Inherited disorders of cobalamin metabolism

Efficacy and tolerance of oral versus parenteral cyanocobalamin supplement in hypocobalaminaemic dogs with chronic enteropathy: a controlled randomised open-label trial

OBJECTIVES

Determine comparative tolerance of daily oral and weekly parenteral cobalamin supplementation, in hypocobalaminaemic dogs with chronic enteropathy. Determine whether oral is as effective as parenteral supplementation at achieving eucobalaminaemia, in hypocobalaminaemic dogs with protein-losing enteropathy, severe hypocobalaminaemia or high canine inflammatory bowel disease activity index at inclusion.

MATERIALS AND METHODS

Thirty-seven client-owned dogs with hypocobalaminaemia and clinical signs of chronic enteropathy were prospectively enrolled in three UK referral centres. Dogs were randomly allocated to daily oral for 12 weeks or weekly parenteral cobalamin supplementation for 6 weeks and one additional dose 4 weeks later. Serum cobalamin, body condition score, canine inflammatory bowel disease activity index and bodyweight were assessed at inclusion, weeks 7 and 13. Serum methylmalonic acid concentration was evaluated at inclusion and at week 13. Owners completed treatment adherence, palatability, tolerance and satisfaction questionnaires at week 13.

RESULTS

Nineteen dogs completed the study. All dogs orally supplemented achieved normal or increased cobalaminaemia at weeks 7 and 13. There was no statistical difference in cobalamin concentration at week 13 in dogs treated with oral or parenteral supplementation, regardless of presence of protein-losing enteropathy, severity of hypocobalaminaemia or canine inflammatory bowel disease activity index at inclusion. Serum methylmalonic acid concentration was not significantly different between oral and parenteral groups, neither were treatment adherence, satisfaction, and tolerance scores at week 13.

CLINICAL SIGNIFICANCE

Oral is as effective and as well-tolerated as parenteral cobalamin supplementation in hypocobalaminaemic dogs with chronic enteropathy and severe clinical or biochemical phenotypes, and should be considered as a suitable treatment option regardless of disease severity.

Expression of the cobalamin transporters cubam and MRP1 in the canine ileum-Upregulation in chronic inflammatory enteropathy

Chronic inflammatory enteropathy (CIE) in dogs, a spontaneous model of human inflammatory bowel disease (IBD), is associated with a high rate of cobalamin deficiency. The etiology of hypocobalaminemia in human IBD and canine CIE remains unknown, and compromised intestinal uptake of cobalamin resulting from ileal cobalamin receptor deficiency has been proposed as a possible cause. Here, we evaluated the intestinal expression of the cobalamin receptor subunits, amnionless (AMN) and cubilin (CUBN), and the basolateral efflux transporter multi-drug resistance protein 1 (MRP1) in 22 dogs with CIE in comparison to healthy dogs. Epithelial CUBN and AMN levels were quantified by confocal laser scanning microscopy using immunohistochemistry in endoscopic ileal biopsies from dogs with (i) CIE and normocobalaminemia, (ii) CIE and suboptimal serum cobalamin status, (iii) CIE and severe hypocobalaminemia, and (iv) healthy controls. CUBN and MRP1 expression was quantified by RT-qPCR. Receptor expression was evaluated for correlation with clinical patient data. Ileal mucosal protein levels of AMN and CUBN as well as mRNA levels of CUBN and MRP1 were significantly increased in dogs with CIE compared to healthy controls. Ileal cobalamin receptor expression was positively correlated with age, clinical disease activity index (CCECAI) score, and lacteal dilation in the ileum, inversely correlated with serum folate concentrations, but was not associated with serum cobalamin concentrations. Cobalamin receptor downregulation does not appear to be the primary cause of hypocobalaminemia in canine CIE. In dogs of older age with severe clinical signs and/or microscopic intestinal lesions, intestinal cobalamin receptor upregulation is proposed as a mechanism to compensate for CIE-associated hypocobalaminemia. These results support oral supplementation strategies in hypocobalaminemic CIE patients.

Review of cobalamin status and disorders of cobalamin metabolism in dogs

Disorders of cobalamin (vitamin B₁₂) metabolism are increasingly recognized in small animal medicine and have a variety of causes ranging from chronic gastrointestinal disease to hereditary defects in cobalamin metabolism. Measurement of serum cobalamin concentration, often in combination with serum folate concentration, is routinely performed as a diagnostic test in clinical practice. While the detection of hypocobalaminemia has therapeutic implications, interpretation of cobalamin status in dogs can be challenging. The aim of this review is to define hypocobalaminemia and cobalamin deficiency, normocobalaminemia, and hypercobalaminemia in dogs, describe known cobalamin deficiency states, breed predispositions in dogs, discuss the different biomarkers of importance for evaluating cobalamin status in dogs, and discuss the management of dogs with hypocobalaminemia.

Anticancer activity of metal complexes: involvement of redox processes

Cells require tight regulation of the intracellular redox balance and consequently of reactive oxygen species for proper redox signaling and maintenance of metal (e.g., of iron and copper) homeostasis. In several diseases, including cancer, this balance is disturbed. Therefore, anticancer drugs targeting the redox systems, for example, glutathione and thioredoxin, have entered focus of interest. Anticancer metal complexes (platinum, gold, arsenic, ruthenium, rhodium, copper, vanadium, cobalt, manganese, gadolinium, and molybdenum) have been shown to strongly interact with or even disturb cellular redox homeostasis. In this context, especially the hypothesis of "activation by reduction" as well as the "hard and soft acids and bases" theory with respect to coordination of metal ions to cellular ligands represent important concepts to understand the molecular modes of action of anticancer metal drugs. The aim of this review is to highlight specific interactions of metal-based anticancer drugs with the cellular redox homeostasis and to explain this behavior by considering chemical properties of the respective anticancer metal complexes currently either in (pre)clinical development or in daily clinical routine in oncology.

Partial characterization of cobalamin deficiency in Chinese Shar Peis

A total of 22,462 serum sample results from dogs being evaluated for gastrointestinal disease at the Gastrointestinal Laboratory, College of Veterinary Medicine, Texas A&M University were evaluated retrospectively. The proportion of dogs with serum cobalamin concentrations below the reference interval and median serum concentrations were compared between Shar Peis and other dog breeds. Serum samples were also obtained prospectively from 22 healthy and 32 Shar Peis with chronic gastrointestinal disease and 59 healthy dogs of other breeds, and serum concentrations of cobalamin, folate, and methylmalonic acid were determined and compared. Overall, 64.0% (89/139) of serum samples from Shar Peis showed serum cobalamin concentrations below the limit of the reference interval and 38.1% (53/139) of these were below the detectable limit for the assay. The median serum cobalamin concentration in Shar Peis was significantly lower than in other breeds. Shar Peis with gastrointestinal disease had significantly lower serum cobalamin and higher serum methylmalonic acid concentrations compared to healthy Shar Peis. Healthy Shar Peis had significantly increased serum methylmalonic acid concentrations compared to healthy dogs of other breeds. There were no meaningful differences in folate concentrations between groups. In conclusion, Shar Peis have a high prevalence of cobalamin deficiency compared to other breeds and healthy Shar Peis may have subclinical cobalamin deficiency.

Thermolability of mutant MMACHC protein in the vitamin B12-responsive cblC disorder

Methylmalonic aciduria and homocystinuria, cblC type, is the most common inborn error of cellular vitamin B12 metabolism. We previously showed that the protein carrying the mutation responsible for late-onset cblC (MMACHC-R161Q), treatable with high dose OHCbl, is able to bind OHCbl with wild-type affinity, leaving undetermined the disease mechanism involved [Froese et al., Mechanism of responsiveness, Mol. Genet. Metab. (2009).]. To assess whether the mutation renders the protein unstable, we investigated the thermostability of the wild-type and mutant MMACHC proteins, either unbound or bound to different cobalamins (Cbl), using differential scanning fluorimetry. We found that MMACHC-wt and MMACHC-R161Q are both very thermolabile proteins in their apo forms, with melting temperatures (T(m)) of 39.3+/-1.0 and 37.1+/-0.7 degrees C, respectively; a difference confirmed by unfolding of MMACHC-R161Q but not MMACHC-wt by isothermal denaturation at 35 degrees C over 120 min. However, with the addition of OHCbl, MMACHC-wt becomes significantly stabilized (Delta T(m max)=8 degrees C, half-maximal effective ligand concentration, AC(50)=3 microM). We surveyed the effect of different cobalamins on the stabilization of the wild-type protein and found that AdoCbl was the most stabilizing, exerting a maximum increase in T(m) of approximately 16 degrees C, followed by MeCbl at approximately 13 degrees C, each evaluated at 50 microM cofactor. The other cobalamins stabilized in the order (CN)(2)Cbi>OHCbl>CNCbl. Interestingly, the AC(50)'s for AdoCbl, MeCbl, (CN)(2)Cbi and OHCbl were similar and ranged from 1-3 microM, which compares well with the K(d) of 6 microM for OHCbl [Froese et al., Mechanism of responsiveness, Mol. Genet. Metab. (2009).]. Unlike MMACHC-wt, the mutant protein MMACHC-R161Q is only moderately stabilized by OHCbl (Delta T(m max)=4 degrees C). The dose-response curve also shows a lower effectivity of OHCbl with respect to stabilization, with an AC(50) of 7 microM. MMACHC-R161Q showed the same order of stabilization as MMACHC-wt, but each cobalamin stabilized this mutant protein less than its wild-type counterpart. Additionally, MMACHC-R161Q had a higher AC(50) for each cobalamin form compared to MMACHC-wt. Finally, we show that MMACHC-R161Q is able to support the base-off transition for AdoCbl and CNCbl, indicating this mutant is not blocked in that respect. Taken together, our results suggest that protein stability, as well as propensity for ligand-induced stabilization, contributes to the disease mechanism in late-onset cblC disorder. Our results underscore the importance of cofactor stabilization of MMACHC and suggest that even small increases in the concentration of cobalamin complexed with MMACHC may have therapeutic benefit in children with the late-onset, vitamin responsive cblC disease.

The reduced folate carrier (RFC) is cytotoxic to cells under conditions of severe folate deprivation. RFC as a double edged sword in folate homeostasis

The reduced folate carrier (RFC), a bidirectional anion transporter, is the major uptake route of reduced folates essential for a spectrum of biochemical reactions and thus cellular proliferation. However, here we show that ectopic overexpression of the RFC, but not of folate receptor alpha, a high affinity unidirectional folate uptake route serving here as a negative control, resulted in an approximately 15-fold decline in cellular viability in medium lacking folates but not in folate-containing medium. Moreover to explore possible mechanisms of adaptation to folate deficiency in various cell lines that express the endogenous RFC, we first determined the gene expression status of the following genes: (a) RFC, (b) ATP-driven folate exporters (i.e. MRP1, MRP5, and breast cancer resistance protein), and (c) folylpoly-gamma-glutamate synthetase and gamma-glutamate hydrolase (GGH), enzymes catalyzing folate polyglutamylation and hydrolysis, respectively. Upon 3-7 days of folate deprivation, semiquantitative reverse transcription-PCR analysis revealed a specific approximately 2.5-fold decrease in RFC mRNA levels in both breast cancer and T-cell leukemia cell lines that was accompanied by a consistent fall in methotrexate influx, serving here as an RFC transport activity assay. Likewise a 2.4-fold decrease in GGH mRNA levels and approximately 19% decreased GGH activity was documented for folate-deprived breast cancer cells. These results along with those of a novel mathematical biomodeling devised here suggest that upon severe short term (i.e. up to 7 days) folate deprivation RFC transport activity becomes detrimental as RFC, but not ATP-driven folate exporters, efficiently extrudes folate monoglutamates out of cells. Hence down-regulation of RFC and GGH may serve as a novel adaptive response to severe folate deficiency.

Oral pharmacologic doses of cobalamin may not be as effective as parenteral cobalamin therapy in reversing hyperhomocystinemia and methylmalonic acidemia in apparently normal subjects

A postmenopausal female evaluated for thrombophilia because of bone infarcts had mild hyperhomocysteinemia, which increased when hormone replacement was discontinued. Serum folate, cobalamin and methylmalonic acid were normal. Compound heterozygosity for C677T/A1298C methylenetetrahydrofolate reductase polymorphisms was present but oral folic acid failed to lower homocysteine and actually increased methylmalonic acid. Oral cobalamin therapy increased serum cobalamin and partially decreased methylmalonic acid but had no effect on homocysteine. Homocysteine remained unchanged after 11 months of oral cobalamin, folic acid and pyridoxine therapy. However, intramuscular cobalamin promptly decreased both metabolites to normal. Thus, parenteral cobalamin therapy may have greater metabolic effects than oral vitamin therapy even in apparently normal subjects.

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.

B12 trafficking in mammals: A for coenzyme escort service

Many coenzymes are vitamins that are assimilated in mammals into their active form from precursors obtained from the diet. They are often both rare and reactive rendering the likelihood low that the cell uses a collision-based strategy for their delivery to dependent enzymes. In humans, there are only two known B12 or cobalamin-dependent enzymes: methionine synthase and methylmalonyl-CoA mutase. However, the pathway for intracellular assimilation and utilization of this cofactor is complex as revealed by careful clinical analyses of fibroblasts from patients with disorders of cobalamin metabolism. In the recent past, six of the eight human genes involved in the B12 pathway have been identified and these have yielded important insights into their roles. The recent literature on the encoded proteins is reviewed, and a model for intracellular B12 trafficking is proposed in which B12 is escorted to its target proteins in the cytoplasmic and mitochondrial compartments in complex with chaperones, thereby averting problems of dilution and adventitious side reactions.

Modulation of intracellular Ca(2+) concentration by vitamin B12 in rat thymocytes

We have studied several novel effects of vitamin B12 (cyanocobalamin) on cellular Ca(2+) homeostasis in rat thymocytes. We determined the effect of various concentrations of vitamin B12 on intracellular Ca(2+) concentration ([Ca(2+)]i) and parameters of Ca(2+)in signaling using the fluorescent dye Fura-2. The basal [Ca(2+)]i in Ca(2+)-containing media was 115 +/- 5 nM but in vitamin B12 (10 nM)-treated thymocytes [Ca(2+)]i was decreased to 60 +/- 15 nM (mean +/- SEM) during the first 5 min. The decline in [Ca(2+)]i was accompanied by an increase in the endoplasmic reticulum Ca(2+) store, presumably as a result of Ca-ATPase activation. At the same time 100 nM-10 mM B12 induced the accumulation of Ca(2+) in mitochondria. Somewhat higher concentrations of B12 (1-10 microM) had no effect on [Ca(2+)]i. A further increase in B12 concentration with range from 50 microM to 1 mM caused a dose-dependent elevation of [Ca(2+)]i from the basal level (115 +/- 5 nM) up to 200 +/- 50 nM in thymocytes, and this elevation was partially blocked in Ca(2+)-free media. This high concentration of vitamin B12 caused a gradual decrease of endoplasmic reticulum Ca(2+) stores by means of Ca-ATPase inhibition. The B12-induced increase in [Ca(2+)]i was not observed after depletion of intracellular Ca(2+) stores, induced by addition of 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ), an inhibitor of endoplasmic reticulum Ca (2+)-ATPase, concanavalin A, or arachidonic acid. These studies show that vitamin B12 regulates [Ca(2+)]i via several different mechanisms at different B12 concentrations. Participation of G proteins and calmodulin activity in B12-mediated [Ca(2+)]i increase is discussed.

Epidermal growth factor as a local mediator of the neurotrophic action of vitamin B(12) (cobalamin) in the rat central nervous system

We have recently demonstrated that the myelinolytic lesions in the spinal cord (SC) of rats made deficient in vitamin B(12) (cobalamin) (Cbl) through total gastrectomy (TG) are tumor necrosis factor-alpha (TNF-alpha)-mediated. We investigate whether or not permanent Cbl deficiency, induced in the rat either through TG or by chronic feeding of a Cbl-deficient diet, might modify the levels of three physiological neurotrophic factors-epidermal growth factor (EGF), vasoactive intestinal peptide (VIP), and somatostatin (SS)-in the cerebrospinal fluid (CSF) of these rats. We also investigated the ability of the central nervous system (CNS) in these Cbl-deficient rats to synthesize EGF mRNA and of the SC to take up labeled Cbl in vivo. Cbl-deficient rats, however the vitamin deficiency is induced, show a selective decrease in EGF CSF levels and an absence of EGF mRNA in neurons and glia in various CNS areas. In contrast, radiolabeled Cbl is almost exclusively taken up by the SC white matter, but to a much higher degree in totally gastrectomized (TGX) rats. Chronic administration of Cbl to TGX rats restores to normal both the EGF CSF level and EGF mRNA expression in the various CNS areas examined. This in vivo study presents the first evidence that the neurotrophic action of Cbl in the CNS of TGX rats is mediated by stimulation of the EGF synthesis in the CNS itself. It thus appears that Cbl inversely regulates the expression of EGF and TNF-alpha genes in the CNS of TGX rats.

Alteration of mitochondrial DNA and RNA level in human fibroblasts with impaired vitamin B12 coenzyme synthesis

Alterations of mitochondrial (mt) nucleic acid metabolism in methylmalonic aciduria (MMA) were studied in two cell lines from skin fibroblasts of patients with mitochondrial (GM00595) or cytosolic (GM10011) defects in the biosynthesis pathways of cobalamin coenzymes. The mtDNA level increased two-fold in GM00595 cells, which carry a mt defect in the adenosylcobalamin synthesis, whereas no appreciable change was found in GM10011 cells. The content of the two rRNAs 16S and 12S mtRNAs, normalized for the mtDNA copy number, decreased by 70% and 50% in GM00595 and GM10011, respectively. The normalized content of ND1, ND2 and CO I mRNAs decreased in GM00595, but was unchanged in GM10011. Respiratory chain complex activities measured in these two cell lines were not different from control activities. These data suggest that the maintenance of the mt function is due to doubling of mtDNA and that this compensatory response takes place only in those cells in which the greater reduction of the level of rRNA might have brought the content of these transcripts below the threshold value for optimal expression of the mt genome.

Transcobalamin II and in vitro proliferation of leukemic cells

We have recently shown that antibodies to transcobalamin II (TCII) inhibit the in vitro growth of human and murine leukemic cells. This antiproliferative strategy targets the uptake of cobalamin (Cbl), an essential cofactor for two biochemical reactions in humans. To date there has been no appropriate cell culture model available to study antagonism of Cbl as a potential antiproliferative strategy. We have established cell culture conditions which allow reproducible measurements of cell proliferation that is dependent on Cbl and its carrier protein, TCII. This bioassay has allowed us to demonstrate that several monoclonal antibodies, raised against TCII, are potent inhibitors of cell proliferation and that excess Cbl abrogates this inhibitory effect. Thus, supporting our hypothesis that interference with Cbl uptake or metabolism will result in inhibition of cell proliferation. Furthermore, Cbl metabolism appears to provide a useful target for antiproliferative strategies which now involve the use of inactive Cbl analogs. In this review, we update our work on the role of targeting TCII and Cbl as an antiproliferative strategy for leukemic cells. We suggest that this strategy may provide a novel direction for anti cancer reagents.

Polyneuropathy due to cobalamin deficiency in the rat

In the present study, we investigated the peripheral nervous system (PNS) (both in terms of its ultrastructure and in terms of its function) of rats made cobalamin (Cbl)-deficient either through total gastrectomy or through prolonged feeding on a Cbl-deficient diet. In both these types of Cbl-deficient neuropathies we found: (a) ultrastructurally, intramyelin and endoneural edema, with no or minimal axonal damage in the PNS, in dorsal root ganglia, and the ventral and dorsal rootlets of the spinal cord; (b) electrophysiologically, a significant reduction in the nerve conduction velocity, consistent with that reported in (a); (c) morphometrically, a significant reduction in the density of myelinated fibers both in the sciatic nerve and in the peroneal nerve. All these pathological changes were reversed by chronic postoperative administration of Cbl into totally gastrectomized (TGX)-rats, hinting at the specificity of the damage itself in relation to the permanent Cbl-deficient status of the TGX-rats. No signs of segmental demyelination or remyelination were found. We also observed a turning of type I fibers into type II fibers in the soleus muscle of all our Cbl-deficient rats, however the Cbl deficiency had been induced. This muscular change was still present in TGX- and Cbl-treated rats, and it cannot be related to a malnutrition status, since it has been observed also in rats fed a Cbl-deficient diet. All these results demonstrate that Cbl deficiency strongly affects rat PNS within different parameters.

Biochemical and clinical response to hydroxocobalamin versus cyanocobalamin treatment in patients with methylmalonic acidemia and homocystinuria (cblC)

OBJECTIVE

To compare the therapeutic effectiveness of hydroxocobalamin and cyanocobalamin in patients with combined methylmalonic acidemia and homocystinuria.

STUDY DESIGN

Analysis of urine methylmalonic acid, plasma homocystine, and growth of two unrelated patients with cobalamin C disease who were initially receiving cyanocobalamin and were subsequently switched to hydroxocobalamin.

RESULTS

Each patient had a significant decrease in urine methylmalonic acid excretion while receiving cyanocobalamin, but levels remained at least 10 times normal. Cyanocobalamin treatment resulted in a decrease of plasma homocystine to near normal in one patient but had no effect on plasma homocystine in the second patient. Each patient was switched to hydroxocobalamin and urine methylmalonic acid levels decreased to the limit of detection. Plasma homocystine values while taking hydroxocobalamin remained < 5 nmol/ml in both patients. In patient 1, who continued to receive cyanocobalamin therapy for more than 1 year, growth rates (height, weight, and head circumference) were very poor. After initiation of hydroxocobalamin, growth parameters normalized with growth rates above normal.

CONCLUSION

Intramuscular cyanocobalamin treatment is inadequate in the treatment of patients with cobalamin C disease. Appropriate management of cobalamin C disease should include only the hydroxocobalamin form of cobalamin.

Antibodies to Transcobalamin II Block In Vitro Proliferation of Leukemic Cells

The plasma protein transcobalamin II (TCII) binds and delivers cobalamin (Cbl; vitamin B12) to all cells, which internalize the TCII/Cbl complex by receptor-mediated endocytosis. Congenital deficiency of TCII results in intracellular Cbl deficiency, one effect of which is to disrupt DNA synthesis, leading to megaloblastic anemia. We report here an in vitro culture system in which cell growth is dependent on delivery of Cbl to cells by TCII. Recombinant human holo-TCII was shown to support in dose-dependent manner the growth of the human erythroleukemic cell line K562 and the murine lymphoma cell line BW5147. Free Cbl also supported cell growth; however, at 100- to 1,000-fold higher concentrations than those effective in the presence of apo-TCII. To determine if cellular depletion of Cbl could be achieved by interfering with interactions between TCII/Cbl and its cell-surface receptor, several monoclonal antibodies raised against human TCII were studied. Three antibodies, found to compete for the same binding site on TCII, proved to be effective inhibitors of TCII/Cbl-dependent cell growth. Our results suggest that monoclonal anti-TCII antibodies that block the function of this protein may prove useful in antitumor therapies.

Antibodies to Transcobalamin II Block In Vitro Proliferation of Leukemic Cells

The plasma protein transcobalamin II (TCII) binds and delivers cobalamin (Cbl; vitamin B12) to all cells, which internalize the TCII/Cbl complex by receptor-mediated endocytosis. Congenital deficiency of TCII results in intracellular Cbl deficiency, one effect of which is to disrupt DNA synthesis, leading to megaloblastic anemia. We report here an in vitro culture system in which cell growth is dependent on delivery of Cbl to cells by TCII. Recombinant human holo-TCII was shown to support in dose-dependent manner the growth of the human erythroleukemic cell line K562 and the murine lymphoma cell line BW5147. Free Cbl also supported cell growth; however, at 100- to 1,000-fold higher concentrations than those effective in the presence of apo-TCII. To determine if cellular depletion of Cbl could be achieved by interfering with interactions between TCII/Cbl and its cell-surface receptor, several monoclonal antibodies raised against human TCII were studied. Three antibodies, found to compete for the same binding site on TCII, proved to be effective inhibitors of TCII/Cbl-dependent cell growth. Our results suggest that monoclonal anti-TCII antibodies that block the function of this protein may prove useful in antitumor therapies.

Cobalamin (coenzyme B12): synthesis and biological significance

This review examines deoxyadenosylcobalamin (Ado-B12) biosynthesis, transport, use, and uneven distribution among living forms. We describe how genetic analysis of enteric bacteria has contributed to these issues. Two pathways for corrin ring formation have been found-an aerobic pathway (in P. denitrificans) and an anaerobic pathway (in P. shermanii and S. typhimurium)-that differ in the point of cobalt insertion. Analysis of B12 transport in E. coli reveals two systems: one (with two proteins) for the outer membrane, and one (with three proteins) for the inner membrane. To account for the uneven distribution of B12 in living forms, we suggest that the B12 synthetic pathway may have evolved to allow anaerobic fermentation of small molecules in the absence of an external electron acceptor. Later, evolution of the pathway produced siroheme, (allowing use of inorganic electron acceptors), chlorophyll (O2 production), and heme (aerobic respiration). As oxygen became a larger part of the atmosphere, many organisms lost fermentative functions and retained dependence on newer, B12 functions that did not involve fermentation. Paradoxically, Salmonella spp. synthesize B12 only anaerobically but can use B12 (for degradation of ethanolamine and propanediol) only with oxygen. Genetic analysis of the operons for these degradative functions indicate that anaerobic degradation is important. Recent results suggest that B12 can be synthesized and used during anaerobic respiration using tetrathionate (but not nitrate or fumarate) as an electron acceptor. The branch of enteric taxa from which Salmonella spp. and E. coli evolved appears to have lost the ability to synthesize B12 and the ability to use it in propanediol and glycerol degradation. Salmonella spp., but not E. coli, have acquired by horizontal transfer the ability to synthesize B12 and degrade propanediol. The acquired ability to degrade propanediol provides the selective force that maintains B12 synthesis in this group.

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.