The lipocalin α1-microglobulin has radical scavenging activity

Abstract

The lipocalin α1-microglobulin (α1m) is a 26-kDa glycoprotein present in plasma and in interstitial fluids of all tissues. The protein was recently shown to have reductase properties, reducing heme-proteins and other substrates, and was also reported to be involved in binding and scavenging of heme and tryptophan metabolites. To investigate its possible role as a reductant of organic radicals, we have studied the interaction of α1m with the synthetic radical, 2,2′-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS radical). The lipocalin readily reacted with the ABTS radical forming reduced ABTS. The apparent rate constant for this reaction was 6.3 ± 2.5 x 103 M-1 s-1. A second reaction product with an intense purple color and an absorbance maximum at 550 nm was formed at a similar rate. This was shown by liquid chromatography/mass spectrometry to be derived from covalent attachment of a portion of ABTS radical to tyrosine residues on α1m. The relative yields of reduced ABTS and the purple ABTS derivative bound to α1m were ∼2:1. Both reactions were dependent on the thiolate group of the cysteine residue in position 34 of the α1m polypeptide. Our results indicate that α1m is involved in a sequential reduction of ABTS radicals followed by trapping of these radicals by covalent attachment. In combination with the reported physiological properties of the protein, our results suggest that α1m may be a radical reductant and scavenger in vivo. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.