Distribution of iodine 125–labeled α 1 -microglobulin in rats after intravenous injection JÖRGEN LARSSON, KARIN WINGÅRDH, TORD BERGGÅRD, JULIA R. DAVIES, LENNART LÖGDBERG, SVEN-ERIK STRAND, and BO ÅKERSTRÖM LUND, SWEDEN, and NEW YORK, NEW YORK The 28-kd plasma protein α 1 -microglobulin is found in the blood of mammals and fish in a free, monomeric form and as high-molecular-weight complexes with molecular masses above 200 kd. In this study, iodine 125–labeled free and high- molecular weight rat α 1 -microglobulin (a mixture of α 1 -microglobulin/α 1 -inhibitor-3 and α 1 -microglobulin/fibronectin complexes) were injected intravenously into rats. The distribution of the proteins was measured by using scintillation camera imag- ing. Both forms of 125 I-labeled α 1 -microglobulin were rapidly cleared from the blood, with a half-life of 2 and 16 minutes for the initial and late phase, respective- ly, for free α 1 -microglobulin; and a half-life of 3 and 130 minutes for the initial and late phase, respectively, for the complexes. After 45 minutes, 6%, 16%, 27%, 13%, and 34% of the free 125 I-labeled α 1 -microglobulin and 18%, 21%, 6%, 10%, and 42% of the 125 I-labeled α 1 -microglobulin complexes were found in the blood, gastroin- testinal tract, kidneys, liver, and the remainder of the body, respectively. The local distribution of injected 125 I-labeled α 1 -microglobulin in intestines and kidneys was investigated by microscopy and autoradiography. In the intestine, both forms were distributed in the basal layers, villi, and luminal contents. The results also suggest- ed intracellular labeling of epithelial cells. Well-defined local regions containing higher concentrations of injected protein could be seen in the intestine. In the kid- neys, both forms were found mostly in the cortex. Free 125 I-labeled α 1 -microglobu- lin was found predominantly in epithelial cells of a subset of the tubules, whereas the 125 I-labeled complexes were more evenly distributed. Intracellular labeling was indicated for both α 1 -microglobulin forms. The results thus indicate a rapid transport of 125 I-labeled α 1 -microglobulin from the blood to most tissues. (J Lab Clin Med 2001;137:165-75) Abbreviations: α 1 m = α 1 -microgloblin; IgG = immunoglobulin G; LDL = low-density lipoprotein; SDS-PAGE = sodium dodecyl sulfate–polyacrylamide gel electrophoresis 165 From the Section for Molecular Signalling, the Section for Molecu- lar Pathogenesis, the Section for Radiation Physics, and the Section for Physical Chemistry, Lund University; and the New York Blood Center. Supported by grants from the Swedish Medical Research Council (project 7144), EU-Biotech (project BIO4-CT98-0420), King Gus- tav V’s 80-year Foundation, the Swedish Society for Medical Research, the Royal Physiographic Society in Lund, the Foundations of Greta and Johan Kock, and Alfred Österlund. Submitted for publication May 4, 2000; revision submitted Novem- ber 13, 2000; accepted November 16, 2000. Reprint requests: Bo Åkerström, MD, Section for Molecular Sig- nalling, Department of Cell and Molecular Biology, Lund Universi- ty, P.O. Box 94, S-221 00 Lund, Sweden. Copyright © 2001 by Mosby, Inc. 0022-2143/2001 $35.00 + 0 5/1/112957 doi:10.1067/mlc.2001.112957 he protein α 1 m is a 28-kd plasma protein first described in urine from patients with tubular pro- teinuria. 1-3 It is also called protein HC. 4 α 1 m con- sists of a 183-amino-acid peptide chain, 5,6 carries car- bohydrates, and has a characteristic brown color attributed to a very tightly bound chromophore. 2,7 α 1 m belongs to the lipocalins, 8 a superfamily of distantly related proteins that show great similarity in their three-dimensional structures. Many lipocalins carry hydrophobic ligands (eg, retinol-binding protein [for a review, see reference 9]), but a ligand for α 1 m has not yet been found. The function of α 1 m is not known, but the protein has many immunoregulatory properties in vitro. It inhibits antigen-induced lymphocyte proliferation, T