Human plasma R-type vitamin B12-binding proteins. I. Isolation and characterization of transcobalamin I. TRANSCOBALAMIN III. and the normal granulocyte vitamin B12-binding protein.

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.     

Human plasma R-type vitamin B12-binding proteins. II. The role of transcobalamin I, transcobalamin III, and the normal granulocyte vitamin B12-binding protein in the plasma transport of vitamin B12.

The normal human granulocyte vitamin B12-binding protein, transcobalamin I, and transcobalamin III, have been labeled with 125I-labeled N-succinimidyl 3-(4-hydroxyphenyl)propionate and utilized for plasma clearance studies performed with rabbits. Both moieties of 125I-labeled granulocyte vitamin B12-binding protein-[57Co]vitamin B12 were cleared rapidly from the plasma (is less than 90% by 5 min) by the liver. After 30 min, the bulk of the 125I reappeared in the plasma in small molecular weight (less than 1000) form and was rapidly excreted in the urine. After 60 min the bulk of the [57Co]vitamin B12 reappeared in the plasma bound to rabbit transcobalamin II and was subsequently taken up by a variety of tissues. Approximately 15% of the 125I-labeled granulocyte vitamin B12-binding protein-[57Co-a1vitamin B12 was excreted intact into the bile during the period from 10 to 80 min after injection. The hepatic uptake of the protein-vitamin B12 complex was blocked by the prior injection of desialyzed fetuin but not by native fetuin. Similar results were obtained with 125I-labeled transcobalamin III-[57Co]vitamin B12. Approximately 90% of both moieties of 125I-labeled transcobalamin I-[57Co]vitamin B12 had prolonged plasma survivals similar to that of 125I-labeled bovine serum albumin. After treatment with neuraminadase, both moieties of the 125I-labeled transcobalamin I-[57Co]vitamin B12 complex were cleared rapidly from the plasma by the liver in a manner that was indistinguishable from that observed in the case of untreated granulocyte vitamin B12-binding protein and transcobalamin III. These observations indicate that desialyzed transcobalamin I and the native forms of the granulocyte vitamin B12-binding protein and transcobalamin III are cleared from plasma by the mechanism elucidated by Ashwell and Morell (Ashwell, G., and Morell A. G. (1974) Adv. Enzymol. 41, 99-128) that is capable of clearing a wide variety of asialoglycoproteins. These observations have implications concerning the function of the human R-type vitamin B12-binding proteins, the nature of the enterohepatic circulation of vitamin B12, the biological significance of the mechanism described by Ashwell and Morell, and the etiology of the increased plasma concentration of human R-type protein that occurs frequently in chronic myelogenous leukemia and occasionally in hepatocellular carcinoma and other solid tumors.     

Malabsorption of protein bound vitamin B12.

Patients with subnormal serum vitamin B12 concentrations were tested for absorption of protein bound vitamin B12 and compared with controls. Absorption of the protein bound vitamin appeared to decrease with increasing age in healthy subjects. Differences between the result of this test and the result of the Schilling test in patients who had undergone gastric surgery were confirmed; such differences were also seen in some patients who had iron deficiency anaemia, an excessive alcohol intake, or folate deficiency. Defective absorption was also found in six patients with an adequate dietary intake of vitamin B12, normal Schilling test results, low serum vitamin concentrations, and tissue changes responding to treatment with vitamin B12. Malabsorption of the vitamin from protein bound sources, which is not detected by the Schilling test, may produce vitamin B12 deficiency of clinical importance.     

The diagnosis of vitamin B 12 deficiency]

In 5 normal persons, 1 patient with pernicious anemia, 5 patients with renal insufficiency in the state of compensation, and in one patient with chronic hemodialysis Schilling tests, activity of 57Co and vitamin B12 serum levels were measured. It was obvicous that activity of 57Co in serum 8 hours after oral application of 0.5 muCi 57Co-vitamin B12 showed reliable results to clear the problem of vitamin B12 malresorption. This method is easier than the Schilling test because the collection of 24-hour-urine is not necessary and an insufficiency of renal function does not influence the result.     

May the Schilling test be already repeated the next day using intrinsic factor?]

The effect of previous administration of a dose of 1000 mu-g vitamin B12 on the Schilling test was examined in 18 patients, repeating the test 24 hrs later. On the first day 57-Co was administered, while on the second day 58-Co labeled vitamin B12 was given. The counting error was less than 2.0% at the 95% confidence level. A decrease in urinary excretion of vitamin B12 of 28.7 plus or minus 22.2% (x plus or minus SD) was found. The mean difference between the two subsequent Schilling test series was statistically significant (p less than 0.05). The excretion data of the first and the second test correlate well (r = 0.86; p less than 0.01; y = 0.66 x + 1.09). Thus the repeated Schilling test with intrinsic factor must not be performed the next day.     

Effect of bile on vitamin B12 absorption.

The standard double-isotope Schilling test was used to study vitamin B12 absorption in seven patients with obstructive jaundice and 10 with T-tube bile duct drainage after cholecystectomy and bile duct exploration. In three and five of these patients respectively absorption was impaired. In the second group six patients were restudied after removal of the T tube, and in each case absorption was improved. Similar results were obtained after bile duct ligation in rats. Bile exclusion produced a 50-60% reduction in renal and hepatic uptake of vitamin B12 from the intestinal lumen. The malabsorption was corrected by replacing bile. These studies suggest that bile plays a part in the normal absorption of vitamin B12.     

Genetic patterns of transcobalamin II and the relationships with congenital defects

The vitamin B12-binding protein, transcobalamin II, is a trace component of plasma with a rapid turnover. This protein is essential for absorption, transport, cellular uptake and for recycling of vitamin B12 (cobalamin). Congenital transcobalamin II deficiency, an inborn error of metabolism is inherited as a recessive trait. The homozygous form of the deficiency is accompanied by severe clinical, hematological and immunological disturbances in the first months of life. Analytical, genetic, biochemical and clinical aspects of transcobalamin II in man and in vertebrates have been reviewed here. A genetic polymorphism for the protein has been found in man, rabbits and mice. Family studies revealed that the genetic patterns in man are determined by four polymorphic and several rare alleles. This genetic variability has been applied in paternity testing and in population studies. Transcobalamin II typing in families of patients with the inherited functional deficiency has led to identification of various deficient alleles in heterozygous carriers of the defects. Applying transcobalamin II typing after bone marrow transplantation demonstrated that this protein originates partly in the bone marrow. Subsequent investigations in cell culture have shown that human skin fibroblasts and cultured bone marrow synthesize and secret isotypes of a transport protein corresponding to the genetic isotypes observed in plasma. Comparison of transcobalamin II types in umbilical cord serum with the maternal types, has proven that the transcobalamin II activity in the cord serum is derived from the fetus. This finding will be of crucial importance in the early diagnosis of the deficiency syndrome.     

Selective vitamin B12 malabsorption in two siblings

Two siblings with megaloblastic anemia responsive to parenteral vitamin B₁₂ were studied to elucidate the cause of the B₁₂ deficiency. Gastric juice from both contained acid and functional intrinsic factor. Serum contained transcobalamin II and lacked antibodies to intrinsic factor. Schilling tests showed vitamin B₁₂ malabsorption uncorrected by hog intrinsic factor or pancreatic extract. Other parameters of small intestinal function were normal. Proteinuria was initially present in both but cleared in one following treatment with B₁₂. These patients with “familial selective vitamin B₁₂ malabsorption” are the first reported from Canada. Only 37 cases have been reported in the world literature to date.     

Biosynthesis of transcobalamin II by adult rat liver parenchymal cells in culture.

The biosynthesis of transcobalamin II was investigated in primary cultures of adult rat liver parenchymal cells maintained in serum-free media. The data indicate that these hepatocytes secrete a vitamin B₁₂-binding substance into the culture medium which is identical to rat serum transcobalamin II as judged by the following criteria: (i) gel filtration on columns of Sephadex G-200; (ii) ion-exchange chromatography on columns of diethyl aminoethyl cellulose and carboxymethyl cellulose; (iii) polyacrylamide-gel electrophoresis at pH 9.5; and (iv) the ability to facilitate cellular vitamin B₁₂ uptake by HeLa cells and mouse L-929 fibroblasts in culture. The secretion of transcobalamin II by the liver parenchymal cells was blocked by cycloheximide, puromycin, and p-fluorophenylalanine. The inhibition by cycloheximide, but not that of the other inhibitors, was partially reversed upon removal of the drug. The liver parenchymal cells incorporated radioactive amino acids into transcobalamin II which was absorbed from the growth medium using affinity chromatography on Sepharose containing covalently linked B₁₂. Collectively, these data indicate that rat liver parenchymal cells, in culture, are capable of the biosynthesis de novo of transcobalamin II and the subsequent secretion of this protein into the culture media.     

Complete disulfide bond assignment of a recombinant immunoglobulin G4 monoclonal antibody

Surface plasmon resonance biosensor analysis was used to evaluate the thermodynamics and binding kinetics of naturally occurring and synthetic cobalamins interacting with vitamin B(12) binding proteins. Cyanocobalamin-b-(5-aminopentylamide) was immobilized on a biosensor chip surface to determine the affinity of different cobalamins for transcobalamin, intrinsic factor, and nonintrinsic factor. A solution competition binding assay, in which a surface immobilized cobalamin analog competes with analyte cobalamin for B(12) protein binding, shows that only recombinant human transcobalamin is sensitive to modification of the corrin ring b-propionamide of cyanocobalamin. A direct binding assay, where recombinant human transcobalamin is conjugated to a biosensor chip, allows kinetic analysis of cobalamin binding. Response data for cyanocobalamin binding to the transcobalamin protein surface were globally fitted to a bimolecular interaction model that includes a term for mass transport. This model yields association and dissociation rate constants of k(a) = 3 x 10(7) M(-1) s(-1) and k(d) = 6 x 10(-4) s(-1), respectively, with an overall dissociation constant of K(D) = 20 pM at 30 degrees C. Transcobalamin binds cyanocobalamin-b-(5-aminopentylamide) with association and dissociation rates that are twofold slower and threefold faster, respectively, than transcobalamin binding to cyanocobalamin. The affinities determined for protein-ligand interaction, using the solution competition and direct binding assays, are comparable, demonstrating that surface plasmon resonance provides a versatile way to study the molecular recognition properties of vitamin B(12) binding proteins.     

Effect of the cobalt-N coordination on the cobamide recognition by the human vitamin B12 binding proteins intrinsic factor, transcobalamin and haptocorrin

The binding of several corrinoids to the binding site of human intrinsic factor, transcobalamin or haptocorrin was investigated, p-Cresolyl cobamide and 2-amino-vitamin B12 are complete corrinoids, whose nucleotide at the lower face of the corrin ring is not coordinated to the cobalt. These corrinoids were greater than or equal to 10(3) times less efficiently recognized by intrinsic factor or transcobalamin than vitamin B12, which contains a Co-coordinated nucleotide. Pseudovitamin B12, with a weak Co-N coordination bond, revealed only moderate affinity to intrinsic factor. From these findings it is concluded that the cobamide binding to intrinsic factor and transcobalamin is strongly affected by the Co-N coordination bonds of their lower cobalt nucleotide ligands. We suggest that the Co-N coordination bond positions the nucleotide at a critical distance to the corrin ring, which is recognized by the binding proteins. Human haptocorrin, however, disclosed to distinctive selectivity regarding the different corrinoid structures. The protein bound all corrinoids with similar efficiency, independent of the strength of their Co-N coordinations, or the structures of their lower Co alpha ligands. Hence, the corrin ring, rather than a structural feature induced by the Co-N coordination, has to be considered responsible for the corrinoid binding to haptocorrin.     

An improved procedure for automated Edman degradation used for determination of the N-terminal amino acid sequence of human transcobalamin I and human intrinsic factor.

An improved procedure for automated Edman degradation is presented. Three programs are described, one with double cleavage and two with single cleavage. The programs presented are characterized by a reversed delivery scheme for buffer and phenyl isothiocyanate, and by reduced cleavage times. The modified procedures applied on automated Edman degradation of the vitamin B12-binding proteins human transcobalamin I and human intrinsic factor, containing approximately 390 and 350 amino residues respectively, gave the following N-terminal amino acid sequences: Human transcobalamin I Glu-Ile-Cys-Glu-Val-Ser-Glu-Glu-Asn-Tyr-Ile-Arg-Leu-Lys-Pro-Leu-Leu-Asn-Thr-Met-Ile-Gln-Ser-Asn-Tyr-Asn-?-Gly- Human intrinsic factor Ser-Thr-Gln-Thr-Gln-Ser-Ser-Cys-Ser-Val-Pro-Ser-Ala-Gln-Glu-Pro-Leu-Val-Asn-Gly-Ile-Gln-?-Leu-Met-Glu-Thr- The background accumulation seems to be related not only to the length of the polypeptide chain being degraded, but also to the content of serine (and possibly threonine). A possible N leads to O acyl shift during the cleavage is a tentative explanation. The programs here represented lead to a significant reduction in background compared to conventional programs and allowed considerable prolongation of the degradations.     

Porcine serum cobalophilin and transcobalamin. Identification, isolation and properties including electrofocusing patterns

Pooled porcine serum was found to contain cobalophilin (also called transcobalamin I) and transcobalamin (also called transcobalamin II). The two proteins were harvested by batchwise absorption with vitamin B-12 covalently coupled to Sepharose, and then separated from each other either by gel filtration or using an immunoadsorbent. Both proteins were finally isolated as single proteins using a second vitamin B-12-Sepharose chromatography step. Cobalophilin and transcobalamin complexed with vitamin B-12 had molecular weights by gel filtration of 135 000 and 38 000 and by the formula of Svedberg 104 000 and 44 000, Stokes radii 4.97 nm and 2.65 nm, and sedimentation coefficients 5.39 S and 3.75 S, respectively. Electrofocusing resolved the cobalophilin complex into three main isoproteins isoelectric at pH 3.23, 3.42 and 3.69, and transcobalamin into only the main component isoelectric at a value as low as pH 3.47. Neither protein was capable of binding to the ileal intrinsic factor receptor.     

Purification of human transcobalamin II-cyanocobalamin by affinity chromatography using thermolabile immobilization of cyanocobalamin.

Transcobalamin II-cyanocobalamin was isolated from Cohn fraction III of pooled human plasma by affinity chromatography on cyanocobalamin-Sepharose and some conventional separation methods. The affinity ligand cyanocobalamin was coupled to AH-Sepharose by a thermolabile linkage. The unsaturated binding protein was absorbed at 4 degrees C and eluted from the column at 37 degrees C as transcobalamin II-cyanocobalamin complex. The final preparation had a specific cyanocobalamin-binding capacity of 0.98 mol cyanocobalamin/mol transcobalamin II, the yield was 55% and the purification index amounted to 1.1 . 10(6). In dodecyl sulphate polyacrylamide gel electrophoresis one major protein band was observed at a molecular weight of 37 000 and a faint band at a molecular weight of 29 000. In polyacrylamide gel isolectric focusing the pure preparation turned out to be heterogeneous with isoelectric points ranging from pH 6.2 to 6.8, possibly by the occurrence of isoproteins.     

Gastric intrinsic factor: The gastric and small intestinal stages of cobalamin absorption. A personal journey

Intrinsic factor (IF) was first identified as a component of the gastric mucosa that reacted with an extrinsic factor, later discovered to be vitamin B12 (VB12). IF has been extensively characterized, and its cloned cDNA used to produce sufficient IF to produce high quality antibodies, and to elucidate its 3-dimensional structure bound to cobalamin (Cbl, VB12). The absorption of the IF–Cbl complex involves internalization by endocytosis, incorporation into multivesicular/lysosomal bodies, release of Cbl by lysosomal proteolysis and pH effects, with subsequent binding to transcobalamin (TC). Hereditary IF deficiency is rare, consistent with the need for IF to absorb Cbl, a vitamin essential for cell replication. When mutations occur, they are most often associated with loss of function, but some mutations occur outside the coding region. The IF-mediated intestinal uptake of Cbl has been harnessed for use as a transporter for peptides, proteins and even nanoparticles. Nanoparticle (NP) technology has produced Cbl-coated NPs that can incorporate peptides (insulin, IgG) that can be absorbed orally to function as hormones and antibodies in rodent models, but these systems are not yet ready for clinical use.     

Allelic forms of mouse transcobalamin 2

Transcobalamin 2 is the only vitamin B12-binding protein found in mouse serum. Two allelic forms of mouse transcobalamin 2 are described. The two forms differ in their mobilities on polyacrylamide gel electrophoresis. The slowly migrating form has been found in serum from 25 inbred mouse strains. The more rapidly migrating form was detected in 3 inbred mouse strains (NZB, ST/bJ, and CPB-WV). Both parental variants were expressed in F₁ progeny of appropriate interstrain crosses, showing codominant expression of the transcobalamin 2 alleles. In backcrosses between F₁ and parental individuals, the two electrophoretic variants were inherited as single Mendelian traits. The strain distribution pattern of the two variants in recombinant inbred lines likewise suggested a single-gene mode of inheritance and indicated a lack of close linkage with a number of genetic loci on chromosomes 1, 2, 4, 5, 6, 7, 9, 12, 14, 15, and 17. We propose the symbol Tcn-2 for the polymorphic gene locus coding for transcobalamin 2 in the mouse and Tcn-2 ^s and Tcn-2 ^f for the two alleles.     

Vitamin B12 transport by rat liver lysosomal membrane vesicles

Vitamin B12 (hydroxycobalamin) is endocytosed by mammalian cells as a complex with transcobalamin II and then processed to free B12 in lysosomes. The mechanism by which free B12 becomes available for subsequent cellular metabolism has been uncertain. Lysosomal transport of cyanocobalamin (B12) was examined using membrane vesicles prepared from Percoll gradient purified lysosomes. B12 uptake by vesicles was dependent upon pH and was inhibited by the protonophore CCCP. Transport exhibited saturation kinetics with a Km of 3.5 microM and temperature dependence with a Q10 of 1.8. Uptake of B12 was dependent upon divalent cations and was inhibited by EDTA. Preparation of vesicles in the presence of 100 microM B12 resulted in stimulation of uptake consistent with a mechanism of countertransport. Excess cyanocobalamin, adenosylcobalamin, methylcobalamin, or cobinamide dicyanide inhibited uptake of B12. Trans-stimulation studies showed that only the first three compounds are actually transported species with cyanocobalamin as the preferred substrate. We conclude that lysosomes have a specific transport system for vitamin B12 that results in release of this enzyme cofactor to the cytoplasm.     

Vitamin B12 absorption from eggs.

The assimilation of 57Co B12 from in vivo labeled eggs was much inferior to that of a comparable amount of crystalline 57Co B12. Furthermore, the absorption varied with the form in which the eggs were served. Judged by the urinary excretion test and the plasma absorption of radioactivity the average absorption from boiled and fried eggs was more than twice that from scrambled whole eggs, but less than half that absorbed from crystalline 57Co B12.     

Isolation of vitamin B-12-binding proteins by combined immuno and affinity chromatography. Comparative studies on the isolated and unisolated proteins

Intrinsic factor or cobalophilin were removed by incubating human gastric juice and pig pyloric extract with purified anti-intrinsic factor and anti-cobalophilin immunoglobulin-G, respectively, covalently coupled to Sepharose. Cobalophilin (transcobalamin I) was also removed from pig serum either by using anti-cobalophilin immunoglobulin-G Sepharose or by Sephadex G-200 chromatography. The one remaining semipurified vitamin B-12-binding protein (intrinsic factor, cobalophilin or transcobalamin II) was then isolated by vitamin B-12-Sepharose affinity chromatography. Intrinsic factors, cobalophilins and transcobalamin II isolated by this two-step procedure were compared by double isotope techniques with the corresponding protein not subjected to affinity chromatography and found to be identical in reaction to antiserum, gel filtration and electrofocusing. The avidity of the isolated and unisolated intrinsic factors for the ileal intrinsic factor receptor was also the same.