Synthesis and secretion of cobalamin binding proteins by opossum kidney cells

Assessment of cellular cobalamin metabolism in Gaucher disease

BACKGROUND

Gaucher disease (GD) is a lysosomal disorder caused by biallelic pathogenic mutations in the GBA1 gene that encodes beta-glucosidase (GCase), and more rarely, by a deficiency in the GCase activator, saposin C. Clinically, GD manifests with heterogeneous multiorgan involvement mainly affecting hematological, hepatic and neurological axes. This disorder is divided into three types, based on the absence (type I) or presence and severity (types II and III) of involvement of the central nervous system. At the cellular level, deficiency of GBA1 disturbs lysosomal storage with buildup of glucocerebroside. The consequences of disturbed lysosomal metabolism on biochemical pathways that require lysosomal processing are unknown. Abnormal systemic markers of cobalamin (Cbl, B₁₂) metabolism have been reported in patients with GD, suggesting impairments in lysosomal handling of Cbl or in its downstream utilization events.

METHODS

Cultured skin fibroblasts from control humans (n = 3), from patients with GD types I (n = 1), II (n = 1) and III (n = 1) and an asymptomatic carrier of GD were examined for their GCase enzymatic activity and lysosomal compartment intactness. Control human and GD fibroblasts were cultured in growth medium with and without 500 nM hydroxocobalamin supplementation. Cellular cobalamin status was examined via determination of metabolomic markers in cell lysate (intracellular) and conditioned culture medium (extracellular). The presence of transcobalamin (TC) in whole cell lysates was examined by Western blot.

RESULTS

Cultured skin fibroblasts from GD patients exhibited reduced GCase activity compared to healthy individuals and an asymptomatic carrier of GD, demonstrating a preserved disease phenotype in this cell type. The concentrations of total homocysteine (tHcy), methylmalonic acid (MMA), cysteine (Cys) and methionine (Met) in GD cells were comparable to control levels, except in one patient with GD III. The response of these metabolomic markers to supplementation with hydroxocobalamin (HOCbl) yielded variable results. The content of transcobalamin in whole cell lysates was comparable in control human and GD patients.

CONCLUSIONS

Our results indicate that cobalamin transport and cellular processing pathways are overall protected from lysosomal storage damage in GD fibroblasts. Extending these studies to hepatocytes, macrophages and plasma will shed light on cell- and compartment-specific vitamin B₁₂ metabolism in Gaucher disease.

Transcellular transport of cobalamin in aortic endothelial cells

Cobalamin [Cbl (or B₁₂)] deficiency causes megaloblastic anemia and a variety of neuropathies. However, homeostatic mechanisms of cyanocobalamin (CNCbl) and other Cbls by vascular endothelial cells are poorly understood. Herein, we describe our investigation into whether cultured bovine aortic endothelial cells (BAECs) perform transcytosis of B₁₂, namely, the complex formed between serum transcobalamin and B₁₂, designated as holo-transcobalamin (holo-TC). We show that cultured BAECs endocytose [⁵⁷Co]-CNCbl-TC (source material) via the CD320 receptor. The bound Cbl is transported across the cell both via exocytosis in its free form, [⁵⁷Co]-CNCbl, and via transcytosis as [⁵⁷Co]-CNCbl-TC. Transcellular mobilization of Cbl occurred in a bidirectional manner. A portion of the endocytosed [⁵⁷Co]-CNCbl was enzymatically processed by methylmalonic aciduria combined with homocystinuria type C (cblC) with subsequent formation of hydroxocobalamin, methylcobalamin, and adenosylcobalamin, which were also transported across the cell in a bidirectional manner. This demonstrates that transport mechanisms for Cbl in vascular endothelial cells do not discriminate between various β-axial ligands of the vitamin. Competition studies with apoprotein- and holo-TC and holo-intrinsic factor showed that only holo-TC was effective at inhibiting transcellular transport of Cbl. Incubation of BAECs with a blocking antibody against the extracellular domain of the CD320 receptor inhibited uptake and transcytosis by ∼40%. This study reveals that endothelial cells recycle uncommitted intracellular Cbl for downstream usage by other cell types and suggests that the endothelium is self-sufficient for the specific acquisition and subsequent distribution of circulating B₁₂ via the CD320 receptor. We posit that the endothelial lining of the vasculature is an essential component for the maintenance of serum-tissue homeostasis of B₁₂.-Hannibal, L., Bolisetty, K., Axhemi, A., DiBello, P. M., Quadros, E. V., Fedosov, S., Jacobsen, D. W. Transcellular transport of cobalamin in aortic endothelial cells.

Mouse transcobalamin has features resembling both human transcobalamin and haptocorrin

In humans, the cobalamin (Cbl) -binding protein transcobalamin (TC) transports Cbl from the intestine and into all the cells of the body, whereas the glycoprotein haptocorrin (HC), which is present in both blood and exocrine secretions, is able to bind also corrinoids other than Cbl. The aim of this study is to explore the expression of the Cbl-binding protein HC as well as TC in mice. BLAST analysis showed no homologous gene coding for HC in mice. Submaxillary glands and serum displayed one protein capable of binding Cbl. This Cbl-binding protein was purified from 300 submaxillary glands by affinity chromatography. Subsequent sequencing identified the protein as TC. Further characterization in terms of glycosylation status and binding specificity to the Cbl-analogue cobinamide revealed that mouse TC does not bind Concanavalin A sepharose (like human TC), but is capable of binding cobinamide (like human HC). Antibodies raised against mouse TC identified the protein in secretory cells of the submaxillary gland and in the ducts of the mammary gland, i.e. at locations where HC is also found in humans. Analysis of the TC-mRNA level showed a high TC transcript level in these glands and also in the kidney. By precipitation to insolubilised antibodies against mouse TC, we also showed that >97% of the Cbl-binding capacity and >98% of the Cbl were precipitated in serum. This indicates that TC is the only Cbl-binding protein in the mouse circulation. Our data show that TC but not HC is present in the mouse. Mouse TC is observed in tissues where humans express TC and/or HC. Mouse TC has features in common with both human TC and HC. Our results suggest that the Cbl-binding proteins present in the circulation and exocrine glands may vary amongst species.

Haptocorrin in humans

BACKGROUND

Evolutionary haptocorrin is the youngest of the cobalamin-binding proteins. It evolved by duplication of the intrinsic factor gene and has been identified in most mammals examined. Its ability to bind both cobalamin and analogues is well established, but apart from that, our knowledge concerning its function and its distribution in adult and foetal life is limited. In this study, we present data on the tissue expression of haptocorrin and on the relation between analogues on haptocorrin and vitamin B(12) status in humans.

METHODS

Polyclonal antibodies towards haptocorrin were used to study the localisation in foetal and adult tissues by immunohistochemistry. Positive immunoreactions were primarily observed in exocrine glands, the gastrointestinal tract and the respiratory system. ELISA was used for measurement of holo- and total haptocorrin in blood samples from individuals diagnosed with vitamin B(12) deficiency, based on measurement of methylmalonic acid (micromol/L) as evident (>0.75, n=61), suspected (0.29-0.75, n=155) or not present (<0.29, n=170). Cobalamins and holotranscobalamin were measured in the same individuals.

RESULTS

Holohaptocorrin was considerably higher than holohaptocorrin-cobalamins (cobalamins minus holotranscobalamin). The median (25th-75th percentile, pmol/L) for holohaptocorrin analogues (holohaptocorrin minus holohaptocorrin-cobalamins) was higher in deficient [200 (130-240)] compared to the non-deficient [140 (80-200)] individuals (analysis of variance and Tukey's multiple comparison test, p<0.01).

CONCLUSIONS

Our results indicate that haptocorrin is widely distributed also in foetal tissues and suggest analogues to accumulate on haptocorrin in vitamin B(12)-deficient individuals, a result that warrants further studies employing methods directly measuring cobalamins and analogues attached to haptocorrin.

The kidney in vitamin B12and folate homeostasis: characterization of receptors for tubular uptake of vitamins and carrier proteins

Over the past 10 years, animal studies have uncovered the molecular mechanisms for the renal tubular recovery of filtered vitamin and vitamin carrier proteins. Relatively few endocytic receptors are responsible for the proximal tubule uptake of a number of different vitamins, preventing urinary losses. In addition to vitamin conservation, tubular uptake by endocytosis is important to vitamin metabolism and homeostasis. The present review focuses on the receptors involved in renal tubular recovery of folate, vitamin B12, and their carrier proteins. The multiligand receptor megalin is important for the uptake and tubular accumulation of vitamin B12. During vitamin load, the kidney accumulates large amounts of free vitamin B12, suggesting a possible storage function. In addition, vitamin B12is metabolized in the kidney, suggesting a role in vitamin homeostasis. The folate receptor is important for the conservation of folate, mediating endocytosis of the vitamin. Interaction between the structurally closely related, soluble folate-binding protein and megalin suggests that megalin plays an additional role in the uptake of folate bound to filtered folate-binding protein. A third endocytic receptor, the intrinsic factor-B12receptor cubilin-amnionless complex, is essential to the renal tubular uptake of albumin, a carrier of folate. In conclusion, uptake is mediated by interaction with specific endocytic receptors also involved in the renal uptake of other vitamins and vitamin carriers. Little is known about the mechanisms regulating intracellular transport and release of vitamins, and whereas tubular uptake is a constitutive process, this may be regulated, e.g., by vitamin status.

Rat transcobalamin: cloning and regulation of mRNA expression

Transcobalamin (TC) has been cloned and used for studying its gene expression in the rat. TC mRNA is distributed widely in adult rat tissues, but at different levels (kidney > liver > lung > yolk sac > intestine > heart > brain > spleen > muscle). TC mRNA levels were 4-fold higher in the jejunum and ileum compared to its levels in the duodenum. During postnatal development, TC mRNA levels in the ileum declined 4-fold from day 4 to day 12, but increased by 5-fold between days 12 and 24. In contrast, TC mRNA levels increased by 2.5-fold in the kidney from day 4 to day 12 and then declined by 2-fold by day 24. Adrenalectomy of adult rats resulted in a 4-fold decline in ileal levels of TC mRNA and a 50% decline in the ileal mucosal formation of the TC-[(57)Co] cobalamin (Cbl) complex following oral administration of [(57)Co]Cbl complexed to gastric intrinsic factor (IF). Cortisone treatment reversed these changes noted in the ileum. In contrast to ileum, kidney TC mRNA levels were not altered significantly in adrenalectomized rats before and after cortisone treatment. Taken together, this study has provided evidence for the regulation of TC gene expression in the rat kidney and intestine during their postnatal development, and cortisone selectively regulates ileal but not kidney TC mRNA levels.

Function and stability of human transcobalamin II: role of intramolecular disulfide bonds C98-C291 and C147-C187

The current studies have investigated the role of three disulfide bonds of human transcobalamin II (TC II), a plasma transporter of cobalamin (Cbl; vitamin B12), in its function and stability. When translated in vitro in the presence or absence of microsomal vesicles, TC II constructs with a single substitution, C3S or C249S, demonstrated synthesis of a stable functional protein. However, TC II synthesized in the presence of microsomal vesicles using constructs with a single (C98S, C147S, C187S, C291S), double (C3/147/S, C98/147/S) or triple (C3/98/147/S) substitution was unstable. In the absence of microsomal vesicles, the percentage of binding to Cbl-Sepharose matrix by TC II expressed by constructs C3S, C3/147/S, C98/147/S, or C3/98/147/S was 100, 49, 52, and 35%, respectively. Upon their reductive alkylation, the binding of TC II expressed by these constructs was reduced to approximately 25-30%. TC II constructs C3S or C249S, when expressed in TC II-deficient fibroblasts, produced a stable functional protein, but those expressed by constructs C147S, C187S, C291S, C3/147/S, C98/147/S, or C3/98/147/S were rapidly degraded. The intracellular degradation of TC II expressed by these constructs was inhibited by lactacystin or MG-132 but not by the lysosomal degradation inhibitors ammonium chloride or chloroquine. These studies suggest that optimal binding of Cbl by human TC II is supported by disulfide bonds C98-C291 and C147-C187 and that their disruption results in loss of Cbl binding and their rapid degradation by the proteasomal machinery.

Megalin- and cubilin-mediated endocytosis of protein-bound vitamins, lipids, and hormones in polarized epithelia

Polarized epithelia have several functional and morphological similarities, including a high capacity for uptake of various substances present in the fluids facing the apical epithelial surfaces. Studies during the past decade have shown that receptor-mediated endocytosis, rather than nonspecific pinocytosis, accounts for the apical epithelial uptake of many carrier-bound nutrients and hormones. The two interacting receptors of distinct evolutionary origin, megalin and cubilin, are main receptors in this process. Both receptors are apically expressed in polarized epithelia, in which they function as biological affinity matrices for overlapping repertoires of ligands. The ability to bind multiple ligands is accounted for by a high number of replicated low-density lipoprotein receptor type-A repeats in megalin and CUB (complement C1r/C1s, Uegf, and bone morphogenic protein-1) domains in cubilin. Here we summarize and discuss the structural, genetic, and functional aspects of megalin and cubilin, with emphasis on their function as receptors for uptake of protein-associated vitamins, lipids, and hormones.

Transcellular Transport of Vitamin B12in LLC-PK1 Renal Proximal Tubule Cells

Abstract. The transcobalamin-vitamin B12complex is responsible for the transport of B12from plasma and into the tissues. The complex is filtered in the renal glomeruli and is a high-affinity ligand for the endocytic receptor megalin expressed in the proximal tubule. This study shows by the use of the proximal tubule LLC-PK1 cell line that transcobalamin-B12is internalized by megalin-mediated endocytosis. After endocytosis and accumulation in endosomes, transcobalamin is degraded and the B12molecule is released from the cells in complex with newly synthesized proteins. The release is polarized in such a way that vitamin in the apical medium is bound to proteins with the size of haptocorrin, whereas the B12released at the basolateral side is complexed to two different proteins with the sizes of transcobalamin and haptocorrin. Furthermore, transcobalamin mRNA was identified by reverse transcription-PCR in LLC-PK1 cells and human and pig kidney, whereas haptocorrin mRNA was identified only in LLC-PK1 cells. The results strongly suggest that megalin located in the proximal tubule cells is important for receptor-mediated tubular reabsorption followed by transcellular transport and release of vitamin B12complexed to newly synthesized carrier proteins. This mechanism is likely to play a significant role in the maintenance of B12homeostasis by returning filtered B12to the pool of circulating vitamin.

Transfer of cobalamin from intrinsic factor to transcobalamin II

The process is obscure by which cobalamin (Cbl) in the endocytosed intrinsic factor (IF)-cobalamin (Cbl) complex is released and transferred to transcobalamin II (TCII) within the enterocyte. Using recombinant IF and TCII, binding of Cbl to IF at pH 5.0 was 70% of binding at pH 7.0, whereas for TCII alone, the value was only 12%. TCII binding activity was lost rapidly at lower pH, but this was not due to protease action. TCII incubated at pH 5.0 with cathepsin L was degraded and could not subsequently bind Cbl. Thus, transfer from IF to TCII is unlikely to occur within an acid compartment. Only 13-15% of bound Cbl was released at pH 5.0 and pH 6.0 from either rat IF, human IF, or human TCII. The K(a) of human or rat IF at pH 7.5 was 2.2 nM; for TCII, the value was 0.34 nM. At pH 7.5, Cbl transfers from IF to TCII, but only to a limited extent (21%), as detected by nondenaturing electrophoresis. Transfer of Cbl from IF to TCII could not be demonstrated at pH values of 5.0 or 6.0. Thus, luminal transfer of Cbl between IF and TCII is likely to be limited, but is possible. The most likely mechanism for intracellular transfer of Cbl from IF to TCII involves initial lysosomal proteolysis of IF, with subsequent Cbl binding to TCII in a more neutral cellular compartment.

Production of gastric intrinsic factor, transcobalamin, and haptocorrin in opossum kidney cells

Opossum kidney epithelial cells were shown previously to synthesize and secrete two cobalamin (Cbl)-binding proteins, presumed to be haptocorrin (Hc) and transcobalamin II (TCII). The present study examines the hypothesis that renal tubular cells also produce intrinsic factor (IF), and this production provides an explanation for the presence of IF in urine. By using antisera raised against human IF and against TCII, the presence of TCII was confirmed, and that of IF discovered in the media of opossum kidney (OK) cells in culture. The apparent molecular weight of IF and TCII was 68 and 43 kDa, respectively. Immunoreactivity on Western blot of the putative IF protein was blocked by recombinant human IF. When proteins secreted into the media were separated electrophoretically under nondenaturing conditions after binding with [(57)Co]Cbl, a broad major band migrated at a relative front independently of recombinant IF or TCII, and probably represents Hc, as the Cbl binding is blocked by cobinamide. Small amounts of bound [(57)Co]Cbl migrated in the position of both IF and TCII, when cobinamide was present. The presence of IF and TCII in OK cells was confirmed by immunohistology. Specific reactivity for IF (blocked by recombinant IF) was found in proximal tubules of opossum kidney, but not in other portions of the nephron, confirming the ability of anti-human IF antiserum to detect opossum IF. A 732-bp fragment of IF, nearly identical in sequence to rat IF, was isolated by RT-PCR from opossum kidney mRNA, and Western blot confirmed the presence of IF protein. The presence of IF was also documented in rat kidney by isolation of an RT-PCR fragment, immunocytochemistry, and Western blot. IF should be added to the list of renal (proximal) tubular antigens that are shared by other epithelia.

Uptake of lipoproteins for axonal growth of sympathetic neurons

Lipoproteins originating from axon and myelin breakdown in injured peripheral nerves are believed to supply cholesterol to regenerating axons. We have used compartmented cultures of rat sympathetic neurons to investigate the utilization of lipids from lipoproteins for axon elongation. Lipids and proteins from human low density lipoproteins (LDL) and high density lipoproteins (HDL) were taken up by distal axons and transported to cell bodies, whereas cell bodies/proximal axons internalized these components from only LDL, not HDL. Consistent with these observations, the impairment of axonal growth, induced by inhibition of cholesterol synthesis, was reversed when LDL or HDL were added to distal axons or when LDL, but not HDL, were added to cell bodies. LDL receptors (LDLRs) and LR7/8B (apoER2) were present in cell bodies/proximal axons and distal axons, with LDLRs being more abundant in the former. Inhibition of cholesterol biosynthesis increased LDLR expression in cell bodies/proximal axons but not distal axons. LR11 (SorLA) was restricted to cell bodies/proximal axons and was undetectable in distal axons. Neither the LDL receptor-related protein nor the HDL receptor, SR-B1, was detected in sympathetic neurons. These studies demonstrate for the first time that lipids are taken up from lipoproteins by sympathetic neurons for use in axonal regeneration.

Vitamin B12 transporters

The uptake of vitamin B12 from the intestine into the circulation is perhaps the most complex uptake mechanism of all the vitamins, involving no less than five separate VB12-binding molecules, receptors and transporters. Each molecule involved in uptake has a separate affinity and specificity for VB12 as well as a separate cell receptor. Thus VB12 is initially bound by haptocorrin in the stomach, then by IF in the small intestine. An IF receptor is then involved in uptake of the IF-VB12 complex by the intestinal epithelial cell, with the subsequent proteolytic release of VB12 and subsequent binding to TcII. The TcII receptor then transports the TcII-VB12 complex across the cell, whence it is released into the circulation. It is surprising, then, that despite its complexity, it has been possible to harness the vitamin VB12 uptake mechanism to enhance the oral uptake of peptides, proteins, and nanoparticles.

Transcobalamin II and its cell surface receptor

Transcobalamin II (TC II), a nonglycoprotein secretory protein of molecular mass 43 kDa, and its plasma membrane receptor (TC II-R), a heavily glycosylated protein with a monomeric molecular mass of 62 kDa, are essential components of plasma cobalamin (Cbl; vitamin B12) transport to all cells. Evidence from studies over the past 10 years has provided some important information on their structure, regulation of expression, and function. Some of the specific findings include (a) identification of the structural relationship of the ligand TC II with other members of the Cbl-binding family of proteins, intrinsic factor (IF) and haptocorrin (HC), (b) regulation of TC II gene expression, (c) molecular basis for human TC II deficiency in patients with a lack of plasma TC II, (d) membrane expression, interactions, and dimerization of TC II-R, and (e) targeting and function of TC II-R in polarized epithelial cells. It is hoped that some of the recent findings presented in this review will provide new insights into the structure and function of these two fascinating proteins and stimulate future research in this area.

Receptor-mediated endocytosis of cobalamin (vitamin B12)

Dietary cobalamin (Cbl) (vitamin B12) is utilized as methyl-Cbl and the coenzyme 5'-deoxyadenosyl Cbl by cells of the body that have the enzymes methionine synthase and methyl malonyl CoA mutase, which convert homocysteine to methionine and methyl malonyl CoA to succinyl CoA, respectively. Prior to conversions and utilizations as the active alkyl forms of Cbl, dietary Cbl is absorbed and transported across cellular plasma membranes by two receptor-mediated events. First, dietary and biliary Cbl bound to gastric intrinsic factor (IF) presented apically to the ileal absorptive enterocytes is transported to the circulation by receptor-mediated endocytosis via apically expressed IF-Cbl receptor. Second, Cbl bound to plasma transcobalamin (TC) II is taken up from the circulation by all cells via a TC II receptor expressed in the plasma membrane of these cells, and in polarized cells via a TC II receptor expressed in the basolateral membranes. This review updates recent work and focuses on (a) the molecular and cellular aspects of Cbl binding protein ligands, IF and TC II, and their cell-surface receptors, IF-Cbl receptor and TC II receptor; (b) the cellular sorting pathways of internalized Cbl bound to IF and TC II in polarized epithelial cells; and (c) the absorption and transport disorders that cause Cbl deficiency.

Bipolar functional expression of transcobalamin II receptor in human intestinal epithelial Caco-2 cells

Transcobalamin II (TC II) receptor is expressed in the apical and basolateral membranes of human intestinal mucosa and in post-confluent human intestinal epithelial Caco-2 cells with a 6-7-fold enrichment in basolateral membranes. Caco-2 cells grown on culture inserts bound (at 5 degrees C) 30 and 180 fmol of the ligand, TC II-[57Co]cobalamin (Cbl), to the apical and the basolateral surfaces, respectively. Within 5 h at 37 degrees C, all apically bound Cbl was internalized and subsequently transcytosed as TC II-Cbl. In contrast, all basolateral surface-bound Cbl was internalized and retained by the cells, but transferred from TC II to other cellular proteins. Chloroquine or leupeptin had no effect on the apical to basolateral transcytosis of either [57Co]Cbl or 125I-TC II. In contrast, following basolateral internalization of the ligand, both chloroquine and leupeptin inhibited the intracellular degradation of 125I-TC II, which resulted in secretion of 60-65% of TC II-Cbl complex into the basolateral medium. When 125I-TC II-Cbl was orally administered to rats, intact labeled TC II was detected in the portal blood 4 and 8 h later. These studies suggest that TC II-Cbl is processed when presented to the (a) apical/luminal side by a hitherto unrecognized non-lysosomal pathway in which both TC II and Cbl are transcytosed and (b) basolateral side by the lysosomal pathway in which TC II is degraded and the released Cbl is utilized.

In vitro and in vivo inactivation of transcobalamin II receptor by its antiserum

Rabbits injected with pure human placental transcobalamin II-receptor (TC II-R) failed to thrive with no apparent tissue or organ damage, but a 2-fold elevation of the metabolites, homocysteine, methylmalonic acid, and the ligand, transcobalamin II, in their plasma. Exogenously added transcobalamin II-[57Co]cyanocobalamin bound very poorly (2-5%) to the affected rabbit liver, kidney, and intestinal total or intestinal basolateral membrane extracts relative to the binding by membrane extracts from normal rabbit tissues. The activity was restored to normal values following a wash of affected rabbit tissue membranes with pH 3 buffer containing 200 mM potassium thiocyanate. Immunoblot analysis of normal and affected rabbit kidney and liver total membranes revealed similar amounts of 124-kDa TC II-R dimer protein. The neutralized and dialyzed extract from the affected rabbit membranes inhibited the binding of the ligand to pure TC II-R and the harvested affected rabbit serum inhibited the uptake of TC II-[57Co]cobalamin (Cbl) from the basolateral side of human intestinal epithelial (Caco-2) cells and decreased the utilization of [57Co]Cbl as coenzymes by the Cbl-dependent enzymes. The loss of exogenously added ligand binding or the binding of 125I-protein A occurred with the intestinal basolateral, but not the apical membranes. Based on these results, we suggest that circulatory antibodies to TC II-R cause its in vivo functional inactivation, suppress Cbl uptake by multiple tissues, and thus cause severe Cbl deficiency and the noted failure to thrive.

Cobalamin

Cobalamin (vitamin B12) is an essential nutrient derived exclusively from bacterial sources. It is an essential cofactor for three known enzymatic reactions. Untreated deficiency, caused by either the autoimmune disease pernicious anemia or nutritional lack, results in a macrocytic anemia and/or subacute combined degeneration of the spinal cord and is eventually fatal. Cobalamin in serum is bound to two proteins, transcobalamin and haptocorrin. The former is responsible for the essential delivery of cobalamin to most tissues. Inadequate tissue availability of cobalamin results in increased concentration of methylmalonic acid and homocyst(e)ine due to inhibition of methylmalonyl-CoA mutase and methionine synthase, respectively. Strict vegetarians have long been known to be at risk of cobalamin deficiency, which develops insidiously over many years. It is now clear that a significant number of the elderly and HIV-positive individuals are also at increased risk of deficiency. Any individual with reduced ability to split cobalamin from food-protein may also become deficient even though intrinsic factor is present. Diagnosis of cobalamin deficiency has frequently relied on total serum cobalamin and the Schilling test. Newer approaches such as analysis of methylmalonic acid, homocyst(e)ine, holotranscobalamin, anti-intrinsic factor antibodies, and serum gastrin may provide more cost-effective testing, as well as identify those with a covert deficiency.

Membrane expression and interactions of human transcobalamin II receptor

Antiserum raised to purified 62-kDa human placental transcobalamin II receptor (TC II-R) has been used to study its synthesis and membrane expression. The antiserum immunoprecipitated a 45-kDa protein from the cell-free translation using human kidney mRNA and recognized a single 124-kDa band on immunoblotting of placental and other human tissue membranes, and quantitation of the blots revealed high levels of TC II-R expression in the human kidney followed by placenta, intestine, and liver. Triton X-100 extraction of placental membranes resulted in the complete (100%) solubilization of the receptor, and immunoblotting of the Triton X-100-soluble fraction revealed a single band of 62 kDa. Lipid extraction of placental membranes with a mixture of chloroformmethanol (2:1) followed by immunoblotting revealed a single band of molecular mass 62 kDa. The molecular mass of the pure Triton X-100-bound receptor increased on SDS-polyacrylamide gel electrophoresis from 62 to 124 kDa upon its insertion in liposomes prepared using egg phosphatidylcholine and cholesterol. Chemical cross-linking of native membrane-or lipid vesicle-bound TC II-R or detergent-soluble extracts of the membrane with 125I-TC II-cobalamin revealed that both the 124- and 62-kDa forms of the receptor were active in ligand binding. Based on these results we suggest that TC II-R is synthesized as a single polypeptide of 45 kDa, and following its maturation (involving N- and O-glycosylation) the 62-kDa mature receptor is expressed in plasma membranes as a noncovalent dimer of 124 kDa. The dimerization of TC II-R in the plasma membranes is due to its interactions with annular lipids.

Expression of transcobalamin II mRNA in human tissues and cultured fibroblasts from normal and transcobalamin II-deficient patients

Transcobalamin II (TCII) is an important plasma transporter of cobalamin (Cbl; vitamin B12). In the present study, TCII gene expression in human and rat tissues and in the fibroblasts of patients with TCII deficiency was investigated. Northern-blot analyses revealed expression of TCII mRNA in many human and rat tissues. In humans, this was 14-fold higher in the kidney than in liver, whereas in the rat the levels of expression were similar in the kidney and liver. Southern-blot analysis of genomic DNA from several species revealed sequence similarity in TCII across species. Metabolic labelling and ribonuclease protection assay revealed a 43 kDa TCII protein and a fully protected TCII mRNA band in normal fibroblasts but not in fibroblasts from three TCII-deficient patients. Southern-blot analysis of genomic DNA from all these fibroblasts revealed identical restriction patterns on BamHI, HindIII, KpnI, MspI and EcoRI digestion. On the basis of these results, we suggest that TCII is expressed in multiple tissues, and its level of expression in tissues varies within the same and across species. Furthermore, the TCII deficiency characterized in this study is due to the absence of TCII protein which in turn is due to the absence or extremely low levels of its mRNA and not to detectable gross alterations in the gene structure.

Intrinsic factor-cobalamin receptor activity in a marsupial, the American opossum (Didelphis virginiana)

1. Significant and specific binding of intrinsic factor-cobalamin occurred in proximal but not in the distal half of the intestine in an adult marsupial, the American opossum. 2. The purified opossum kidney receptor, like rat and canine kidney receptors, revealed a single band of M(r) approximately 230 on SDS-PAGE. However, unlike the rat and canine receptors, the opossum receptor was sensitive to both Endoglycosidase H and peptide-N-glycosidase F. 3. The opossum intrinsic factor-cobalamin receptor demonstrated a ten-fold higher affinity for intrinsic factor-cobalamin complex when the source of IF was from the opossum pancreas, rather than rat stomach. 4. The opossum kidney receptor had low immune-crossreactivity with anti-serum raised to rat and canine kidney receptor. 5. These studies suggest that intrinsic factor-cobalamin receptor expressed in the American opossum, though conserved, appears to be structurally different from the rat and canine receptors.

Leupeptin and ammonium chloride inhibit intrinsic factor mediated transcytosis of [57Co]cobalamin across polarized renal epithelial cells

The [125I] intrinsic factor (IF) mediated transcytosis of [57Co]Cyanocobalamin (Cbl) by polarized opossum kidney cells was inhibited (greater than 80%) by preincubation of the cells with lysosomotropic agents leupeptin or ammonium chloride. Inhibition of Cbl transcytosis resulted in the intracellular accumulation of both [125I]IF (48 kDa) and [57Co]Cbl. Intracellular degradation of [125I]IF occurred during normal cellular transcytosis of [57Co]Cbl and in one h following internalization the major intracellular degradation products of IF were two polypeptides of Mr 29 kDa and 19 kDa. The size of the major degradation product of IF in the basolateral media was 10 kDa. Based on these results, we suggest that IF is internalized by the renal epithelial cells and is degraded by leupeptin-sensitive acid proteases during Cbl transcytosis.

Functional expression of transcobalamin II cDNA in Xenopus laevis oocytes

The products of in vitro transcription of human transcobalamin II (TC II) cDNA when microinjected into Xenopus laevis oocytes yielded a single secretory protein of 43 kDa. The mobility of the 43 kDa band did not change following digestion with peptide N-glycosidase F. [57Co]Cbl bound to the medium was immunoprecipitated with anti-serum to human TC II, but not to other Cbl binders. In addition, the [57Co]Cbl complex also bound to placental microsomes. These results suggest that TC II mRNA transcribed encodes TC II which contains both the Cbl and receptor binding domains. Furthermore, Xenopus oocytes can be used as a screening system to define structural elements important in TC II's secretion and binding reactions.