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In platelets, agonists that stimulate phosphoinositide turnover cause the rapid phosphorylation of a protein of apparent relative molecular mass (Mr) 40-47,000, called P47, by protein kinase C (PKC). Diverse identities have been ascribed to P47 including lipocortin, inositol 1,4,5-trisphosphate 5-phosphomonoesterase, pyruvate dehydrogenase alpha subunit and an actin regulatory protein. We have isolated human P47 clones by immunological screening of a lambda gt11 complementary DNA library from HL-60 cells, a human promyelocytic leukaemia cell line. P47 recombinants thus identified hybridized to a 3.0 kilobase (kb) messenger RNA in mature white blood cell lines; the same mRNA was induced in HL-60 cells during differentiation. A 1,050 base pair (bp) open reading frame that could encode a protein of Mr40,087 was confirmed by comparison with peptide sequences from platelet P47, and by expression of the putative recombinant P47 in E. coli and in vitro. The P47 sequence appears to have been conserved throughout vertebrate evolution, and is not similar to any other known sequence including human lipocortin and the alpha subunit of pyruvate dehydrogenase. The P47 protein contains a potential Ca2+-binding 'EF-hand' structure and a region that strongly resembles known PKC phosphorylation sites.

Human leukocyte proteins from more than 150 patients with rheumatoid arthritis, together with age- and sex-matched controls, were analyzed by use of the ISO-DALT technique in two-dimensional polyacrylamide gel electrophoresis. Patients with ankylosing spondylitis, polymyalgia rheumatica, psoriatic arthritis, calcium tendinitis, post-infectious arthritis, and asymmetrical seronegative arthritis were also included as positive controls. Synthesis of several proteins, referred to by number as members of the "Rheuma" set, is shown to increase in the leukocyte preparations from patients with classical rheumatoid arthritis. Several of these proteins are specific to monocytes or granulocytes; others are of unknown cellular origin, but appear to be unique to rheumatoid arthritis. The Rheuma proteins appear to be indicators of disease activity, because their increased synthesis can be correlated with sedimentation rate and other clinical indices of rheumatoid disease activity.

Protein with a molecular mass of 53000 daltons undetectable in healthy persons was identified by electrophoresis in peripheral blood leukocytes of patients with Down's syndrome. The protein was completely extracted with 0.4 N HCl from leukocyte homogenates and was found to be identical, as regards electrophoretic mobility, to protein detected in two patients with chronic myeloblastic leukemia. The causes of discrepancy between theoretically expected and electrophoresis-revealed differences in protein composition of normal and trisomal cells.

This paper describes information-handling aspects of the TYCHO I analysis system (Clin, Chem. 27: 1807--1820, 1981), which analyzes two-dimensional electrophoresis gels, matches the individual protein spots with those in a reference pattern, and stores various information--including spot measurements, identifications, treatment profiles, set memberships, and comments--in a computerized database. This and additional information such as amino acid composition and cellular localization is then accessible from an interactive program that includes a pictorial user interface and presents much of the data in graphical form. Use of the TYCHO I system is illustrated by examples drawn from analyses of gel patterns from human leukocytes.

1. A granulocyte-rich fraction was isolated from blood of a patient with chronic granulocytic leukaemia and from blood of a normal subject and the cells were disrupted in isotonic sucrose. The nuclei were removed by low-speed centrifugation and the post-nuclear supernatant was fractionated by centrifugation on sucrose density gradients. 2. The subcellular organelles in the gradients were detected with marker enzymes by the use of highly sensitive assay techniques. Similar results were obtained with granulocytes from both subjects. 3. Unsaturated vitamin B12-binding proteins were almost exclusively localized to the specific granules. Chromatographic analysis of these proteins showed them to have approximately equal proportions of transcobalamins I and III. Vitamin B12 assayed microbiologically could not be detected in the specific granule fractions.

6 normal females were found to have a level of folic-acid-binding protein (FABP) in their sera (mean level 345 pg/ml). Lysates of their various leucocytes were assayed and FABP found exclusively in the polymorph fraction. The sera and lysates were fractionated by Sephadex G-200 gel column chromatography and the molecular weight of the FABP appeared to be approximately 40,000. A hitherto unreported inhibitory effect of heparin on folate binding was noticed. Studies have been performed on folic-acid-binding protein (FABP) in various pathological states, but the levels found in normal people are usually considered too low to permit investigation. In an earlier study [3], 6 normal females were found to have levels in excess of 300 pg/ml whilst the mean level of 94 controls was 76 pg/ml. Sera and leucocyte lysates from these people have been used to estimate the molecular weight of FABP in normal people.

The concentrations and molecular forms of prealbumin, orosomucoid, albumin, alpha 1-antitrypsin and haptoglobin in leucocytes were studied in patients with alpha 1-antitrypsin deficiency, in acute phase and with leukaemia, by rocket and crossed immunoelectrophoresis. Alpha 1-antitrypsin was the only protein being synthesized by bone marrow cells, blast cells and leucocytes present in the blood during cytotoxic treatment. The other proteins are taken up by the cells. The concentration of alpha 1-antitrypsin in serum and cells was correlated in alpha 1-antitrypsin deficiency and the acute phase, suggesting either that the same signal for synthesis is used in the liver and leucocytes, or that a balance is kept by means of leucocyte uptake. Chronic lymphocytic leukaemia lymphocytes contained only traces of albumin, which may be related to the decreased membrane mobility of these cells. Polycythemia leucocytes, however, contained increased concentrations of all plasma proteins, especially orosomucoid and haptoglobin2 . These two protein forms were found to be markers of mature granulocytes. The cellular concentrations of prealbumin, albumin and transferrin were about 35% of normal in acute myeloid leukaemia cells and no orosomucoid2 and haptoglobin2 was present. Alpha 1-antichymotrypsin was present in a heterogeneous form and the molecular form of alpha 1-antitrypsin (alpha 1 AT4 ) was the same as in monocytes. These findings suggest the existence of a distinct 'AML'-type protein pattern which could be of functional and diagnostic importance.

Sixteen plasma proteins were studied in the supernatants from 20 successive washings of human leucocytes by a new sensitive immunoelectrophoretic method. In the first few supernatants a fast and parallel decrease in concentration of all proteins took place, but for 11 proteins this was followed by a phase of slow decrease due to cellular release. Eight proteins could be quantitated throughout the 20 washings: prealbumin, albumin, orosomucoid, alpha 1-antitrypsin, transferrin, C3, haptoglobin, and IgG. These proteins and trace amounts of eight other plasma proteins were present in the cell lysate after the washings. In comparison with plasma, the leucocytes contained relatively higher concentrations of prealbumin, orosomucoid, alpha 1-antitrypsin, and haptoglobin than of albumin. In crossed immunoelectrophoresis these same four proteins were present as molecular variants differing from their plasma counterpart in electrophoretic mobility and morphology of the precipitate.

We describe here a computer system for the analysis of high-resolution two-dimensional gel-electrophoresis patterns, with some initial applications. The system (called TYCHO) comprises programs for image acquisition, background subtraction and smoothing, spot detection, gaussian spot modeling, and pattern matching and comparison. It is based on a conventional minicomputer, but makes extensive use of a high-speed array processor in the image-processing and -modeling steps. Used in concert with the ISO-DALT two-dimensional electrophoresis system (Anal. Biochem. 85:331-354, 1978), TYCHO allows quantitative measurement of hundreds of proteins in complex biological samples, and constitutes the initial data-reduction system required for work towards a Human Protein Index.

Leukocyte extracts contain enzymes that digest fibrinogen and release a fibrinopeptide A-containing fragment. This study was undertaken to identify the responsible proteinase and to characterize the fibrinopeptide A-containing fragment so that it could be used as an index of enzyme activity. Both the fibrinogenolytic activity and the release of the fibrinopeptide A-containing fragment mediated by the leukocyte extracts were shown to be due to human neutrophil elastase (HNE) by the following criteria: activity was completely blocked by a specific HNE inhibitor or by adsorbing HNE from the extracts with a monospecific antibody and reconstitution with purified HNE restored the ability to release the fibrinopeptide A-containing fragment. This fragment was not released by a variety of other proteinases or by HNE-inhibitor complexes indicating that, at least with respect to the enzymes tested, it is a specific product of HNE and its release requires the free enzyme. By separating the products of HNE digestion of fibrinogen using high performance liquid chromatography, identifying the immunoreactive fractions and subjecting them to amino acid analysis, the fragment was identified as A alpha 1-21, indicating an HNE cleavage site at the Val(A alpha 21)-Glu(A alpha 22) bond. The mean plasma A alpha 1-21 level was markedly higher in patients with alpha 1-proteinase inhibitor deficiency as compared to healthy controls (0.2 nM vs. 7.9 nM; P less than 0.0001), consistent with increased in vivo HNE activity in these individuals.

Cytosol proteins from normal lymphocytes, leukemic lymphocytes, and different cultured lymphoid cell lines were separated by two-dimensional mini gel electrophoresis. By staining with Coomassie blue, specific protein patterns were obtained. Very similar gel maps were produced by the cytosol proteins of chronic lymphocytic leukemia cells, hairy cells, and of in vitro grown B cells. Protein 36/6.2 (molecular weight/isoelectric point), consistently present in these cells, could not be demonstrated in normal lymphocytes. For the comparison of control Raji cells--an Epstein-Barr-Virus (EBV)-DNA carrying Burkitt's lymphoma cell line--with Raji cells induced for early antigen (EA) production, 35S-methionine-labelled total cell lysates were analyzed. In the induced cells, an additional protein (100/5.5) was found; this might be one of the immunologically defined EBV-associated antigens. These results demonstrate that two-dimensional mini gel electrophoresis can be useful for the characterization of leukemic cells in addition to the morphological, cytochemical, and surface marker analyses.

Transcobalamin I and transcobalamin III have been purified approximately 6,000,000- and 3,000,000-fold, respectively, from normal human plasma using a purification scheme consisting of immunoadsorption, dialysis against 7.5 M guanidine HCl to remove endogenous vitamin B12, and affinity chromatography on vitamin B12-Sepharose. The two proteins were separated from each other subsequently by chromatography on DEAE-cellulose. The vitamin B12-binding protein present in granulocytes obtained from normal subjects has been purified approximately 5000-fold using affinity chromatography on vitamin B12-Sepharose as the sole purification technique. The final preparations of all three proteins were homogeneous based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Transcobalamin I and transcobalamin III belong to the R-typed class of vitamin B12-binding proteins and are indistinguishable from each other, and from the human granulocyte, milk, and saliva R-type vitamin B12-binding proteins, when studied by immunodiffusion with rabbit anti-human milk vitamin B12-binding protein sera. The carbohydrate compositions, expressed as moles of carbohydrate per mole of vitamin B12, of transcobalamin I, transcobalamin III, and the normal granulocyte vitamin B12-binding protein, respectively, are: sialic acid, 18, 11, 11; fucose, 9, 20, 24; galactose, 41, 51, 46; mannose, 24, 22, 20; galactosamine, 2, 2, 2; and glucosamine, 46, 54, 46. The high sialic acid content of transcobalamin I appears to account for the fact that this protein elutes after transcobalamin III and the normal granulocyte vitamin B12-binding protein during chromatography on DEAE-cellulose. This observation provides support for the hypothesis that differences among the R-type vitamin B12-binding proteins are due to differences in carbohydrate content. The similarities in carbohydrate composition and other properties of transcobalamin III and the granulocyte vitamin B12-binding protein provide support for the hypothesis that human plasma transcobalamin III is derived from granulocytes. The differences observed between transcobalamin I and the normal granulocyte vitamin B12-binding protein suggest that transcobalamin I may not be derived from granulocytes.

Using polyacrylamide gel electrophoresis an additional protein band was found in the leukocyte preparation obtained from patients with Down syndrome as compared with the cells of healthy donors. Impairments in gene expression, due to alteration in the ratio between content of histone genes and of nuclear DNA, were apparently responsible for qualitative alterations in the cell protein composition under conditions of aneuploidy as well as for the quantitative alterations, deviating from the principle of "gene dosage".

Preparation techniques are outlined for blood samples from cattle, which permit rapid separation of several constant fractions within a large number of animals. Electrophoretical examination of the blood fractions, the methods for which are described, rendered possible the phenotyping of 5 new polymorphic protein systems. Furthermore the electrophoresis of erythrocyte carboanhydrase was tested. In about 550 cattle, including familial material of the breed 'Deutsche Schwarzbunte', the observed phenotypes, segregation data and allele frequencies of 7 polymorphisms, are described. We were able to find new polymorphisms of serum posttransferrins (Ptf), leucocytic proteins 1 and 2 (Leu1 and Leu2), and erythrocytic proteins 1 and 2 (Ery1, and Ery2). Moreover, data of carbonic anhydrase isoenzymes stained with protein dyes, as well as identified as esterases, are given.

The R-type vitamin B12-binding proteins, here called cobalophilin, in human plasma, serum, leukocytes, gastric juice, amniotic fluid, and milk, were characterized by isoelectric focusing and gel filtration. Cobalophilin from all these sources in microheterogenous, consisting of several isoproteins with isoelectric points (pI) between 2.3 and 5.0. Isoproteins with pI values of 3.0, 3.3, 3.6, 3.9, and 4.2 were found in most of the cells and fluids studied, but in different proportions. Milk contains isoproteins with pI values mainly between 4.0 and 4.7, and such components also occur in considerable amounts in saliva. The cobalophilin in these fluids also has a smaller Stokes radius than that from other sources. On the basis of the isoelectric patterns and the Stokes radii the isoproteins of cobalophilin are tentatively divided into two populations, a glandular one occurring in milk and saliva and a myelogenic one occurring in all the cells and fluids studied but only occasionally in milk.

Plasma levels of a novel leucocyte protein (L1) were determined in a series of 176 febrile patients and compared with CRP, BLC, and BSR. Among 82 patients with bacterial infection, 81 had L1 levels above the 0.95 interval. Levels of 40 to 130 times the normal mean were seen frequently during life-threatening infections like septicaemia, meningitis, or pneumonia. In contrast, 23 of 27 patients with viral infections had normal L1 levels. A poor correlation was found between L1 levels, CRP, BLC, and BSR, suggesting that these parameters reflect different aspects of the response to tissue damage and inflammation. L1 and CRP seem equal in distinguishing between bacterial and viral infections. Low or normal L1 levels argue strongly against a bacterial infection, while elevated L1 levels discriminate poorly between bacterial and non-infectious inflammatory or malignant diseases.

Peripheral blood leukocytes from 30 patients with classic or definite rheumatoid arthritis (RA) and 18 healthy controls were separated into lymphocytes, monocytes and granulocytes and labelled with 35S-methionine. The integral membrane proteins with an amphiphilic nature were separated from hydrophilic proteins by use of the nonionic detergent Triton X-114. Both whole cells and membrane proteins were isotope labelled and separated by high resolution two-dimensional electrophoresis under denaturing conditions. Polypeptide spots were visualised by autoradiography. No consistent differences were found when leukocytes from RA patients were compared to the controls.

Human platelets were tested for the presence of proteins immunologically cross-reactive with red cell spectrin and protein 4.1. As assessed by indirect immunofluorescence microscopy, platelets were specifically reactive with affinity-purified rabbit antisera against red cell spectrin and protein 4.1. The immunoreactive platelet constituents were further analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by electrophoretic transfer to nitrocellulose paper and immunoperoxidase staining. We found that whole platelets, membranes, and cytoskeletal preparations isolated by Triton X-100 extraction contain small amounts of proteins reacting with anti-spectrin or anti-protein 4.1 antiserum. The immunoreactive spectrin-like platelet protein has an apparent molecular weight of 240,000 and comigrates with the alpha-subunit of red cell spectrin. The major immunoreactive protein 4.1-like constituent has an apparent molecular weight of 78,000, which is slightly less than that of red cell protein 4.1. We conclude that platelets contain a spectrin-like protein which, by analogy with red cell spectrin, may have a role in membrane-cytoskeletal attachment. The properties and function of the platelet protein 4.1-like constituent are not yet known.

Mo1 and gp160-gp93 are two surface membrane glycoprotein heterodimers present on granulocytes and monocytes derived from humans and guinea pigs, respectively. We purified both antigens and found that their alpha subunits had identical N-termini which were significantly homologous to the alpha subunit of the human adhesion platelet glycoprotein IIb/IIIa.

Protein (m.v. 53 kilodalton) with electrophoretic mobility identical to that of protein detected in leukocytes of patients with Down's syndrome and of protein p53 obtained from mouse ascites carcinoma was demonstrated by polyacrylamide gel electrophoresis in healthy donors' leukocytes cultured with PHA and in peripheral blood of patients with chronic myeloleukemia. Appearance of the class p53 proteins in leukocytes of patients may be connected with the presence in their blood of the population of immature or transformed, proliferating or merely DNA-synthesizing leukocytes, particularly those amplifying the gene that codes protein p53.

We have further characterized osteoclast activating factor (OAF) using a bioassay for bone resorption which utilizes the release of previously incorporated (45)Ca from fetal rat long bones in organ culture. When supernatant media from activated leukocyte cultures were concentrated on Amicon PM10 membranes (assigned molecular weight cutoff 10,000 daltons) and chromatographed on Sephadex G-50 columns, the bone-resorbing activity eluted between the molecular weight markers chymotrypsinogen (25,000 daltons) and cytochrome c (12,500 daltons). This peak of biological activity has been called big OAF. When filtrates from the PM10 membranes were concentrated on Amicon UM2 membranes (assigned molecular weight cutoff 1,000 daltons) and chromatographed on Sephadex G-50 columns, some of the biological activity eluted between the molecular weight markers chymotrypsinogen and cytochrome c (big OAF), but there was a separate peak of biological activity which eluted with [(3)H]proline (140 daltons). This second peak has been called little OAF. Little OAF was eluted from Bio-Gel P6 columns between the molecular weight markers calcitonin (approximately 3,500 daltons) and vitamin B(12) (1,330 daltons), but was retained by Spectrapor dialysis tubing (nominal molecular weight cutoff 3,500 daltons). Big OAF was converted to little OAF by equilibration in 1 M NaCl or 2 M urea. Little OAF was self-associated back to big OAF by equilibration in buffers of low ionic strength (Tris-HCl 10-50 mM). Little OAF was extracted into the organic phase in ethyl acetate after acidification of the sample to pH 3.5. The biological activity remained in the aqueous phase after ethyl acetate extraction at pH 7.5-8.4. Little OAF has been purified more than 6,000-fold compared with the original material so that bone-resorbing activity is maximal in a sample with a protein concentration of 80 ng/ml.

In paroxysmal nocturnal hemoglobinuria (PNH), an acquired hemolytic anemia, deficiency of decay accelerating factor (DAF) renders blood cells susceptible to increased deposition of autologous complement activation fragments (C3b) and complemented-mediated injury. To investigate the mechanism of the DAF defect, DNA and mRNA from normal and PNH leukocytes were compared in blot hybridization assays by using DAF cDNA and oligonucleotide probes. Southern analyses of DNA from normal cells revealed a single gene spanning approximately equal to 35 kilobases of DNA. Six HindIII banding patterns were distinguishable among normal individuals. In family studies, the patterns segregated as three homozygous and three heterozygous genotypes deriving from three haplotypes: A, B, and C with frequencies of 0.47, 0.36, and 0.17, respectively. Oligonucleotide mapping localized the polymorphic HindIII sites to two noncoding regions in the vicinity of exons encoding (i) the protein oligosaccharide-rich domain and (ii) the mRNA 3'-untranslated region. Analyses of DNA from DAF-negative leukocytes of eight PNH patients demonstrated restriction fragment profiles identical to those of normal individuals for all enzymes studied. Three patients had the BC (normals = 3/32), three patients had the AA (normals = 6/32), and two patients had the AC (normals = 8/32) HindIII genotype. Of the three PNH patients exhibiting the BC genotype, family studies of two demonstrated the expected inheritance patterns, and RNA gel blot analyses of two showed mRNA transcripts indistinguishable from those in normal cells. The absence of DAF gene or mRNA alterations in affected PNH cells that lack other glycolipid-anchored proteins as well as DAF argues that the lesion underlying PNH cells resides in the glycolipid-anchor pathway.

In 40 patients with untreated lung cancer cytochemical studies of the peripheral blood leukocytes were conducted by means of a cytological method for the simultaneous staining of nucleoproteids (RNP and DNP) and some cathionic proteins (after Zvetkova and Zvetkov [60]). Changes were detected in the RNP cytoplasmic contents of lymphocytes, of which the most outstanding were the reduction and uneven distribution of RNP granules, their frequent extracellular expulsion by means of microclasmatoses, as well as changes in the staining of cathionic proteins of RNP accompanied by an increased nuclear chromatin condensation in the small and medium-sized lymphocytes. Parellel to reducing of the percentage of these cells in the peripheral blood of patients with advanced neoplastic disease an increased number of lymphoblastoid and monoblastoid cells is established with RNP diffusely stained, but reduced in quantity and localized in the cytoplasmic periphery and projections (compared to Downey type II atypical cells). By means of one of the variants of the method (modified type of Feulgen's reaction) a characteristic distribution and structuring of the nuclear chromatin is established in mono- and polymorphonuclear cells, most clearly expressed in the nuclei of monocytes and monoblastoid cells, as well as in nuclei of neutrophil granulocytes. In these cellular types a more specific nuclear modelling (microhypersegmentation) is observed resulting in multiple irregular nuclear projections on the nuclear surface, probably caused by subkaryolemal distribution of uneven chromatin thickenings. The changes are also recorded in the cathionic protein containing secondary cytoplasmic granules in granulocytes-neutrophils and eosinophils, probably associated with changes in the lysosomal and phagocytic functions of these cells in neoplastic diseases. The authors discuss the importance of the obtained results in connection with data on the participation of lymphocytes and neutrophils in the immune response to tumour antigenic stimuli during the course of the neoplastic process, as well as with data on the suppressive effect of antigenic (serum, viral) factors, possibly affecting the synthesis and the transport of cellular nucleoproteids (RNP and DNP) in leukocytes of cancer patients.

The level of intraleukocytic cationic proteins is studied in rats with the transplanted Pliss lymphosarcoma and sarcoma induced by 7,12-dimethylbenz(a)anthracene (DMBA). The amount of cationic proteins was found to decrease in granulocytes both of mongrel and Wistar rats with transplantable tumours. The phenomenon is typical of the rats with the DMBA-induced tumour.

The present study deals with the conditions necessary for the specific demonstration of nuclear acidic HCl insoluble proteins is smeared peripheral leucocytes by means of a simple cytochemical procedures. These proteins can be very easily selectively demonstrated by staining with Azure C or Toluidine blue above pH 6, after the fixation of smears with methanol, extraction of histones, nucleic acids (with HCl, TCA) and treatment with formaldehyde. The double fixation with methanol and formaldehyde facilitated specific extraction of nuclear acidic HCl insoluble proteins of smeared leucocytes with pepsin. The nuclear acidic HCl insoluble proteins are present in leucocytes in interchromatin areas and their isoelectric point determined by cytochemical procedure is pH 6.

Lactoferrin is contained in cytoplasmic granules of human polymorphonuclear leukocytes. Upon centrifugation, it sediments in a band of granules that also contain 50% of the lysozyme activity. This granule class is distinct from others associated with alkaline phosphatase and peroxidase. The granules are latent for lactoferrin as only lysed granules have the capacity specifically to inhibit antigen binding by anti-lactoferrin serum.

Separation of homogenates of human polymorphonuclear leukocytes (PMN) into different fractions by sedimentation in centrifugal fields that ranged from 126 x g to 50,000 x g resulted in a differential distribution of the lysosomal enzymes. Peroxidase, lysozyme, beta-glucuronidase, and acid phosphatase activity were separated from each other. This demonstrates that the lysosomes of human PMN comprise at least three and possibly four physically and chemically different cytoplasmic particles. Proteins which are more cationic than lysozyme and which may be analogous to cationic lysosomal protein of rabbit PMN were associated with lysozyme and beta-glucuronidase rich granules. Antibacterial activity was present in four of the five cell fractions which this work produced. These results are significant because they differ from those obtained with rabbits and because they directly influence future experimental design and interpretation, in attempts to analyze antibacterial, scavenging, and inflammatory capacities of human PMN. Since lysosomes differ physically, biochemically, and morphologically, they may well differ with respect to their function in the PMN.

Analysis of the primary structure of beta(2)-microglobulin indicates that this human protein is homologous in sequence to the constant portion of immunoglobulin light chains (C(L)), and to the homology regions (C(H)1, C(H)2, and C(H)3) of the constant portion of gamma1 (heavy) chains of immunoglobulin G. Homology with the C(H)3 region is particularly striking. No convincing homology could be demonstrated by similar comparisons with the variable regions of immunoglobulin light and heavy chains. beta(2)-Microglobulin contains an intrachain disulfide loop of 57 amino-acid residues that is similar in size to disulfide loops found in the constant regions of immunoglobulin G. These findings suggest that beta(2)-microglobulin is a free immunoglobulin domain, possibly serving an effector function similar to that of the C(H)3 domain of gamma1 chains of immunoglobulin G.