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

alpha(1)-Microglobulin (alpha(1)m) 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 alpha(1)m can bind heme and that a C-terminally processed form of alpha(1)m degrades heme. It is shown here that alpha(1)m has catalytic reductase and NADH-dehydrogenase-like activities. Cytochrome c, nitroblue tetrazolium (NBT), methemoglobin, and ferricyanide were reduced by alpha(1)m. 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 alpha(1)m, which has much less chromophore than plasma and urine alpha(1)m, was a stronger reductant than the latter alpha(1)m 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 alpha(1)m may provide a physiological protection mechanism against extracellularly exposed heme groups and other oxidants. (C) 2004 Elsevier Inc. All rights reserved.