alpha(1)-Microglobulin: a yellow-brown lipocalin

alpha (1)-Microglobulin, also called protein HC, is a lipocalin with immuno suppressive properties. The protein has been found in a number of vertebrat e species including frogs and fish. This review summarizes the present know ledge of its structure, biosynthesis, tissue distribution and immunoregulat ory properties. alpha (1)-Microglobulin has a yellow-brown color and is siz e and charge heterogeneous. This is caused by an array of small chromophore prosthetic groups, attached to amino acid residues at the entrance of the lipocalin pocket. A gene in the lipocalin cluster encodes alpha (1)-microgl obulin together with a Kunitz-type proteinase inhibitor, bikunin. The gene is translated into the alpha (1)-microglobulin-bikunin precursor, which is subsequently cleaved and the two proteins secreted to the blood separately. alpha (1)-Microglobulin is found in blood and in connective tissue in most organs. It is most abundant at interfaces between the cells of the body an d the environment, such as in lungs, intestine, kidneys and placenta. alpha (1)-Microglobulin inhibits immunological functions of white blood cells in vitro, and its distribution is consistent with an anti-inflammatory and pr otective role in vivo. (C) 2000 Elsevier Science B.V. All rights reserved.