Fragmentation of the human transplantation antigen heavy chain by limited proteolysis, acid cleavage, and cyanogen bromide treatment

Highly purified, papain-solubilized HLA-A, -B, and -C antigens comprising a mixture of a great number of allelic forms from at least three loci have been fragmented by limited proteolysis, acid cleavage, and cyanogen bromide treatment. Limited proteolysis of 125I-labeled HLA-A, -B, and -C antigens with trypsin, chymotrypsin, thermolysin, and pepsin resulted in the production of two large fragments. One fragment was associated with beta 2-microglobulin and contained all of the carbohydrate. The other fragment, which had a molecular weight of about 13,000, is most probably derived from the COOH-terminal part of the heavy chain. Acid cleavage of the HLA antigen heavy chain gave rise to two main fragments with molecular weights of 22,000 and 11,000. Both fragments contained disulfide bonds. Two minor components, representing further cleavage products of the 22,000-dalton fragment, were also observed. Cleavage of the HLA antigen heavy chain at methionyl residues gave rise to one carbohydrate-containing, cysteine-free 14,000-dalton fragment and one 20,000-dalton fragment that contained all cysteines but no carbohydrate. NH2-terminal amino acid sequence analyses demonstrated that the 22,000-dalton acid cleavage fragment and the 14,000-dalton cyanogen bromide fragment were derived from the NH2-terminal part of the HLA antigen heavy chain.

Isolation and properties of detergent-solubilized HLA antigens obtained from platelets

Deoxycholate-solubilized HLA antigens have been isolated from platelets and comprised a mixture of 43,000- and 39,000-dalton polypeptide chains associated with beta2-microglobulin. Limited proteolysis experiments suggested that the 39,000-dalton chain is a fragment of the intact 43,000-dalton chain. Further proteolysis of the 39,000-dalton fragment yields a 33,000-dalton component. The 39,000-dalton molecule is more acidic than both the 43,000- and the 33,000-dalton chains. Differences in the amino acid compositions of the 43,000- and 39,000-dalton species demonstrate that the peptide(s) released on generation of the 39,000-dalton component are charged. The proteolytic split most probably occurs in the COOH-terminal end, which, owing to its content of charged amino acids, most probably is not integrated into the hydrocarbon matrix of the membrane. The 39,000- and 43,000-dalton components bind detergent in micellar form and can be incorporated into liposomes. The 33,000-dalton fragment has lost the ability to bind detergent micelles and is not incorporated into liposomes.

Expression of Ia antigen-like molecules on cells in the corneal epithelium

Ia antigens, known to be expressed preferentially on cells of the immune system, have been shown to be present on cells in the cornea. By immunofluorescence studies, it was shown that a specific rabbit anti-Ia antigen serum stained cells in the corneal epithelium. The stained cells were well integrated into the corneal epithelial architecture and displayed short dendritic processes. Their location in the cornea and corneal limbus and their morphological appearance make it reasonable to suggest that the Ia antigen-expressing cells of the cornea are equivalents of the Ia antigen-bearing Langerhans' cells of the epidermis.

Zinc status and serum levels of retinol-binding protein, tocopherol and lower density lipoproteins in male and female rats fed on semi-purified diets containing rapeseed protein or casein

Experiments were carried out in order to evaluate the effect of various levels of dietary zinc supplementation on the zinc status, growth and contents of retinol-binding protein (RBP), tocopherol and lower density lipoproteins (VLDL + LDL) in serum of rats fed on diets containing rapeseed protein concentrate (RPC) or casein as the sole source of protein. In male rats fed on RPC diet at a 10% protein 5% fat level, a highly significant correlation was found between dietary zinc content and the total tibia zinc content (r = 0.946) in the dietary zinc range of 35-137 microgram/mg. In contrast, the casein rats attained almost maximal bone zinc contents even at the lower level of dietary zinc. No correlation was found between zinc status and the protein efficiency ratio obtained for the rapeseed protein. In female rats fed on a zinc-supplemented RPC diet at a 20% protein and 10% fat level, the serum levels of tocopherol and VLDL + LDL were reduced in comparison to the levels observed in female rats fed on a corresponding casein diet. No diet-related changes in serum levels of RBP were found. However, male rats showed significantly higher RBP values than the female rats.

Amino acid sequence homology between HLA-A,B,C antigens, beta2-microglobulin and immunoglobulins

Papain-solubilized HLA-A,B,C antigen heavy chains have been cleaved by combined acid and CNBr treatment to yield three large fragments. A 14,000-dalton peptide comprises the NH2-terminal portion of the molecule, less a five-membered peptide. The 14,000-dalton fragment is followed in the linear sequence by a 9000-dalton peptide connected through an aspartyl-prolyl bond to the COOH-terminal 11,000-dalton fragment. The 9000- and 11,000-dalton fragments contain disulphide bridges that are immunoglobulin-like inasmuch as they encompass some fifty-five to sixty amino acid residues. The NH2-terminal portion of the HLA antigen heavy chain is devoid of cysteine. NH2-terminal amino acid sequence analyses do not reveal homologies between the 14,000- and 9000-dalton fragments, beta2-microglobulin, and the constant immunoglobulin domains. However, the NH2-terminal sequence of the 11,000-dalton fragment is as homologous to beta2-microglobulin and the constant immunoglobulin domains as they are to one another.

In vitro uptake of vitamin A from the retinol-binding plasma protein to mucosal epithelial cells from the monkey's small intestine

The in vitro uptake of retinol from its plasma carrier protein, the retinol-binding protein (RBP), to the cells of the monkey's small intestine has been studied. [3H]Retinol was readily delivered from the RBP to the cells without a concomitant cellular uptake of the RBP. The [3H]retinol accumulation in the cells was dependent on the temperature and was virtually abolished at 0 degrees. Several metabolic inhibitors could not impede the uptake process. The cellular [3H]retinol accumulation was linear for about 45 min and exhibited characteristic saturation kinetics. The uptake of [3H]retinol by the cells could be inhibited by RBP containing unlabeled retinol, vitamin A-depleted RBP, and Fab' fragments against RBP. In contrast, free, unlabeled retinol and the metabolite form of RBP, lacking retinol and affinity for prealbumin, were inactive. It is therefore suggested that there is a receptor for vitamin A on the cell surface which recognizes the protein part of the protein-ligand complex.

Formation and properties of retinylphosphate galactose

Crude cell membrane fractions from a number of tissues can form acidic glycolipids. The formation of acidic galactose lipid and mannose lipid was greatly reduced in vitamin A deficiency, primarily in tissues known to be mucus-producing. Mouse mastocytoma tissue was active in forming acidic galactose lipids with UDP-galactose as substrate. One of the products was identified as retinylphosphate galactose. The synthetase reaction producing this compound exhibited an apparent pH optimum at 6.3. The presence of detergent and retinol stimulated the synthetase reaction, which exhibited an absolute requirement for Mn2+ or Mg2+. The synthetase reaction was readily reversible. Incubation of particulate enzyme with retinylphosphate galactose and UDP yielded UDP-galactose and a compound tentatively identified as retinylphosphate. The galactose lipid was isolated by column chromatography on DEAE-cellulose and silica gel. The retinylphosphate galactose was homogeneous when examined by thin layer chromatography. Mild acid hydrolysis of labeled retinylphosphate galactose yields [14C]galactose, whereas alkaline hydrolysis and hydrogenolysis produced [14C]galactose 1-phosphate. Retinylphosphate galactose bound to vitamin A-depleted, retinol-binding protein.

Independent expression of the two HL-A antigen polypeptide chains

It is now well established that beta2-microglobulin constitutes one of the two HL-A antigen subunits. In this study support was obtained for the previous notion that the human lymphoma Daudi does not produce beta2-microglobulin (beta2m). Papain-solubilized as well as Nonidet P-40-solubilized Daudi HL-A antigens do not contain any beta2m or any detectable structural analogue of this protein. The chemical and physico-chemical characteristics of highly purified HL-A antigens derived from Daudi cells are indistinguishable from those of the HL-A antigen-carrying polypeptide chain isolated from the P3HRIK cell line. Like P3HRIK-derived HL-A antigens, the HL-A antigens derived from Daudi cells are composed of two identical heavy, alloantigenic polypeptide chains with a molecular weight of about 50 000 each, which are held together by disulfide bridge(s). The HL-A antigens of P3HRIK cells contain, in contrast to Daudi HL-A antigens, two molecules of beta2m. Although no evidence was obtained suggesting any beta2m synthesis in Daudi cells it was apparent that these cells express the HL-A alloantigenic polypeptide chain in amounts similar to those of other cell lines which produce beta2m. The present data suggest [1] that beta2m and the alloantigenic HL-A polypeptide chain are under separate genetic regulation [2], that the cell surface integration of the HL-A antigen-carrying polypeptide chain is independent of the presence of beta2m and [3] that beta2m does not constitute a membrane component absolutely necessary to the integrity of the cell membrane.

Structural and immunological similarities between HLA antigens from three loci

HLA antigens are coded for by three closely linked loci. The gene products of the first and second locus are known to be very similar in overall structure. Antigens coded for by the third locus were examined with regard to their chemical and immunological relationship to the first and second loci gene products. It is demonstrated that the third locus antigens are composed of two types of polypeptide chains, the smaller of which is identical to beta2-microglobulin. The alloantigenic polypeptide chain has an apparent molecular weight of 48 000 when solubilized by detergen treatment and about 35 000 when released from the cell membrane by proteolysis. These data together with size and shape analyses by gel chromatography and sedimentation velocity determinations show that HLA antigens from the three subloci are indistinguishable with the techniques employed. Suggestive evidence was obtained that the third locus antigens are more sensitive to proteolysis than the first and second locus antigens. The similarity between the three gene products was also evident on cyanogen bromide fragmentation of the polypeptide chains. Furthermore, rabbit antibodies against antigens coded for by the first locus cross-reacted extensively with antigens from the second and third locus. This study therefore lends strong credence to the view that the three HLA antigen subloci have arisen by gene duplications of a common ancestral gene.

The subunit structure of thymus leukemia antigens

EDTA-containing buffer solubilizes thymus leukemia antigens (TLa) from crude thymocyte membrane fractions. The TL antigens consist mainly of molecules of a size similar to immunoglobulin G when gel chromatography analyses were performed under physiological conditions. A single component of TLa was apparent on sucrose density gradient ultracentrifugation of solubilized thymocyte membrane macromolecules as monitored by indirect immunoprecipitation. The sedimentation constant for the TL antigens (5.8 S) was considerably less than that for immunoglobulin G. The gel chromatography and ultracentrifugation data suggest an apparent molecular weight for TLa of about 120000. TLa isolated by indirect immunoprecipitation is composed of two types of polypeptide chains. The smaller subunit was identified as beta2-microglobulin. The larger polypeptide chain carried the alloantigenic determinants and displayed a molecular weight of about 50000 after reduction and alkylation. TLa subjected to molecular weight determination under denaturing conditions was composed of two components. The smaller component was beta2-microglobulin which evidently is linked to the larger polypeptide chain by noncovalent interactions only. The larger component had a size greater than reduced and alkylated immunoglobulin G heavy chains. Upon reduction and alkylation of the latter component its size was reduced and it appeared to have a molecular weight of about 50000. Consequently, TLa is composed of two disulfide linked heavy polypeptide chains and two beta2-microglobulin molecules. TLa solubilized by papain digestion comprises two polypeptide chains, one of which is beta2-microglobulin. The larger 37000-dalton subunit is a fragment of the heavy polypeptide chain. This was demonstrated by digesting solubilized 120000-dalton TLa with papain. The proteolytic fragments obtained were indistinguishable from those directly released from the cell surface by proteolysis. The papain-derived TLa fragment exhibited most if not all the alloantigenic determinants.

Studies on thyroid hormone-binding proteins. II. Binding of thyroid hormones, retinol-binding protein, and fluorescent probes to prealbumin and effects of thyroxine on prealbumin subunit self association

Vitamin A in human plasma is transported by its specific carrier protein, the retinol-binding protein. Under physiological conditions the protein forms a stable protein-protein complex with the tetrameric plasma protein, the thyroxine-binding prealbumin. Human prealbumin was shown to interact with the fluorescent probes 1,8-anilinonaphthalene sulfonate (ANS) and 2-p-toluidinylnaphthalene-6-sulfonate (TNS). ANS bound to the protein at two independent sites with the apparent association constant 3 X 10(5) M-1, whereas TNS interacted with a single site with the binding constant 5 X 10(k) M-1. The fluorescent yield of protein-bound ANS was 0.95, a more than 200-fold enhancement compared with that of ANS in aqueous solutions. TNS enhanced its quantum yield nearly 500-fold to 0.37. On addition of thyroid hormones the fluorescent probes could be quantitatively displaced from the protein. This finding suggested that triiodthyronine, thyroxine, and the probes bound to a common site in prealbumin, which is likely to have a strongly hydrophobic character. The association constants for the interaction between prealbumin and the thyroid hormones could be calculated by using the hormones as competitive inhibitors in the TNS-prealbumin titrations. The data from the competition experiments together with those obtained from equilibrium dialysis revealed one major hormone binding site on the protein. The calculated association constant were 9 X 10(6) M-1 and 1 X 10(8) M-1 for triiodothyronine and thyroxine, respectively. Prealbumin monomers were bound to Sepharose by covalent attachment, and their properties were examined. Evidence was obtained demonstrating that the retinol-binding protein could interact with a single subunit of prealbumin. The estimated apparent association constant for the interaction of the protein and the matrix-bound monomeric prealbumin was 3 X 10(4) M-1, approximately 250-fold lower than that measured for protein and matrix-bound tetrameric prealbumin. The data, however, strongly suggest that there are four retinol-binding protein sites per prealbumin molecule. Using the technique of sedimentation equilibrium ultracentrifugation the prealbumin subunit self-association has been studied. The energy of the interaction for the prealbumin subunits is very high, and various concentrations of guanidine hydrochloride has to be used to perturb the equilibrium. All experiments indicated that prealbumin dissociates directly into monomers without the presence of intermediate forms. Thyroxine perturbed the chemical equilibrium of the prealbumin monomer-tetramer system by strengthening the interaction between the subunits.

The concentrations of retinol-binding protein, prealbumin, and transferrin in the sera of newly delivered mothers and children of various ages

Umbilical-cord blood from 50 full-term newborns and capillary or venous blood from 226 children varying in age from 6 weeks to 16 years was examined for content of retinol-binding protein (RBP), prealbumin, and transferrin. The mean levels of the three proteins in the newborns were 21 mug/ml, 128 mug/ml, and 1.65 mg/ml, respectively. The corresponding maternal levels were 39 mug/ml, and 3.2 mg/ml. Paired serum values of mothers and newborns showed a correlation for both RBP and prealbumin but not for transferrin. The low levels of RBP at birth (about 50 per cent of that of nonpregnant women) increased rapidly over the first 6 months to a maximum of 31 mug/ml but declined thereafter to a mean level of about 26 mug/ml, which was maintained until the age of puberty when adult values were gradually attained. In serum, the prealbumin levels showed a similar development, whereas the transferrin values reached constant adult levels as early as 2 years of age.