Synthesis of alpha 1-microglobulin by guinea-pig liver

The lipocalin α1-microglobulin binds heme in different species

The lipocalin alpha(1)-microglobulin (alpha(1)m), found in plasma and tissues of various vertebrates, is brown, forms complexes with other proteins and has immunomodulatory effects in vitro, but the physiological function is not yet established. Human alpha(1)m was recently shown to bind heme and, after cleavage of a C-terminal tetrapeptide, initiate heme degradation, thus suggesting a heme-scavenger function. In this work the heme-binding of alpha(1)m was characterized using heme immobilized on agarose beads, spectrophotometry, and electrophoresis. alpha(1)m, both in plasma and in purified form, displayed a concentration-dependent binding to heme-agarose. The apparent dissociation-constant was estimated to be around 2 x 10(-6)M for both free alpha(1)m and the IgA-alpha(1)m complex. Incubation with free heme resulted in two forms of alpha(1)m with different electrophoretic mobility. alpha(1)m, identified on Western blotting, was found in eluates from heme-agarose after incubation with human biological fluids as well as sera from non-human species, indicating evolutionary conservation of the heme-binding property. Heme-binding could be instrumental for isolating new alpha(1)m-homologues.

Distribution of iodine 125-labeled alpha1-microglobulin in rats after intravenous injection

The 28-kd plasma protein alpha(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 alpha(1)-microglobulin (a mixture of alpha(1)-microglobulin/alpha(1)-inhibitor-3 and alpha(1)-microglobulin/fibronectin complexes) were injected intravenously into rats. The distribution of the proteins was measured by using scintillation camera imaging. Both forms of (125)I-labeled alpha(1)-microglobulin were rapidly cleared from the blood, with a half-life of 2 and 16 minutes for the initial and late phase, respectively, for free alpha(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 alpha(1)-microglobulin and 18%, 21%, 6%, 10%, and 42% of the (125)I-labeled alpha(1)-microglobulin complexes were found in the blood, gastrointestinal tract, kidneys, liver, and the remainder of the body, respectively. The local distribution of injected (125)I-labeled alpha(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 suggested intracellular labeling of epithelial cells. Well-defined local regions containing higher concentrations of injected protein could be seen in the intestine. In the kidneys, both forms were found mostly in the cortex. Free (125)I-labeled alpha(1)-microglobulin 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 alpha(1)-microglobulin forms. The results thus indicate a rapid transport of (125)I-labeled alpha(1)-microglobulin from the blood to most tissues.

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

alpha(1)-Microglobulin, also called protein HC, is a lipocalin with immunosuppressive properties. The protein has been found in a number of vertebrate species including frogs and fish. This review summarizes the present knowledge of its structure, biosynthesis, tissue distribution and immunoregulatory properties. alpha(1)-Microglobulin has a yellow-brown color and is size 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)-microglobulin 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 and 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 protective role in vivo.

Alpha 1-microglobulin is mitogenic for guinea pig lymphocytes

Guinea pig alpha 1-microglobulin (alpha 1-m) was found to exert mitogenic effects on guinea pig lymphocytes, i.e. peritoneal exudate lymphocytes (PEL) or T lymphocytes from regional or mesenteric lymph nodes (LNL). On the other hand, spleen T lymphocytes did not respond to alpha 1-m. Stimulation of LNL required the presence of accessory cells. The stimulatory effect was strain-specific: alpha 1-m clearly induced proliferation of lymphocytes from Strain 2 guinea pigs, whereas Strain 13 lymphocytes responded poorly or not at all. Moreover, alpha 1-m inhibited antigen-driven proliferation of guinea pig lymphocytes. None of the effects described were species-specific, e.g. human alpha 1-m exerted similar effects on the guinea pig lymphocytes. These data indicate that alpha 1-m may be involved in the regulation of lymphocyte activation.

Rat alpha 1-microglobulin. Purification from urine and synthesis by hepatocyte monolayers

Rat alpha 1-microglobulin was isolated from the urine of rats treated with sodium chromate, and was purified by the use of gel chromatography, affinity chromatography on concanavalin-A-Sepharose and ion-exchange chromatography. The protein was heterogeneous in charge, had a tendency to form dimers, and was associated with a brown-coloured chromophore. The size of the protein (25 kDa) was similar to guinea pig alpha 1-microglobulin but smaller than the human protein, when measured with sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Immunological cross-reaction with human and guinea pig alpha 1-microglobulin was demonstrated. The concentration of alpha 1-microglobulin in rat serum was 16.4 mg/l (SD = 8.5 mg/l, n = 13) and rat serum alpha 1-microglobulin was eluted from a gel chromatography column at two different positions corresponding to monomeric alpha 1-microglobulin and IgA. The latter alpha 1-microglobulin activity could be absorbed by anti-IgA serum. Rat alpha 1-microglobulin and albumin were continuously released into the medium of rat hepatocyte monolayers, and alpha 1-microglobulin was isolated from the medium by the use of immunoprecipitation with anti-(alpha 1-microglobulin). Tritiated leucine, added to the medium, was incorporated into the protein, suggesting a de novo synthesis of alpha 1-microglobulin by the hepatocytes. The size of hepatic alpha 1-microglobulin was similar to that of purified urinary rat alpha 1-microglobulin, when determined with sodium dodecyl sulfate/polyacrylamide gel electrophoresis.

Alpha 1-microglobulin: a new antigenic component of the epidermo-dermal junction in normal human skin

Monoclonal antibodies to human alpha 1-microglobulin (alpha 1-m), a glycoprotein present in most biological fluids, reacted with the basement membrane of the epidermo-dermal junction and that of eccrine sweat glands in normal human skin. This immunofluorescence was specific in that control antibodies of known well-defined reactivity did not stain these structures. Therefore, alpha 1-m seems to be a new antigenic component of the junction and it may be an interesting antigen to study in diseases of this area.

Localization of alpha 1-microglobulin (HC protein) in normal human tissues: an immunohistochemical study using monoclonal antibodies

Alpha-1-microglobulin is a low molecular weight (approximately 30 000 d) glycoprotein present in biological fluids. It is heterogeneous in charge. A monoclonal antibody was used to investigate the tissue distribution of the protein in normal human tissues and cell lines by indirect immunofluorescence and immunoperoxidase techniques. The protein was demonstrable in cells of the monocyte-macrophage lineage, in thymus and T cell dependent areas of spleen, lymph node and tonsils. It was detected in several lymphoid or nonlymphoid cell lines but not in peripheral blood lymphocytes. The microglobulin was also detectable in the cytoplasm of hepatocytes. Finally, it was observed in glandular secretions (sudoral glands and mucosal glands of the digestive tract) where it may be associated with IgA. Possible explanations for the highly divergent results previously reported with polyclonal antisera to alpha 1 microglobulin are discussed.

Comparative biochemistry of the guinea-pig: a partial checklist

A great deal is known about guinea-pig biochemistry, but the information is scattered and difficult to assemble. The guinea-pig also possesses a number of unusual biochemical features which add to its interest. For these reasons we have compiled a list of biochemical characteristics of the guinea-pig, organized in a series of tables, with brief discussions of some of the entries.