Distribution of iodine 125-labeled alpha(1)-microglobulin in rats after intravenous injection

The 28-kd plasma protein al-microglobulin is found in the blood of mammals and fish in a free, monomeric form and as high-molecular-weight complexes w ith molecular masses above 200 kd. In this study, iodine 125-labeled free a nd high-molecular weight rat alpha (1)-microglobulin (a mixture of alpha (1 )-microglobulin/alpha (1) -inhibitor-3 and alpha (1)-microglobulin/fibronec tin complexes) were injected intravenously into rats. The distribution of t he proteins was measured by using scintillation camera imaging. Both forms of (125)l-labeled alpha (1)-microglobulin were rapidly cleared from the blo od, with a half-life of 2 and 16 minutes for the initial and late phase, re spectively, for free alpha (1)-microglobulin; and a half-life of 3 and 130 minutes for the initial and late phase, respectively, for the complexes. Af ter 45 minutes, 6%, 16%, 27%, 13%, and 34% of the free (125)l-labeled alpha (1)-microglobulin and 18%, 21%, 6%, 10%, and 42% of the (125)l-labeled alp ha (1)-microglobulin complexes were found in the blood, gastrointestinal tr act, kidneys, liver, and the remainder of the body, respectively. The local distribution of injected (125)l-labeled alpha (1)-microglobulin in intesti nes and kidneys was investigated by microscopy and autoradiography. In the intestine, both forms were distributed in the basal layers, villi, and lumi nal contents. The results also suggested intracellular labeling of epitheli al cells. Well-defined local regions containing higher concentrations of in jected protein could be seen in the intestine. In the kidneys, both forms w ere found mostly in the cortex. Free (125)l-labeled al-microglobulin was fo und predominantly in epithelial cells of a subset of the tubules, whereas t he (125)l-labeled complexes were more evenly distributed. Intracellular lab eling was indicated for both al-microglobulin forms. The results thus indic ate a rapid transport of (125)l-labeled alpha (1)-microgiobulin from the bl ood to most tissues.