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J. Biol. Chem., Vol. 282, Issue 43, 31493-31503, October 26, 2007

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The Lipocalin ₁-Microglobulin Has Radical Scavenging Activity^*

Bo Åkerström¹, Ghassan J. Maghzal, Christine C. Winterbourn^¶, and Anthony J. Kettle^¶

From the Department of Clinical Sciences, Lund University, 22184 Lund, Sweden, the Centre for Vascular Research, Department of Pathology, Faculty of Medicine, University of Sydney, New South Wales 2006, Australia, and the ^¶Free Radical Research Group, Department of Pathology, Christchurch School of Medicine and Health Sciences, Christchurch 8140, New Zealand

The lipocalin ₁-microglobulin (₁m) 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 ₁m 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 ₁m. The relative yields of reduced ABTS and the purple ABTS derivative bound to ₁m were 2:1. Both reactions were dependent on the thiolate group of the cysteine residue in position 34 of the ₁m polypeptide. Our results indicate that ₁m 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 ₁m may be a radical reductant and scavenger in vivo.

Received for publication, March 27, 2007 , and in revised form, August 24, 2007.

^* This work was supported by grants from the Swedish Research Council (Project 7144), the Health Research Council of New Zealand and The New Zealand Centre of Research Excellence for Growth and Development, the Swedish Society for Medical Research, the Royal Physiographic Society in Lund, the Foundations of Greta and Johan Kock and Alfred Österlund, the Swedish Foundation for International Cooperation in Research and Higher Education (STINT), and the Blood and Defense Network, Lund University. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 46-46-222-8578; Fax: 46-46-157756; E-mail: bo.akerstrom{at}med.lu.se.

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