductase and NADH-dehydrogenase-like activities. Cytochrome c, nitroblue tetrazolium (NBT), methemoglobin, and ferricyanide were reduced by α1m. Comparison of the reduction rates suggests that methemoglobin is a better substrate than cytochrome c, NBT, and ferricyanide. The reactions with cytochrome c and NBT were mediated by superoxide anions since they were inhibited by superoxide dismutase. The addition of the biological electron donors NADH, NADPH, or ascorbate enhanced the reduction rate of cytochrome c approximately 30-fold. Recombinant α1m, which has much less chromophore than plasma and urine α1m, was a stronger reductant than the latter α1m forms. Site-directed mutagenesis of C34, K92, K118, and K130 and thiol group chemistry showed that the C34 thiol group was involved in the redox reaction but relies upon cooperation with the lysyl residues. The redox properties of α1m may provide a physiological protection mechanism against extracellularly exposed heme groups and other oxidants.