α1-microglobulin is found both in blood and in most tissues

In this study we demonstrate that, in addition to blood, alpha(1)-microglobulin (alpha(1)m) is present in most tissues, including liver, heart, eye, kidney, lung, pancreas, and skeletal muscle. Western blotting of perfused and homogenized rat tissue supernatants revealed aim in its free, monomeric form and in high molecular weight forms, corresponding to the complexes fibronectin-alpha(1)m and alpha(1)-inhibitor-3-alpha(1)m, which have previously been identified in plasma. The liver also contained a series of alpha(1)m isoforms with apparent molecular masses between 40 and 50 kD. These bands did not react with anti-inter-alpha-inhibitor antibodies, indicating that they do not represent the alpha(1)m-bikunin precursor protein. Similarly, the heart contained a 45-kD alpha(1)m band and the kidney a 50-kD alpha(1)m band. None of these alpha(1)m isoforms was present in plasma. Immunohistochemical analysis of human tissue demonstrated granular intracellular labeling of alpha(1)m in hepatocytes and in the proximal epithelial cells of the kidney. In addition, alpha(1)m immunoreactivity was detected in the interstitial connective tissue of heart and lung and in the adventitia of blood vessels as well as on cell surfaces of cardiocytes. alpha(1)m mRNA was found in the liver and pancreas by polymerase chain reaction, suggesting that the protein found in other tissues is transported via the bloodstream from the production sites in liver and pancreas. The results of this study indicate that in addition to its role in plasma, aim may have Important functions in the interstitium of several tissues.